Prof. Dr. Gerhard Dehm

Structure and Nano-/Micromechanics of Materials
Max-Planck-Institut für Eisenforschung

Author IDs

  • Phase formation and electrical properties of reactively sputtered Fe1−x O thin films
    Evertz, S. and Nicolin, N. and Cheng, N. and Primetzhofer, D. and Best, J.P. and Dehm, G.
    Journal of Physics D: Applied Physics 57 (2024)
    Wüstite, Fe1−x O, is a crucial phase for the transition to CO2-free steel manufacturing as well as promising for electrochemical applications such as water splitting and ammonia synthesis. To study the effect of interfaces in these applications, thin-film model systems with defined interfaces are ideal. Previous studies lack a description of the growth mechanism to obtain Fe1−x O thin films. Here, we investigate the phase formation of metastable Fe1−x O during reactive magnetron sputtering while systematically varying the O2/Ar flow ratio from 1.8% to 7.2% and the pressure-distance product between 3.5 and 7.2 Pa cm. If bulk diffusion is minimized, thin films containing 96 vol.% wüstite and 4 vol.% Fe as impurity phase were achieved. Therefore, the wüstite phase formation appears to be surface diffusion dominated. To reveal the influence of impurity phases in wüstite on the electrical resistivity, systematic electrical resistivity measurements while cooling in situ were performed for the first time. The electrical resistivity was lower than that of single crystals of the respective iron oxides. This is attributed to the formation of Fe-rich layers at the substrate-film interface, which serve as additional conduction paths. © 2023 The Author(s). Published by IOP Publishing Ltd
    view abstract10.1088/1361-6463/ad0a3e
  • A novel microwall sliding test uncovering the origin of grain refined tribolayers
    Xia, W. and Patil, P.P. and Liu, C. and Dehm, G. and Brinckmann, S.
    Acta Materialia 246 (2023)
    view abstract10.1016/j.actamat.2023.118670
  • Advancing strength and counteracting embrittlement by displacive transformation in heterogeneous high-entropy alloys containing sigma phase
    Lu, W. and Guo, W. and Wang, Z. and Li, J. and An, F. and Dehm, G. and Raabe, D. and Liebscher, C.H. and Li, Z.
    Acta Materialia 246 (2023)
    view abstract10.1016/j.actamat.2023.118717
  • Atomic motifs govern the decoration of grain boundaries by interstitial solutes
    Zhou, Xuyang and Ahmadian, Ali and Gault, Baptiste and Ophus, Colin and Liebscher, Christian H. and Dehm, Gerhard and Raabe, Dierk
    Nature Communications 14 (2023)
    Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive, especially with respect to light interstitial solutes like B and C. Here, we use Fe alloyed with B and C to exploit the strong interdependence of interface structure and chemistry via charge-density imaging and atom probe tomography methods. Direct imaging and quantifying of light interstitial solutes at grain boundaries provide insight into decoration tendencies governed by atomic motifs. We find that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries. This insight not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure. © 2023, The Author(s).
    view abstract10.1038/s41467-023-39302-x
  • Atomic-resolution observations of silver segregation in a [111] tilt grain boundary in copper
    Langenohl, Lena and Brink, Tobias and Richter, Gunther and Dehm, Gerhard and Liebscher, Christian H.
    Physical Review B 107 (2023)
    Alloying a material and hence segregating solutes to grain boundaries is one way to tailor a material to the demands of its application. Direct observation of solute segregation is necessary to understand how the interfacial properties are altered. In this study, we investigate the atomic structure of a high-angle grain boundary both in pure copper and upon silver segregation by aberration-corrected scanning transmission electron microscopy and spectroscopy. We further correlate the experiments to atomistic simulations to quantify the local solute excess and its impact on grain boundary properties. We observe that the grain boundary structure remains intact upon silver segregation and up to five different positions within a structural unit serve as segregation sites. By combining the atomic-resolution observation with atomistic modeling, we are able to quantify the local silver concentration and elucidate the underlying segregation mechanism. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the ""Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.
    view abstract10.1103/PhysRevB.107.134112
  • Author Correction: Dual phase patterning during a congruent grain boundary phase transition in elemental copper (Nature Communications, (2022), 13, 1, (3331), 10.1038/s41467-022-30922-3)
    Langenohl, L. and Brink, T. and Freitas, R. and Frolov, T. and Dehm, G. and Liebscher, C.H.
    Nature Communications 14 (2023)
    view abstract10.1038/s41467-023-36850-0
  • Automated classification of granular bainite and polygonal ferrite by electron backscatter diffraction verified through local structural and mechanical analyses
    Jentner, R.M. and Tsai, S.P. and Welle, A. and Scholl, S. and Srivastava, K. and Best, J.P. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 38 (2023)
    Differentiation of granular bainite and polygonal ferrite in high-strength low-alloy (HSLA) steels possesses a significant challenge, where both nanoindentation and chemical analyses do not achieve an adequate phase classification due to the similar mechanical and chemical properties of both constituents. Here, the kernel average misorientation from electron backscatter diffraction (EBSD) was implemented into a Matlab code to differentiate and quantify the microstructural constituents. Correlative electron channeling contrast imaging (ECCI) validated the automated phase classification results and was further employed to investigate the effect of the grain tolerance angle on classification. Moreover, ECCI investigations highlighted that the grain structure of HSLA steels can be subdivided into four grain categories. Each category contained a different nanohardness or substructure size that precluded a nanoindentation-based phase classification. Consequently, the automated EBSD classification approach based on local misorientation achieved a reliable result using a grain tolerance angle of 5°. Graphical abstract: [Figure not available: see fulltext.] © 2023, The Author(s).
    view abstract10.1557/s43578-023-01113-7
  • Chemical evolution of polycrystalline cementite (Fe3C) during single-pass sliding wear: An investigation by surface spectroscopy
    Tsybenko, H. and Prabhakar, J.M. and Rohwerder, M. and Dehm, G. and Brinckmann, S.
    Acta Materialia 245 (2023)
    view abstract10.1016/j.actamat.2022.118614
  • Decoupling the electrical resistivity contribution of grain boundaries in dilute Fe-alloyed Cu thin films
    Bishara, H. and Langenohl, L. and Zhou, X. and Gault, B. and Best, J.P. and Dehm, G.
    Scripta Materialia 230 (2023)
    view abstract10.1016/j.scriptamat.2023.115393
  • Effect of stiff substrates on enhancing the fracture resistance of Barium Titanate thin films
    Mathews, Nidhin George and Lambai, Aloshious and Mohanty, Gaurav and Venkataramani, N. and Dehm, Gerhard and Jaya, Balila Nagamani
    Materials and Design 235 (2023)
    Damage tolerance of a thin film attached to a substrate is dependent on several parameters such as film thickness, film orientation, residual stresses, nature of interfaces, microstructure and defects present. Here we study the fracture resistance and crack growth trajectory in BaTiO3 thin films attached to elasticially stiff substrates using micromechanical experiments and finite element modeling. Microcantilever fracture tests are carried out on bi-layered systems of BaTiO3 film on Pt-Si and SrTiO3 substrates to study the effect of interfaces, texture and elastic mismatch on fracture toughness. The substrates in the bi-layers are chosen to force the crack to experience a shielding effect in terms of a decrease in the crack driving force. Experiments revealed an unexpectedly large increase in fracture toughness when the crack tip is closer to a stiffer substrate, qualitatively matching the predictions from the numerical model. The bi-layered films attached to the substrate showed 125% (on Pt-Si) and 160% (on SrTiO3) increase in fracture toughness compared to the free-standing films, for the first time, revealing the significant effect of elastic modulus of the substrate on improving the fracture resistance at such micrometer length scales. © 2023 The Authors
    view abstract10.1016/j.matdes.2023.112440
  • Green laser powder bed fusion based fabrication and rate-dependent mechanical properties of copper lattices
    Kang, Sung-Gyu and Gainov, Ramil and Heußen, Daniel and Bieler, Sören and Sun, Zhongji and Weinberg, Kerstin and Dehm, Gerhard and Ramachandramoorthy, Rajaprakash
    Materials and Design 231 (2023)
    Additive manufacturing of pure copper (Cu) via laser-powder bed fusion (L-PBF) is challenging due to the low energy absorptivity under infra-red laser. As a result, 3-dimensional architectures, known for excellent load-bearing and energy absorption capabilities, have not been fabricated in pure Cu, so far. This study, for the first time, Cu lattice structures are fabricated through laser-powder bed fusion (L-PBF) with green laser (λ = 515 nm). Structural and microstructural analysis confirm that the lattice structures consist of well-defined unit-cells and show dense microstructure. The deformation behavior is investigated under a wide range of strain rates from ∼0.001 /s to ∼1000 /s. The stress–strain curves exhibit a smooth and continuous deformation without any post-yield softening, which can be attributed to the intrinsic mechanical properties of Cu. Correlated with post-mortem microscopy examination, the rate-dependent deformation behavior of pure Cu lattice structures is investigated and rationalized. The current work suggests that the complex Cu architectures can be fabricated by L-PBF with green laser and are suitable for dynamic loading applications. © 2023 The Author(s)
    view abstract10.1016/j.matdes.2023.112023
  • Hardening effect of diffusible hydrogen on BCC Fe-based model alloys by in situ backside hydrogen charging
    Rao, Jing and Lee, Subin and Dehm, Gerhard and Duarte, María Jazmin
    Materials and Design 232 (2023)
    Hydrogen embrittlement is common in metallic materials and a critical issue in industries involving hydrogen-related processes. Here we investigate the mechanical response upon hydrogen loading of ferritic Fe-16Cr, Fe-21Cr and Fe-4Al alloys. We use a novel in situ setup for electrochemical backside hydrogen charging during nanoindentation. Single-phase ferritic Fe-Cr binary alloys with high hydrogen diffusivity and low solubility, are ideal for in situ studies during hydrogen charging, particularly the effect of diffusible and lightly trapped hydrogen is targeted. The hardness increases linearly with increasing hydrogen content until a quasi-equilibrium state between hydrogen absorption and desorption is reached while Young's modulus remains unaffected. Above this transient region, the slope of the absolute hardness experiences a drastic decrease. The hardness variation in Fe-21Cr is anisotropic as determined for (1 0 0), (1 1 0) and (1 1 1) oriented grains. Increasing the Cr content enhances the hardening effect in (1 0 0) orientation: a 16.7 % hardness increase is observed in Fe-21Cr, while Fe-16Cr, shows an increment of 10.8 %. A Fe-4Al alloy increases slightly in hardness by only 4.3 % at the applied current density of 3 mA/cm2. The hardening effect is caused by enhancing dislocation density, as revealed by studying the cross-section underneath the nanoindentation imprints. © 2023 The Authors
    view abstract10.1016/j.matdes.2023.112143
  • Interstitial Segregation has the Potential to Mitigate Liquid Metal Embrittlement in Iron
    Ahmadian, Ali and Scheiber, Daniel and Zhou, Xuyang and Gault, Baptiste and Romaner, Lorenz and Kamachali, Reza D. and Ecker, Werner and Dehm, Gerhard and Liebscher, Christian H.
    Advanced Materials 35 (2023)
    The embrittlement of metallic alloys by liquid metals leads to catastrophic material failure and severely impacts their structural integrity. The weakening of grain boundaries (GBs) by the ingress of liquid metal and preceding segregation in the solid are thought to promote early fracture. However, the potential of balancing between the segregation of cohesion-enhancing interstitial solutes and embrittling elements inducing GB de-cohesion is not understood. Here, the mechanisms of how boron segregation mitigates the detrimental effects of the prime embrittler, zinc, in a Σ5 [001] tilt GB in α-Fe (4 at.% Al) is unveiled. Zinc forms nanoscale segregation patterns inducing structurally and compositionally complex GB states. Ab initio simulations reveal that boron hinders zinc segregation and compensates for the zinc-induced loss in GB cohesion. The work sheds new light on how interstitial solutes intimately modify GBs, thereby opening pathways to use them as dopants for preventing disastrous material failure. © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adma.202211796
  • Local strength of bainitic and ferritic HSLA steel constituents understood using correlative electron microscopy and microcompression testing
    Jentner, R.M. and Scholl, S. and Srivastava, K. and Best, J.P. and Kirchlechner, C. and Dehm, G.
    Materials and Design 236 (2023)
    HSLA steels with different polygonal ferrite and granular bainite contents resulting from two different cooling rates were investigated. Micropillar compression tests, electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) experiments were performed to reveal microscopic strength differences and their origin. The obtained results indicate that a higher cooling rate caused a smaller granular bainite substructure size and a higher dislocation density for both ferrite and bainite. In addition, the critical resolved shear stress (CRSS) values for both phases were found to be higher for the faster cooling process. This is ascribed to the increased dislocation density for faster cooling rather than the grain size as will be discussed in the manuscript. Interestingly, the macroscopic yield strength can be closely estimated by the CRSS obtained from micropillar compression considering the corresponding phase fractions. The achieved results can be used in future as input variables for crystal plasticity models. © 2023 The Authors
    view abstract10.1016/j.matdes.2023.112507
  • Mechanical properties and thermal stability of ZrCuAlx thin film metallic glasses: Experiments and first-principle calculations
    Poltronieri, C. and Brognara, A. and Bignoli, F. and Evertz, S. and Djemia, P. and Faurie, D. and Challali, F. and Li, C.H. and Belliard, L. and Dehm, G. and Best, J.P. and Ghidelli, M.
    Acta Materialia 258 (2023)
    In this work, we provide a holistic picture about the relationship between atomic structure, mechanical properties, and thermal stability of ZrCuAlx thin film metallic glasses (TFMGs) varying the Al content from 0 to 12 at.%, carrying out a broad characterization involving experiments and ab initio molecular dynamic simulations (AIMD). We show that the addition of Al resulted in a change of average interatomic distances by ∼10 pm with the formation of shorter bonds (Al-Zr and Al-Cu), influencing the mechanical response (shear/elastic moduli and hardness) which increases by ∼15% for 12 at.% Al. Moreover, tensile tests on polymer substrate revealed a maximum value for the crack initiation strain of 2.1% for ZrCuAl9, while the strain-to-failure rapidly decreases at higher Al contents. The observed reduction in damage tolerance is correlated to a transition in atomic configuration. Specifically, a maximum in density of full and defective icosahedral cluster population is observed at 9 at.% Al, inducing a more shear-resistant behavior to the material. Thermal stability is investigated by high-energy and conventional x-ray diffraction and electrical resistivity measurements as a function of the temperature. Glass transition (Tg) and crystallization (Tx) temperature increase by Al addition reaching 450 and 500 °C, respectively for ZrCuAl12. The increase in thermal stability is related to the reduction in atomic mobility due to the formation of shorter chemical bonds, inhibiting atomic reconfiguration during crystallization. In conclusion, we provide guidelines to the design of compositional-tailored ZrCuAlx TFMGs with tuned mechanical properties and thermal stability with potential impact on industrial applications. © 2023
    view abstract10.1016/j.actamat.2023.119226
  • Melt pool signatures of TiN nanoparticle dry-coated Co25Cr25Fe25Ni25 metal powder in laser-powder-bed-fusion
    Gärtner, E. and Witte, A. and Peter, N.J. and Devulapalli, V. and Ellendt, N. and Dehm, G. and Jägle, E.A. and Uhlenwinkel, V. and Mädler, L.
    Materials and Design 226 (2023)
    view abstract10.1016/j.matdes.2023.111626
  • Microstates and defects of incoherent Σ3 [111] twin boundaries in aluminum
    Ahmad, S. and Brink, T. and Liebscher, C.H. and Dehm, G.
    Acta Materialia 243 (2023)
    view abstract10.1016/j.actamat.2022.118499
  • Microstructure and physical properties of dual-phase soft magnetic Fe-Co-Ti-Ge alloys
    Zhang, X. and Han, L. and Dehm, G. and Liebscher, C.H.
    Journal of Alloys and Compounds 945 (2023)
    view abstract10.1016/j.jallcom.2023.169282
  • Numerical and experimental studies on crack nucleation and propagation in thin films
    Harandi, Ali and Rezaei, Shahed and Karimi Aghda, Soheil and Du, Chaowei and Brepols, Tim and Dehm, Gerhard and Schneider, Jochen M. and Reese, Stefanie
    International Journal of Mechanical Sciences 258 (2023)
    The prediction of damage and cracking patterns in ceramic thin films plays a vital role in the optimal design thereof. In this study, we focus on developing a numerical framework to predict fracture in ceramic thin films. For accurate and efficient modeling, the fracture energy and the material strength (ultimate stress) are taken into account by the cohesive phase-field damage model. Moreover, we argue that the orientation of the grain morphology induces a preferential direction for the crack which serves as a weak spot for crack initiation and propagation. A novel equivalent fracture energy is introduced into the formulation to account for the effects of microstructure on the cracking behavior of thin films. On the experimental side, tensile tests on (V,Al)N and (V,Al)(O,N) thin films deposited on ductile substrates are performed. It has been shown that this approach is a fast and efficient tracking tool for determining the mode-I fracture properties. To evaluate the accuracy of the anisotropic cohesive phase-field damage model proposed in this study, the crack patterns and crack density values of two chemically different thin films are compared for both numerical and experimental setups. This work provides new insights into the effect of different material parameters on the damage behavior of thin films. The key parameters can be summarized as the ultimate strength, fracture energy as well as the existing residual stress within the film. © 2023 Elsevier Ltd
    view abstract10.1016/j.ijmecsci.2023.108568
  • Preferred corrosion pathways for oxygen in Al2Ca – twin boundaries and dislocations
    Peter, N.J. and Zander, D. and Cao, X. and Tian, C. and Zhang, S. and Du, K. and Scheu, C. and Dehm, G.
    Journal of Alloys and Compounds 936 (2023)
    view abstract10.1016/j.jallcom.2022.168296
  • Preventing Hydrogen Embrittlement: The Role of Barrier Coatings for the Hydrogen Economy
    Wetegrove, Marcel and Duarte, Maria Jazmin and Taube, Klaus and Rohloff, Martin and Gopalan, Hariprasad and Scheu, Christina and Dehm, Gerhard and Kruth, Angela
    Hydrogen (Switzerland) 4 (2023)
    Hydrogen barrier coatings are protective layers consisting of materials with a low intrinsic hydrogen diffusivity and solubility, showing the potential to delay, reduce or hinder hydrogen permeation. Hydrogen barrier coatings are expected to enable steels, which are susceptible to hydrogen embrittlement, specifically cost-effective low alloy-steels or light-weight high-strength steels, for applications in a hydrogen economy. Predominantly, ceramic coating materials have been investigated for this purpose, including oxides, nitrides and carbides. In this review, the state of the art with respect to hydrogen permeation is discussed for a variety of coatings. Al2O3, TiAlN and TiC appear to be the most promising candidates from a large pool of ceramic materials. Coating methods are compared with respect to their ability to produce layers with suitable quality and their potential for scaling up for industrial use. Different setups for the characterisation of hydrogen permeability are discussed, using both gaseous hydrogen and hydrogen originating from an electrochemical reaction. Finally, possible pathways for improvement and optimisation of hydrogen barrier coatings are outlined. © 2023 by the authors.
    view abstract10.3390/hydrogen4020022
  • Probing porosity in metals by electrical conductivity: Nanoscale experiments and multiscale simulations
    Kaiser, T. and Dehm, G. and Kirchlechner, C. and Menzel, A. and Bishara, H.
    European Journal of Mechanics, A/Solids 97 (2023)
    view abstract10.1016/j.euromechsol.2022.104777
  • Role of Ag segregation on microscale strengthening and slip transmission in an asymmetric Σ5 copper grain boundary
    Bhat, Mohammed Kamran and Sukumar, Prithiv Thoudden and Langenohl, Lena and Best, James P. and Dehm, Gerhard
    Acta Materialia 255 (2023)
    Micropillar compression was used to investigate whether Ag segregation to an asymmetric Σ5[001] grain boundary will lead to measurable strength differences compared to the pure copper bicrystal. Ag segregation was accomplished by deposition and subsequent annealing of an Ag thin-film applied on the surface of the Cu bicrystal. Atom probe tomography analysis indicated Ag segregation at the grain boundary with a peak concentration of 2.3 at.%. While the pristine Σ5 grain boundary shows a yield strength of 288 ± 18 MPa when compressing 1 µm diameter pillars along 〈001〉, micropillars containing an Ag-segregated Σ5 grain boundary demonstrated an increased yield strength of 318 ± 17 MPa. In addition, post-deformation electron microscopy was carried out to examine the active slip systems and slip transmission across Ag-free and Ag-containing bicrystals. The results are compared to reference measurements of the adjacent single crystal grains. The 1 µm pillar diameter promoted deformation governed by dislocation-grain boundary interactions for the bicrystalline pillars. This is the first time that changes in flow stress associated with grain boundary segregation have been quantified locally without interference from other mechanisms such as solid solution strengthening, formation of precipitates or changes in stacking fault energy. The results clearly indicate that purely geometrical models for slip transmission are not sufficient as the local atomic structure and composition influence dislocation transmission through grain boundaries. © 2023
    view abstract10.1016/j.actamat.2023.119081
  • Segregation-enhanced grain boundary embrittlement of recrystallised tungsten evidenced by site-specific microcantilever fracture
    Tian, Chunhua and Ma, Yan and Ghafarollahi, Alireza and Patil, Piyush and Dehm, Gerhard and Bitzek, Erik and Rasinski, Marcin and Best, James P.
    Acta Materialia 259 (2023)
    Tungsten stands a prime candidate for plasma-facing applications in fusion reactors, attributed to its capacity to withstand high temperatures and intensive particle fluxes. The operational heat flux, however, can induce recrystallisation of the initial microstructure, increasing the brittle-to-ductile transition temperature. Although such a phenomenon is thought to result from impurity segregation to grain boundaries, direct evidence of impurity-induced grain boundary embrittlement has not yet been reported. Addressing this, our study employs microcantilever testing, coupled with local chemical analysis via atom probe tomography, to unveil the impact of impurity segregation on the fracture toughness of recrystallised tungsten with a purity of 99.98 at.%. The in situ fracture toughness measurements were performed with the notch placed directly at random high-angle grain boundaries, revealing brittle failure regardless of grain boundary misorientation or grain orientation. Notably, both single-crystalline microcantilevers and the as-received material exhibited significant plasticity before failure, with instances without crack propagation. In contrast, recrystallised grain boundaries displayed a fracture toughness of 4.7 ± 0.4 MPa·√m, determined using a linear elastic approach - notably lower than for cleavage plane fracture in tungsten microcantilevers. Local atom probe analysis of the high-angle grain boundaries exposed phosphorous segregation exceeding 2 at.% at the recrystallised interfaces, stemming from recrystallisation. Atomistic simulations confirmed the role of phosphorous in embrittling high-angle grain boundaries in tungsten, while additionally revealing mechanisms of crack-grain boundary interactions and their dependence on phosphorous segregation. © 2023
    view abstract10.1016/j.actamat.2023.119256
  • Stability and Failure Mechanisms of Al2O3|Al Bilayer Coatings Exposed to 300 Bar Hydrogen at 673 K
    Hieke, Stefan Werner and Frank, Anna and Duarte, Maria Jazmin and Gopalan, Hariprasad and Patil, Piyush and Wetegrove, Marcel and Rohloff, Martin and Kruth, Angela and Pistidda, Claudio and Dornheim, Martin and Taube, Klaus and Dehm, Gerhard and Scheu, Christina
    Advanced Engineering Materials (2023)
    Hydrogen barrier coatings are important for future hydrogen economy to enable materials for applications in hydrogen tanks. In the present study, coatings consisting of amorphous Al2O3 (≈100 nm) synthesized by plasma ion-assisted deposition on top of crystalline metallic Al (≈100 nm) are exposed to 300 bar hydrogen pressure at 673 K for 6 days. This is done to mimic and accelerate conditions in hydrogen storage containers for metallic hydrides. They remain intact after such harsh conditions, although changes do occur. Blister-like features are observed consisting of a buckled oxide layer while the metallic Al layer underneath is retracted. As these features are also found for coatings annealed under 1 bar Ar atmosphere it is concluded that they are not related to the formation of gas bubbles but they form due to solid-state dewetting. This is different to literature observation where H2 bubbles are reported as a consequence of interface diffusion of H/H+ species present due to the initial precursor used for film deposition. The mechanical properties of the coatings, which are evaluated from nanoindentation load–displacement curves, change only moderately. Overall, the study shows that Al2O3|Al coatings are suitable candidates to prevent hydrogen ingress, but dewetting due to long-term exposure at elevated temperatures must be prevented. © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adem.202300619
  • The role of incoherent twin boundaries on the plasticity of Cu micropillars
    Hosseinabadi, Reza and Brognara, Andrea and Kirchlechner, Christoph and Best, James P. and Dehm, Gerhard
    Materials and Design 232 (2023)
    The role of a ∑3{1 1 2} incoherent twin boundary (ITB) on the shear stress of Cu at the micron scale has been investigated through microcompression of bi-crystalline pillars containing ITB, as well as single-crystalline pillars, in two different compression directions. The Cu sample containing ITBs was synthesized using magnetron sputtering on a sapphire substrate. Firstly, pillars along [1 1 1] compression direction were milled on the film surface. As multiple slip systems were activated upon loading, the dislocation-ITB interaction in this direction was dominated by the dislocation–dislocation interactions. Another set of pillars was milled from the side of the film (in the thickness of the film) in a nominally [134¯] compression direction. Compression in this direction activated a single slip in each grain, which facilitated the investigation of the interaction between dislocations and ITBs. Post-mortem images showed that slip traces were not distinctly connected at the boundary unlike ideal slip transmission in pillars containing a coherent twin boundary. Moreover, bi-crystalline pillars in the single slip direction are stronger than single-crystalline pillars. The observations indicate that ITBs are not impenetrable for dislocations, but the boundary demonstrates some resistance to transmission. © 2023 The Authors
    view abstract10.1016/j.matdes.2023.112164
  • Universality of grain boundary phases in fcc metals: Case study on high-angle [111] symmetric tilt grain boundaries
    Brink, T. and Langenohl, L. and Bishara, H. and Dehm, G.
    Physical Review B 107 (2023)
    view abstract10.1103/PhysRevB.107.054103
  • Unsupervised clustering of nanoindentation data for microstructural reconstruction: Challenges in phase discrimination
    Jentner, R.M. and Srivastava, K. and Scholl, S. and Gallardo-Basile, F.J. and Best, J.P. and Kirchlechner, C. and Dehm, G.
    Materialia 28 (2023)
    view abstract10.1016/j.mtla.2023.101750
  • Deformation and phase transformation in polycrystalline cementite (Fe3C) during single- and multi-pass sliding wear
    Tsybenko, H. and Tian, C. and Rau, J. and Breitbach, B. and Schreiber, P. and Greiner, C. and Dehm, G. and Brinckmann, S.
    Acta Materialia 227 (2022)
    Cementite (Fe3C) plays a major role in the tribological performance of rail and bearing steels. Nonetheless, the current understanding of its deformation behavior during wear is limited because it is conventionally embedded in a matrix. Here, we investigate the deformation and chemical evolution of bulk polycrystalline cementite during single-pass sliding at a contact pressure of 31 GPa and reciprocating multi-pass sliding at 3.3 GPa. The deformation behavior of cementite was studied by electron backscatter diffraction for slip trace analysis and transmission electron microscopy. Our results demonstrate activation of several deformation mechanisms below the contact surface: dislocation slip, shear band formation, fragmentation, grain boundary sliding, and grain rotation. During sliding wear, cementite ductility is enhanced due to the confined volume, shear/compression domination, and potentially frictional heating. The microstructural alterations during multi-pass wear increase the subsurface nanoindentation hardness by up to 2.7 GPa. In addition, we report Hägg carbide (Fe5C2) formation in the uppermost deformed regions after both sliding experiments. Based on the results of electron and X-ray diffraction, as well as atom probe tomography, we propose potential sources of excess carbon and mechanisms that promote the phase transformation. © 2022 The Author(s)
    view abstract10.1016/j.actamat.2022.117694
  • Dislocation-enhanced electrical conductivity in rutile TiO2 accessed by room-temperature nanoindentation
    Bishara, H. and Tsybenko, H. and Nandy, S. and Muhammad, Q.K. and Frömling, T. and Fang, X. and Best, J.P. and Dehm, G.
    Scripta Materialia 212 (2022)
    Dislocation-enhanced electrical conductivity is an emerging topic for ceramic oxides. In contrast to the majority of present studies which focus on large-scale crystal deformation or thin film fabrication to introduce dislocations, we use a nanoindentation “pop-in stop” method to locally generate 〈011〉 edge-type dislocations at room temperature, without crack formation, on the (100) surface of a rutile TiO2 single-crystal. Ion beam assisted deposition of microcontacts allowed for both deformed and non-deformed zones to be locally probed by impedance spectroscopy. Compared to the dislocation-free region, a local enhancement of the electrical conductivity by 50% in the dislocation-rich regions is found. The study paves the way for local “mechanical-doping” of ceramics and oxide materials, allowing for the use of dislocations to tune the local conductivity with high spatial resolution. © 2022 The Author(s)
    view abstract10.1016/j.scriptamat.2022.114543
  • Dislocation-mediated electronic conductivity in rutile
    Muhammad, Q.K. and Bishara, H. and Porz, L. and Dietz, C. and Ghidelli, M. and Dehm, G. and Frömling, T.
    Materials Today Nano 17 (2022)
    It has been recently shown that doping-like properties can be introduced into functional ceramics by inducing dislocations. Especially for TiO2, donor and acceptor-like behavior were observed depending on the type of introduced mesoscopic dislocation network. However, these early reports could not fully elucidate the mechanism behind it. In this work, we rationalize the electrical properties of dislocations by targeted microelectrode impedance measurements, local conductivity atomic force microscopy, and Kelvin probe force microscopy on deformed single crystals, comparing dislocation-rich and deficient regions. With the help of finite element method calculations, a semi-quantitative model for the effect of dislocations on the macroscopic electrical properties is developed. The model describes the dislocation bundles as highly conductive regions in which respective space charges overlap and induce temperature-independent, highly stable electronic conductivity. We illustrate the mechanism behind unique electrical properties tailored by introducing dislocations and believe that these results are the cornerstone in developing dislocation-tuned functionality in ceramics. © 2021 Elsevier Ltd
    view abstract10.1016/j.mtnano.2021.100171
  • Dual phase patterning during a congruent grain boundary phase transition in elemental copper
    Frommeyer, L. and Brink, T. and Freitas, R. and Frolov, T. and Dehm, G. and Liebscher, C.H.
    Nature Communications 13 (2022)
    view abstract10.1038/s41467-022-30922-3
  • Dynamic cryo-mechanical properties of additively manufactured nanocrystalline nickel 3D microarchitectures
    Schwiedrzik, J. and Ramachandramoorthy, R. and Edwards, T.E.J. and Schürch, P. and Casari, D. and Duarte, M.J. and Mohanty, G. and Dehm, G. and Maeder, X. and Philippe, L. and Breguet, J.-M. and Michler, J.
    Materials and Design 220 (2022)
    view abstract10.1016/j.matdes.2022.110836
  • Effect of composition and nanostructure on the mechanical properties and thermal stability of Zr100-xCux thin film metallic glasses
    Brognara, A. and Best, J.P. and Djemia, P. and Faurie, D. and Dehm, G. and Ghidelli, M.
    Materials and Design 219 (2022)
    view abstract10.1016/j.matdes.2022.110752
  • Effect of hybridization in PdAlY-(Ni/Au/Ir) metallic glasses thin films on electrical resistivity
    Bishara, H. and Kontis, P. and Dehm, G. and Schneider, J.M. and Evertz, S.
    Scripta Materialia 214 (2022)
    view abstract10.1016/j.scriptamat.2022.114681
  • Elucidating dynamic precipitation and yield strength of rolled Mg–Al–Ca–Mn alloy
    Li, J. and Zhou, X. and Su, J. and Breitbach, B. and Chwałek, M.L. and Wang, H. and Dehm, G.
    Materials Science and Engineering A 856 (2022)
    view abstract10.1016/j.msea.2022.143898
  • Elucidation of formation and transformation mechanisms of Ca-rich Laves phase in Mg-Al-Ca-Mn alloys
    Li, J. and Zhou, X. and Breen, A. and Peng, Z. and Su, J. and Kürnsteiner, P. and Correa, M.J.D. and Chwałek, M.L. and Wang, H. and Holec, D. and Mayer, J. and Dehm, G.
    Journal of Alloys and Compounds 928 (2022)
    view abstract10.1016/j.jallcom.2022.167177
  • Free, flexible and fast: Orientation mapping using the multi-core and GPU-accelerated template matching capabilities in the Python-based open source 4D-STEM analysis toolbox Pyxem
    Cautaerts, N. and Crout, P. and Ånes, H.W. and Prestat, E. and Jeong, J. and Dehm, G. and Liebscher, C.H.
    Ultramicroscopy 237 (2022)
    view abstract10.1016/j.ultramic.2022.113517
  • Implication of grain-boundary structure and chemistry on plasticity and failure
    Dehm, G. and Cairney, J.
    MRS Bulletin 47 (2022)
    view abstract10.1557/s43577-022-00378-3
  • Influence of crystal orientation on twinning in austenitic stainless-steel during single micro-asperity tribology and nanoindentation
    Patil, P. and Lee, S. and Dehm, G. and Brinckmann, S.
    Wear 504-505 (2022)
    view abstract10.1016/j.wear.2022.204403
  • Massive interstitial solid solution alloys achieve near-theoretical strength
    Liu, C. and Lu, W. and Xia, W. and Du, C. and Rao, Z. and Best, J.P. and Brinckmann, S. and Lu, J. and Gault, B. and Dehm, G. and Wu, G. and Li, Z. and Raabe, D.
    Nature Communications 13 (2022)
    Interstitials, e.g., C, N, and O, are attractive alloying elements as small atoms on interstitial sites create strong lattice distortions and hence substantially strengthen metals. However, brittle ceramics such as oxides and carbides usually form, instead of solid solutions, when the interstitial content exceeds a critical yet low value (e.g., 2 at.%). Here we introduce a class of massive interstitial solid solution (MISS) alloys by using a highly distorted substitutional host lattice, which enables solution of massive amounts of interstitials as an additional principal element class, without forming ceramic phases. For a TiNbZr-O-C-N MISS model system, the content of interstitial O reaches 12 at.%, with no oxides formed. The alloy reveals an ultrahigh compressive yield strength of 4.2 GPa, approaching the theoretical limit, and large deformability (65% strain) at ambient temperature, without localized shear deformation. The MISS concept thus offers a new avenue in the development of metallic materials with excellent mechanical properties. © 2022, The Author(s).
    view abstract10.1038/s41467-022-28706-w
  • Metallographic preparation methods for the Mg based system Mg-Al-Ca and its Laves phases
    Andre, D. and Freund, M. and Rehman, U. and Delis, W. and Felten, M. and Nowak, J. and Tian, C. and Zubair, M. and Tanure, L. and Abdellaoui, L. and Springer, H. and Best, J.P. and Zander, D. and Dehm, G. and Sandlöbes-Haut, S. and Korte-Kerzel, S.
    Materials Characterization 192 (2022)
    view abstract10.1016/j.matchar.2022.112187
  • Microstructure and residual stress evolution in nanocrystalline Cu-Zr thin films
    Chakraborty, J. and Oellers, T. and Raghavan, R. and Ludwig, A. and Dehm, G.
    Journal of Alloys and Compounds 896 (2022)
    Grazing incidence X-ray diffraction (GIXRD) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) were employed to study the microstructure evolution and stress development in the nanocrystalline Cu100−X-ZrX (2.5 at% ≤ x ≤ 5.5 at%) alloy thin films. Small Zr additions to Cu led to significant lattice parameter anisotropy in the as-deposited Cu-Zr thin films both due to macroscopic lattice strain and stacking faults in the Cu matrix. Strain free lattice parameters obtained after the XRD stress analysis of Cu-Zr thin films confirmed formation of a supersaturated substitutional Cu-Zr solid solution. For the first time, the study of film microstructure by XRD line profile analysis (XLPA) confirmed progressive generation of dislocations and planar faults with increasing Zr composition in Cu-Zr alloy films. These microstructural changes led to the generation of tensile stresses in the thin films along with considerable stress gradients across the films thicknesses which are quantified by the traditional dψhkl−Sin2ψ and GIXRD stress measurement methods. The origin of tensile stresses and stress gradients in the Cu-Zr film are discussed on the basis of film growth and heterogeneous microstructure with changing Zr composition. © 2021
    view abstract10.1016/j.jallcom.2021.162799
  • Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films
    Devulapalli, V. and Hans, M. and Sukumar, P.T. and Schneider, J.M. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 238 (2022)
    view abstract10.1016/j.actamat.2022.118180
  • Multiscale characterization of damage tolerance in barium titanate thin films
    Mathews, N.G. and Saxena, A.K. and Venkataramani, N. and Dehm, G. and Jaya, B.N.
    Journal of Applied Physics 132 (2022)
    view abstract10.1063/5.0095139
  • Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study
    Šestan, A. and Sreekala, L. and Markelj, S. and Kelemen, M. and Zavašnik, J. and Liebscher, C.H. and Dehm, G. and Hickel, T. and Čeh, M. and Novak, S. and Jenuš, P.
    Acta Materialia 226 (2022)
    Tungsten-tungsten carbide (W/W2C) composites are considered as possible structural materials for future nuclear fusion reactors. Here, we report on the effect of helium (He) implantation on microstructure evolution of polycrystalline W/W2C composite consolidated by field-assisted sintering technique (FAST), homogenously implanted at room temperature with 1 MeV 4He+ ions at the fluence of 8 × 1016 ions cm−2 and annealed at 1873 K for 20 minutes. Samples were analysed by scanning and transmission electron microscopy to study the presence and size of He bubbles. Monomodal He bubbles in W (30-80 nm) are limited to point defects and grain boundaries, with a considerable void denuded zone (150 nm). Bubbles do not form in W2C, but at the W|W2C interface and are considerably larger (200-400 nm). The experimental observations on He behaviour and migration in W and W2C were assessed by density functional theory (DFT) calculations, suggesting He migration and accumulation in the composite are determined by the effective He-He binding in clusters, which will give rise to decohesion. In the presence of He clusters, the decohesion of bulk W into free surfaces is energetically highly favourable but not sufficient in the W2C; hence bubbles are only observed in W grains and interfaces and not within bulk W2C. © 2022
    view abstract10.1016/j.actamat.2021.117608
  • Obtaining different orientation relationships for Cu films grown on (0001) α-Al2O3substrates by magnetron sputtering
    Dehm, G. and Edongué, H. and Wagner, T. and Oh, S.H. and Arzt, E.
    International Journal of Materials Research 96 (2022)
    Cu films were grown on (0001) α-Al2O3 single-crystals by magnetron sputtering. The growth behavior was manipulated by Ar+-ion sputter cleaning of the substrates at kinetic energies between 100 and 500 eV, changing the sputter rate from 0.75 to 1.1 nm/s, and using nominal substrate temperatures of 100 and 200 °C, respectively. Polycrystalline Cu films formed on α-Al2O3 substrates after an Ar+-ion bombardment at 500 eV, while epitaxial Cu films evolved when Ar+-ion energies of 100 and 200 eV were used. The epitaxial Cu films always consisted of two twin-related growth variants. However, two different orientation relationships emerged which differ by a 30° in-plane rotation of the (111) oriented Cu films when the deposition rate is changed from 0.75 to 1.1 nm/s. The results will be discussed on the basis of differences in the growth process. © 2005 Carl Hanser Verlag, München.
    view abstract10.3139/ijmr-2005-0045
  • On the interplay between microstructure, residual stress and fracture toughness of (Hf-Nb-Ta-Zr)C multi-metal carbide hard coatings
    Gopalan, H. and Marshal, A. and Hans, M. and Primetzhofer, D. and Cautaerts, N. and Breitbach, B. and Völker, B. and Kirchlechner, C. and Schneider, J.M. and Dehm, G.
    Materials and Design 224 (2022)
    view abstract10.1016/j.matdes.2022.111323
  • Size scaling in bi-crystalline Cu micropillars containing a coherent twin boundary
    Hosseinabadi, R. and Riesch-Oppermann, H. and Best, J.P. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 230 (2022)
    view abstract10.1016/j.actamat.2022.117841
  • Strain rate dependent deformation behavior of BCC-structured Ti29Zr24Nb23Hf24 high entropy alloy at elevated temperatures
    Cao, T. and Guo, W. and Lu, W. and Xue, Y. and Lu, W. and Su, J. and Liebscher, C.H. and Li, C. and Dehm, G.
    Journal of Alloys and Compounds 891 (2022)
    The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10−3 to 10 s−1, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10−3 s−1. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s−1, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening. © 2021 Elsevier B.V.
    view abstract10.1016/j.jallcom.2021.161859
  • Strategies for damage tolerance enhancement in metal/ceramic thin films: Lessons learned from Ti/TiN
    Mishra, A.K. and Gopalan, H. and Hans, M. and Kirchlechner, C. and Schneider, J.M. and Dehm, G. and Jaya, B.N.
    Acta Materialia 228 (2022)
    Most functional microelectronic devices as well as hard coatings use brittle ceramics like Titanium Nitride (TiN) in nanostructured, thin film form. Damage tolerance is critical to their deployment in service, and life extension. In this study, we explore multilayering to enhance the damage tolerance of such material systems. Ti/TiN is a model metal/ceramic system with a strong interface, where elastic-plastic mismatch could potentially be used to modify the crack driving force. We carry out systematic numerical simulations of crack driving force in Ti/TiN multilayers with changing layer spacing. Micro-cantilever experiments are then carried out on a selected set of multilayers to determine the fracture toughness as a function of the number of interfaces. The 50 layer multilayer exhibits a fracture toughness that is 82% higher than the single layer TiN while maintaining a comparable hardness to the latter. The weak intercolumnar boundaries of sputtered films are found to be a limitation in fully exploiting the advantage of the shielding effect due to alternating stiff and compliant layers. The results are discussed in the context of design parameters for multilayering in metal/ceramic thin film systems in general. © 2022 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2022.117777
  • The origin of jerky dislocation motion in high-entropy alloys
    Utt, D. and Lee, S. and Xing, Y. and Jeong, H. and Stukowski, A. and Oh, S.H. and Dehm, G. and Albe, K.
    Nature Communications 13 (2022)
    view abstract10.1038/s41467-022-32134-1
  • Ultralong one-dimensional plastic zone created in aluminum underneath a nanoscale indent
    Nie, Z.-Y. and Sato, Y. and Ogata, S. and Duarte, M.J. and Dehm, G. and Li, J. and Ma, E. and Xie, D.-G. and Shan, Z.-W.
    Acta Materialia 232 (2022)
    view abstract10.1016/j.actamat.2022.117944
  • Aluminum depletion induced by co-segregation of carbon and boron in a bcc-iron grain boundary
    Ahmadian, A. and Scheiber, D. and Zhou, X. and Gault, B. and Liebscher, C.H. and Romaner, L. and Dehm, G.
    Nature Communications 12 (2021)
    The local variation of grain boundary atomic structure and chemistry caused by segregation of impurities influences the macroscopic properties of polycrystalline materials. Here, the effect of co-segregation of carbon and boron on the depletion of aluminum at a Σ5 (3 1 0)[0 0 1] tilt grain boundary in a α − Fe-4 at%Al bicrystal is studied by combining atomic resolution scanning transmission electron microscopy, atom probe tomography and density functional theory calculations. The atomic grain boundary structural units mostly resemble kite-type motifs and the structure appears disrupted by atomic scale defects. Atom probe tomography reveals that carbon and boron impurities are co-segregating to the grain boundary reaching levels of >1.5 at%, whereas aluminum is locally depleted by approx. 2 at.%. First-principles calculations indicate that carbon and boron exhibit the strongest segregation tendency and their repulsive interaction with aluminum promotes its depletion from the grain boundary. It is also predicted that substitutional segregation of boron atoms may contribute to local distortions of the kite-type structural units. These results suggest that the co-segregation and interaction of interstitial impurities with substitutional solutes strongly influences grain boundary composition and with this the properties of the interface. © 2021, The Author(s).
    view abstract10.1038/s41467-021-26197-9
  • Automated Crystal Orientation Mapping by Precession Electron Diffraction-Assisted Four-Dimensional Scanning Transmission Electron Microscopy Using a Scintillator-Based CMOS Detector
    Jeong, J. and Cautaerts, N. and Dehm, G. and Liebscher, C.H.
    Microscopy and Microanalysis 27 (2021)
    The recent development of electron-sensitive and pixelated detectors has attracted the use of four-dimensional scanning transmission electron microscopy (4D-STEM). Here, we present a precession electron diffraction-assisted 4D-STEM technique for automated orientation mapping using diffraction spot patterns directly captured by an in-column scintillator-based complementary metal-oxide-semiconductor (CMOS) detector. We compare the results to a conventional approach, which utilizes a fluorescent screen filmed by an external charge charge-coupled device camera. The high-dynamic range and signal-to-noise characteristics of the detector greatly improve the image quality of the diffraction patterns, especially the visibility of diffraction spots at high scattering angles. In the orientation maps reconstructed via the template matching process, the CMOS data yield a significant reduction of false indexing and higher reliability compared to the conventional approach. The angular resolution of misorientation measurement could also be improved by masking reflections close to the direct beam. This is because the orientation sensitive, weak, and small diffraction spots at high scattering angles are more significant. The results show that fine details, such as nanograins, nanotwins, and sub-grain boundaries, can be resolved with a sub-degree angular resolution which is comparable to orientation mapping using Kikuchi diffraction patterns. Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of the Microscopy Society of America.
    view abstract10.1017/S1431927621012538
  • Combinatorial exploration of B2/L21 precipitation strengthened AlCrFeNiTi compositionally complex alloys
    Wolff-Goodrich, S. and Marshal, A. and Pradeep, K.G. and Dehm, G. and Schneider, J.M. and Liebscher, C.H.
    Journal of Alloys and Compounds 853 (2021)
    Using both novel high-throughput screening via combinatorial thin film deposition and conventional bulk alloy synthesis techniques, a large region of the AlCrFeNiTi composition space has been probed for alloys that could serve as low cost alternatives to nickel-base superalloys for medium-to-high temperature structural applications. Phase formation trends in this highly complex alloying system have been determined using characterisation techniques that span multiple length scales—from bulk X-ray diffraction and differential scanning calorimetry to atomically resolved scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. A large region of stability for both disordered A2 and ordered B2/L21 type phases is observed, with several compositions exhibiting fine-scaled precipitation structures of these two phases. For alloys with ≥20 at.% Al, the precipitation structure was further refined to a nano-scale lamellar arrangement of A2 and B2/L21 phases. Formation of C14 Laves phase, especially for compositions with >10 at.% Ti, has consistently been observed. We include a screening of the mechanical properties based on nanoindentation and macroscopic hardness test data correlated with scanning electron microscope (SEM) observations of the hardness indents. The phase formation trends observed by both combinatorial thin film deposition and bulk alloy synthesis are discussed in detail for samples in the as-deposited and as-cast conditions, respectively. © 2020 Elsevier Ltd
    view abstract10.1016/j.jallcom.2020.156111
  • Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance
    Luo, T. and Serrano-Sánchez, F. and Bishara, H. and Zhang, S. and Villoro, B. and Kuo, J.J. and Felser, C. and Scheu, C. and Snyder, G.J. and Best, J.P. and Dehm, G. and Yu, Y. and Raabe, D. and Fu, C. and Gault, B.
    Acta Materialia 217 (2021)
    Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries. © 2021 The Authors
    view abstract10.1016/j.actamat.2021.117147
  • Effect of synthesis temperature on the phase formation of NiTiAlFeCr compositionally complex alloy thin films
    Marshal, A. and Singh, P. and Music, D. and Wolff-Goodrich, S. and Evertz, S. and Schökel, A. and Johnson, D.D. and Dehm, G. and Liebscher, C.H. and Schneider, J.M.
    Journal of Alloys and Compounds 854 (2021)
    The synthesis temperature dependent phase formation of Ni10Ti10Al25Fe35Cr20 thin films is compared to a bulk processed sample of identical composition. The as-cast alloy exhibits a dual-phase microstructure which is composed of a disordered BCC phase and AlNiTi-based B2- and/or L21-ordered phase(s). Formation of the BCC phase as well as an ordered AlNi-based B2 phase is observed for a thin film synthesised at 500 °C (ratio of synthesis temperature of thin film to melting temperature of bulk alloy: T/Tm = 0.49), which is attributed to both surface and bulk diffusion mediated growth. Post deposition annealing at 900 °C (T/Tm = 0.75) of a thin film deposited without intentional heating results in the formation of NiAlTi-based B2 and/or L21-phase(s) similar to the bulk sample, which is attributed to bulk diffusion. Depositions conducted at room temperature without intentional substrate heating (T/Tm = 0.20) resulted in the formation of an X-ray amorphous phase, while a substrate temperature increase to 175 °C (T/Tm = 0.28) causes the formation of a BCC phase. Atom probe tomography of the thin films deposited without intentional substrate heating and at 175 °C indicates the formation of ∼5 nm and ∼10 nm FeAl-rich domains, respectively. This can be rationalized based on the activation energy for surface diffusion, as Ti and Ni exhibt 2.5 to 4 times larger activation energy barriers than Al, Fe and Cr. It is evident from the homologous temperature that the phase formation observed at 500 °C (T/Tm = 0.49) is a result of both surface and bulk diffusion. As the temperature is reduced, the formation of FeAl-rich domains can be understood based on the differences in activation energy for surface diffusion and is consistent with kinetically limited thin film growth. © 2020 Elsevier B.V.
    view abstract10.1016/j.jallcom.2020.155178
  • Faceting diagram for Ag segregation induced nanofaceting at an asymmetric Cu tilt grain boundary
    Peter, N.J. and Duarte, M.J. and Kirchlechner, C. and Liebscher, C.H. and Dehm, G.
    Acta Materialia 214 (2021)
    In this work, we experimentally establish the isothermal nanofacet evolution at an asymmetric ∑5 tilt grain boundary in the Cu-Ag system using a diffusion couple approach. We investigate the nanofacet formation along the grain boundary in dependence of the Ag solute excess concentration. The initial grain boundary dissociates into asymmetric Ag-lean segments and Ag-rich symmetric (210) segments. Increasing Ag excess leads to an increase in Ag-rich facet segment length, while the length of the asymmetric facets remains constant. From this, we construct a grain boundary nanofaceting diagram deduced from our experiments relating local atomic structure, overall inclination and Ag solute excess. © 2021 The Author(s)
    view abstract10.1016/j.actamat.2021.116960
  • In situ nanoindentation during electrochemical hydrogen charging: a comparison between front-side and a novel back-side charging approach
    Duarte, M.J. and Fang, X. and Rao, J. and Krieger, W. and Brinckmann, S. and Dehm, G.
    Journal of Materials Science 56 (2021)
    The effects of hydrogen in metals are a pressing issue causing severe economic losses due to material deterioration by hydrogen embrittlement. A crucial understanding of the interactions of hydrogen with different microstructure features can be reached by nanoindentation due to the small volumes probed. Even more, in situ testing while charging the sample with hydrogen prevents the formation of concentration gradients due to hydrogen desorption. Two custom electrochemical cells for in situ testing were built in-house to charge the sample with hydrogen during nanoindentation: “front-side” charging with the sample and the indenter tip immersed into the electrolyte, and “back-side” charging where the analyzed region is never in contact with the solution. During front-side charging, surface degradation often occurs which also negatively influences analyses after hydrogen charging. The back-side charging approach proposed in this work is a promising technique for studying in situ the effects of hydrogen in alloys under mechanical loads, while completely excluding the influence of the electrolyte on the nanoindented surface. Hydrogen diffusion from the charged back-side toward the testing surface is here demonstrated by Kelvin probe measurements in ferritic FeCr alloys, used as a case study due to the high mobility of hydrogen in the bcc lattice. During nanoindentation, a reduction on the shear stress necessary for dislocations nucleation due to hydrogen was observed using both setups; however, the quantitative data differs and a contradictory behavior was found in hardness measurements. Finally, some guidelines for the use of both approaches and a summary of their advantages and disadvantages are presented. Graphical abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
    view abstract10.1007/s10853-020-05749-2
  • Influence of strain rate on the activation of {110}, {112}, {123} slip in ferrite of DP800
    Tian, C. and Dehm, G. and Kirchlechner, C.
    Materialia 15 (2021)
    We have performed micro pillar compression to investigate the influence of strain rate on the activation of three slip plane families, namely {110}, {112} and {123}, in ferrite of a dual phase steel. The critical resolved shear stress of all three slip plane families rises with increased strain rate. The strain rate sensitivity drops with increasing strain. Increasing strain rate does not reduce the number of activated slip systems, instead resulting in slip plane activation outside of that predicted by Schmid´s law. The activation volume of 13b³ to 16b³ suggests that the Peierl's process is the rate controlling mechanism in ferrite of DP800. © 2020
    view abstract10.1016/j.mtla.2020.100983
  • Influence of substrates and e-beam evaporation parameters on the microstructure of nanocrystalline and epitaxially grown Ti thin films
    Devulapalli, V. and Bishara, H. and Ghidelli, M. and Dehm, G. and Liebscher, C.H.
    Applied Surface Science 562 (2021)
    Titanium thin films were deposited on silicon nitride (SiNx) coated Si, NaCl, and sapphire substrates varying the deposition conditions using e-beam evaporation to investigate thin film growth modes. The microstructure and texture evolution in dependence of substrate, deposition rate, film thickness, and substrate temperature were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Thin films obtained on SiNx and NaCl substrates were nanocrystalline, while the films deposited on sapphire transformed from nanocrystalline to single crystalline at deposition temperatures above 200 °C. Predominantly, a surface plane orientation of (0002) was observed for the single crystalline films due to the minimization of surface energy. The orientation relationship of epitaxial single crystalline films grown on C-plane sapphire substrate is found to be (0002)Ti ‖ (0006)Sapphire,〈112¯0〉Ti ‖ 〈033¯0〉Sapphire. In this orientation relationship, both the total surface and strain energy of the film are minimized. The results were complemented by resistivity measurements using the four-point probe method reporting an increase from ~60 μ Ω cm to ~95 μΩ cm for single crystalline and nanocrystalline films, respectively. © 2021 Elsevier B.V.
    view abstract10.1016/j.apsusc.2021.150194
  • Investigation of the orientation relationship between nano-sized G-phase precipitates and austenite with scanning nano-beam electron diffraction using a pixelated detector
    Cautaerts, N. and Rauch, E.F. and Jeong, J. and Dehm, G. and Liebscher, C.H.
    Scripta Materialia 201 (2021)
    Scanning nano-beam electron diffraction with a pixelated detector was employed to investigate the orientation relationship of nanometer sized, irradiation induced G-phase (M6Ni16Si7) precipitates in an austenite matrix. Using this detector, the faint diffraction spots originating from the small G-phase particles could be resolved simultaneously as the intense matrix reflections. The diffraction patterns were analyzed using a two-stage template matching scheme, whereby the matrix is indexed first and the precipitates are indexed second after subtraction of the matrix contribution to the diffraction patterns. The results show that G-phase forms with orientation relationships relative to austenite that are characteristic of face-centered cubic (FCC) to body-centered cubic (BCC) transformations. This work demonstrates that nano-beam electron diffraction with a pixelated detector is a promising technique to investigate orientation relationships of nano-sized precipitates with complex crystal structures in other material systems with relative ease. © 2021 The Author(s)
    view abstract10.1016/j.scriptamat.2021.113930
  • Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc?
    Kini, M.K. and Lee, S. and Savan, A. and Breitbach, B. and Addab, Y. and Lu, W. and Ghidelli, M. and Ludwig, Al. and Bozzolo, N. and Scheu, C. and Chatain, D. and Dehm, G.
    Surface and Coatings Technology 410 (2021)
    The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.126945
  • Nanoindentation pop-in in oxides at room temperature: Dislocation activation or crack formation?
    Fang, X. and Bishara, H. and Ding, K. and Tsybenko, H. and Porz, L. and Höfling, M. and Bruder, E. and Li, Y. and Dehm, G. and Durst, K.
    Journal of the American Ceramic Society (2021)
    Most oxide ceramics are known to be brittle macroscopically at room temperature with little or no dislocation-based plasticity prior to crack propagation. Here, we demonstrate the size-dependent brittle to ductile transition in SrTiO3 at room temperature using nanoindentation pop-in events visible as a sudden increase in displacement at nominally constant load. We identify that the indentation pop-in event in SrTiO3 at room temperature, below a critical indenter tip radius, is dominated by dislocation-mediated plasticity. When the tip radius increases to a critical size, concurrent dislocation activation and crack formation, with the latter being the dominating process, occur during the pop-in event. Beyond the experimental examination and theoretical justification presented on SrTiO3 as a model system, further validation on α-Al2O3, BaTiO3, and TiO2 are briefly presented and discussed. A new indentation size effect, mainly for brittle ceramics, is suggested by the competition between the dislocation-based plasticity and crack formation at small scale. Our finding complements the deformation mechanism in the nano-/microscale deformation regime involving plasticity and cracking in ceramics at room temperature to pave the road for dislocation-based mechanics and functionalities study in these materials. © 2021 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.17806
  • On the fracture behavior of Cr2AlC coatings
    Völker, B. and Stelzer, B. and Mráz, S. and Rueß, H. and Sahu, R. and Kirchlechner, C. and Dehm, G. and Schneider, J.M.
    Materials and Design 206 (2021)
    Bulk MAX phase materials were investigated heavily in the last decades due to their advantageous combination of metallic and ceramic properties. In recent years, MAX phases also gained the interest of the protective coatings community. Cr2AlC is a very promising material, since the crystalline MAX phase can be deposited at comparatively low (550 °C) substrate temperatures. Another advantage of the Cr2AlC MAX phase is its self-healing ability. The goal of this investigation was to characterize the fracture toughness of Cr2AlC protective coatings using in situ SEM micro-cantilever tests and to determine the influence of different microstructures on the fracture behavior. Surprisingly, the fracture toughness is only moderately affected by the microstructure of the crystalline samples investigated here, which reveal a fracture toughness ranging from 1.8 ± 0.1 MPam1/2 to 2.4 ± 0.2 MPam1/2. In contrast to that, it could be shown that there is a significant increase in fracture toughness for the amorphous coating with identical chemical composition (4.1 ± 0.5 MPam1/2) of almost twice the fracture toughness compared to the crystalline coatings. The detrimental influence of grain boundaries in the crystalline coating and the lack of grain boundaries in the amorphous sample might explain the formidable fracture toughness. © 2021 The Authors
    view abstract10.1016/j.matdes.2021.109757
  • On the role of pre-existing defects in influencing hardness in nanoscale indentations — Insights from atomistic simulations
    Chauniyal, A. and Dehm, G. and Janisch, R.
    Journal of the Mechanics and Physics of Solids 154 (2021)
    Using in-situ nanoindentation experiments it is possible to study the dislocation mechanisms which unfold under an indenter.Large-scale atomistic simulations of the same are possible due to similarities in length scale, provided that defects can be included in the simulation. Yet, nanoindentation simulations have so far been mostly undertaken on defect free samples, while studies with pre-existing defects are few. The latter show that the average hardness is not affected by the presence of pre-existing defects, which justifies the use of ideal crystals in such simulations. However, this observation is counter-intuitive, as indenter-defect interactions should lead to work hardening and manifest themselves in hardness calculations. Our simulations along with a new look at the evolution of dislocations under the indenter, show for the first time, that hardness in atomistic simulations is influenced by pre-existing defects in the sample. Utilizing a face-centred tetragonal TiAl bicrystal with misfit dislocations at the interface, to populate the sample with defects, we correlate the contact-pressure variations to defect-indenter interactions. We show that the measured contact-pressure is affected by the presence and nature of defects under the indenter. Dislocation pile ups lead to intermittent rise in contact pressure, while seamless growth leads to steady convergence. The sensitivity to detect such defect interactions depends upon indenter size while convergence to average hardness is a result of curvature accommodation near the surface. Our findings prove that pre-existing defects have a profound influence on calculated hardness in indentation simulations which also corroborates with experimental observations in the literature. © 2021 Elsevier Ltd
    view abstract10.1016/j.jmps.2021.104511
  • Phase decomposition in nanocrystalline Cr0.8Cu0.2 thin films
    Chakraborty, J. and Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 888 (2021)
    Metastable Cr0.8Cu0.2 alloy thin films with nominal thickness of 360 nm have been deposited on Si(100) substrate by co-evaporation of Cu and Cr using molecular beam epitaxy (MBE). Phase evolution, microstructure, stress development, and crystallographic texture in Cr0.8Cu0.2 thin films have been investigated by X-ray diffraction (XRD), atom probe tomography (APT) and transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) during annealing of the films in the temperature range 200–450 °C. X-ray diffraction of the as-deposited thin film shows single phase bcc crystal structure of the film whereas APT observation of fine precipitates in the film matrix due to inherent compositional fluctuation indicates onset of phase separation via spinodal decomposition regime. XRD (in-situ) and APT investigation of 300 °C annealed film reveals that the early stage of phase separation involves localized formation of metastable intermediate bcc precipitate phase having 60 at% Cr and 40 at% Cu approximately (~Cr0.6Cu0.4). For longer duration of annealing at temperature ≥350 °C, such metastable bcc precipitates act as heterogeneous nucleation sites for the onset of precipitation of Cu rich fcc Cu(Cr) phase which indicates a change of phase separation mechanism from ‘spinodal decomposition’ to ‘nucleation and growth’. Annealing of the film at temperature ≥400 °C for longer duration leads to the formation of a two phase structure with Cu rich fcc precipitate phase in a Cr rich bcc matrix. Observed phase decomposition is accompanied by significant changes in the microstructure, residual stress and crystallographic texture in the Cr rich bcc film matrix which leads to the minimization of both surface and strain energies and thereby a reduction of total Gibbs free energy of the thin film. Thermodynamic model calculation has been presented in order to understand the nucleation pathway of Cu rich stable fcc Cu(Cr) precipitates via non-classical nucleation of metastable intermediate bcc Cr0.6Cu0.4 phase. © 2021 Elsevier B.V.
    view abstract10.1016/j.jallcom.2021.161391
  • Reactive wear protection through strong and deformable oxide nanocomposite surfaces
    Liu, C. and Li, Z. and Lu, W. and Bao, Y. and Xia, W. and Wu, X. and Zhao, H. and Gault, B. and Liu, C. and Herbig, M. and Fischer, A. and Dehm, G. and Wu, G. and Raabe, D.
    Nature Communications 12 (2021)
    Wear-related energy and material loss cost over 2500 Billion Euro per year. Traditional wisdom suggests that high-strength materials reveal low wear rates, yet, their plastic deformation mechanisms also influence their wear performance. High strength and homogeneous deformation behavior, which allow accommodating plastic strain without cracking or localized brittle fracture, are crucial for developing wear-resistant metals. Here, we present an approach to achieve superior wear resistance via in-situ formation of a strong and deformable oxide nanocomposite surface during wear, by reaction of the metal surface with its oxidative environment, a principle that we refer to as ‘reactive wear protection’. We design a TiNbZr-Ag alloy that forms an amorphous-crystalline oxidic nanocomposite surface layer upon dry sliding. The strong (2.4 GPa yield strength) and deformable (homogeneous deformation to 20% strain) nanocomposite surface reduces the wear rate of the TiNbZr-Ag alloy by an order of magnitude. The reactive wear protection strategy offers a pathway for designing ultra-wear resistant alloys, where otherwise brittle oxides are turned to be strong and deformable for improving wear resistance. © 2021, The Author(s).
    view abstract10.1038/s41467-021-25778-y
  • Reducing cohesion of metal powders for additive manufacturing by nanoparticle dry-coating
    Gärtner, E. and Jung, H.Y. and Peter, N.J. and Dehm, G. and Jägle, E.A. and Uhlenwinkel, V. and Mädler, L.
    Powder Technology 379 (2021)
    Additive manufacturing processes, such as laser powder bed fusion, require steady powder processing but often exhibit poor flowability and low powder bed densities. Reducing the attractive Van-der-Waals force through nanoparticle coating can enhance initially poor flowability. We investigated the effect of dry-coating nanosized SiO2 on gas-atomized CoCrFeNi powders containing different amounts of particles < 20 μm with respect to nanoparticle concentration and mixing time. The dynamic angle of repose of a 0–90 μm powder reduced 50% and bulk powder density increased 30% with nanoparticle concentrations up to 0.153 wt.-%. The granular Bond-number was correlated with the powder flowability and porosity. The effect of mixing time was investigated with mixing two fractions 20–90 μm and 0–90 μm at a constant nominal nanoparticle surface area coverage of 128% for 2 to 1440 min. Short mixing times improved the flowability, while extensive mixing resulted in nanoparticle reagglomeration and deteriorated flow. © 2020
    view abstract10.1016/j.powtec.2020.10.065
  • Scratch hardness at a small scale: Experimental methods and correlation to nanoindentation hardness
    Tsybenko, H. and Farzam, F. and Dehm, G. and Brinckmann, S.
    Tribology International 163 (2021)
    Nanoindentation and scratch experiments probe the hardness by leaving a permanent imprint on the material's surface. The scratch hardness, however, is less used due to its unclear relation to the nanoindentation hardness and discrepancies in the evaluation methods. We investigate which scratch hardness evaluation methods lead to consistent results and for which materials and load-ranges the contact area can be estimated by the Hertz solution or by the nanoindentation hardness. Finally, we address the relation of nanoindentation and scratch hardness. Secondary influences on the scratch hardness (tip size, scratch depth and elliptical shape) are addressed. To evaluate the general applicability of the findings, we study 5 materials with significantly different deformation behavior: aluminum, copper, soda-lime glass, cementite, and silicon. © 2021 Elsevier Ltd
    view abstract10.1016/j.triboint.2021.107168
  • Structure and hardness of in situ synthesized nano-oxide strengthened CoCrFeNi high entropy alloy thin films
    Lee, S. and Chatain, D. and Liebscher, C.H. and Dehm, G.
    Scripta Materialia 203 (2021)
    In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing its hardness by 14% compared to that of the film without precipitates, through the Orowan-type strengthening mechanism. Our novel approach paves the way to design medium- and high-entropy alloys with high strength by making use of oxide phases. © 2021
    view abstract10.1016/j.scriptamat.2021.114044
  • Symbiotic crystal-glass alloys via dynamic chemical partitioning
    Wu, G. and Liu, C. and Brognara, A. and Ghidelli, M. and Bao, Y. and Liu, S. and Wu, X. and Xia, W. and Zhao, H. and Rao, J. and Ponge, D. and Devulapalli, V. and Lu, W. and Dehm, G. and Raabe, D. and Li, Z.
    Materials Today (2021)
    The design of high performance structural materials is always pursuing combinations of excellent yet often mutually exclusive properties such as mechanical strength, ductility and thermal stability. Although crystal-glass composite alloys provide better ductility compared to fully amorphous alloys, their thermal stability is poor, due to heterogeneous nucleation at the crystal-glass interface. Here we present a new strategy to develop thermally stable, ultrastrong and deformable crystal-glass nanocomposites through a thermodynamically guided alloy design approach, which mimics the mutual stabilization principle known from symbiotic ecosystems. We realized this in form of a model Cr-Co-Ni (crystalline)/Ti-Zr-Nb-Hf-Cr-Co-Ni (amorphous) laminate composite alloy. The symbiotic alloy has an ultrahigh compressive yield strength of 3.6 GPa and large homogeneous deformation of ∼15% strain at ambient temperature, values which surpass those of conventional metallic glasses and nanolaminate alloys. Furthermore, the alloy exhibits ∼200 K higher crystallization temperature (TX > 973 K) compared to that of the original TiZrNbHf-based amorphous phase. The elemental partitioning among adjacent amorphous and crystalline phases leads to their mutual thermodynamic and mechanical stabilization, opening up a new symbiotic approach for stable, strong and ductile materials. © 2021 Elsevier Ltd
    view abstract10.1016/j.mattod.2021.10.025
  • Understanding Grain Boundary Electrical Resistivity in Cu: The Effect of Boundary Structure
    Bishara, H. and Lee, S. and Brink, T. and Ghidelli, M. and Dehm, G.
    ACS Nano 15 (2021)
    Grain boundaries (GBs) in metals usually increase electrical resistivity due to their distinct atomic arrangement compared to the grain interior. While the GB structure has a crucial influence on the electrical properties, its relationship with resistivity is poorly understood. Here, we perform a systematic study on the resistivity-structure relationship in Cu tilt GBs, employing high-resolution in situ electrical measurements coupled with atomic structure analysis of the GBs. Excess volume and energies of selected GBs are calculated using molecular dynamics simulations. We find a consistent relation between the coincidence site lattice (CSL) type of the GB and its resistivity. The most resistive GBs are in the high range of low-angle GBs (14°-18°) with twice the resistivity of high angle tilt GBs, due to the high dislocation density and corresponding strain fields. Regarding the atomistic structure, GB resistivity approximately correlates with the GB excess volume. Moreover, we show that GB curvature increases resistivity by ∼80%, while phase variations and defects within the same CSL type do not considerably change it. © 2021 The Authors. Published by American Chemical Society.
    view abstract10.1021/acsnano.1c06367
  • Approaches to Measure the Resistivity of Grain Boundaries in Metals with High Sensitivity and Spatial Resolution: A Case Study Employing Cu
    Bishara, H. and Ghidelli, M. and Dehm, G.
    ACS Applied Electronic Materials 2 (2020)
    It is well-known that grain boundaries (GBs) increase the electrical resistivity of metals due to their enhanced electron scattering. The resistivity values of GBs are determined by their atomic structure; therefore, assessing the local resistivity of GBs is highly significant for understanding structure-property relationships. So far, the local electrical characterization of an individual GB has not received much attention, mainly due to the limited accuracy of the applied techniques, which were not sensitive enough to detect the subtle differences in electrical resistivity values of highly symmetric GBs. Here, we introduce a detailed methodology to probe in situ or ex situ the local resistivity of individual GBs in Cu, a metallic model system we choose due to its low resistance. Both bulk Cu samples and thin films are investigated, and different approaches to obtain reliable and accurate resistivity measurements are described, involving the van der Pauw technique for macroscopic measurements as well as two different four-point-probe techniques for local in situ measurements performed inside a scanning electron microscope. The in situ contacts are realized with needles accurately positioned by piezodriven micromanipulators. Resistivity results obtained on coincidence site lattice GBs (incoherent ς3 and asymmetric ς5) are reported and discussed. In addition, the key experimental details as well as pitfalls in the measurement of individual GB resistivity are addressed. © 2020 American Chemical Society.
    view abstract10.1021/acsaelm.0c00311
  • Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstract10.1016/j.actamat.2019.11.004
  • Atomistic deformation behavior of single and twin crystalline Cu nanopillars with preexisting dislocations
    Ko, W.-S. and Stukowski, A. and Hadian, R. and Nematollahi, A. and Jeon, J.B. and Choi, W.S. and Dehm, G. and Neugebauer, J. and Kirchlechner, C. and Grabowski, B.
    Acta Materialia 197 (2020)
    Molecular dynamics simulations are performed to investigate the impact of a coherent Σ3 (111) twin boundary on the plastic deformation behavior of Cu nanopillars. Our work reveals that the mechanical response of pillars with and without the twin boundary is decisively driven by the characteristics of initial dislocation sources. In the condition of comparably large pillar size and abundant initial mobile dislocations, overall yield and flow stresses are controlled by the longest, available mobile dislocation. An inverse correlation of the yield and flow stresses with the length of the longest dislocation is established and compared to experimental data. The experimentally reported subtle differences in yield and flow stresses between pillars with and without the twin boundary are likely related to the maximum lengths of the mobile dislocations. In the condition of comparably small pillar size, for which a reduction of mobile dislocations during heat treatment and mechanical loading occurs, the mechanical response of pillars with and without the twin boundary can be clearly distinguished. Dislocation starvation during deformation is more pronounced in pillars without the twin boundary than in pillars with the twin boundary because the twin boundary acts as a pinning surface for the dislocation network. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.07.029
  • Bulk nanostructured AlCoCrFeMnNi chemically complex alloy synthesized by laser-powder bed fusion
    Jung, H.Y. and Peter, N.J. and Gärtner, E. and Dehm, G. and Uhlenwinkel, V. and Jägle, E.A.
    Additive Manufacturing 35 (2020)
    We report the synthesis of a bulk nanostructured alloy using laser-powder bed fusion. The equiatomic AlCoCrFeMnNi chemically complex alloy forms a nanoscale modulated structure, which is homogeneously distributed in the as-built condition. The nanostructure consists of Al & Ni-rich ordered and Cr & Fe-rich disordered BCC phases. The two phases form an interconnected phase network with coherent interface boundaries. Atom probe tomography and aberration-corrected scanning transmission electron microscopy analysis of the spatial distribution of the modulated structure suggests the occurrence of nano-scale spinodal decomposition. These results introduce a direct synthesis of bulk nanostructured alloys with promising geometric flexibility. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101337
  • Composition dependence of hardness and elastic modulus of the cubic and hexagonal NbCo2 Laves phase polytypes studied by nanoindentation
    Luo, W. and Kirchlechner, C. and Li, J. and Dehm, G. and Stein, F.
    Journal of Materials Research 35 (2020)
    Regarding the effect of composition on the mechanical properties of intermetallic phases such as Laves phases, there is conflicting information in the literature. Some authors observed defect hardening when deviating from stoichiometric Laves phase composition, whereas others find defect softening. Here, we present a systematic investigation of the defect state, hardness, and elastic modulus of cubic and hexagonal NbCo2 Laves phases as a function of crystal structure and composition. For this purpose, diffusion couples were prepared which exhibit diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases, with concentration gradients covering their entire homogeneity ranges from 24 to 37 at.% Nb. Direct observations of dislocations and stacking faults in the diffusion layers as a function of composition were performed by electron channeling contrast imaging, and the hardness and elastic modulus were probed in the diffusion layers along the concentration gradients by nanoindentation. © 2020 Materials Research Society.
    view abstract10.1557/jmr.2019.384
  • Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2019.11.010
  • Crystal structure and composition dependence of mechanical properties of single-crystalline NbCo2 Laves phase
    Luo, W. and Kirchlechner, C. and Zavašnik, J. and Lu, W. and Dehm, G. and Stein, F.
    Acta Materialia 184 (2020)
    Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019
    view abstract10.1016/j.actamat.2019.11.036
  • Crystal–Glass High-Entropy Nanocomposites with Near Theoretical Compressive Strength and Large Deformability
    Wu, G. and Balachandran, S. and Gault, B. and Xia, W. and Liu, C. and Rao, Z. and Wei, Y. and Liu, S. and Lu, J. and Herbig, M. and Lu, W. and Dehm, G. and Li, Z. and Raabe, D.
    Advanced Materials 32 (2020)
    High-entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple-principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high-entropy nanotwinned crystalline phase and the glass-forming-ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co-deformation of the two regions. This crystal–glass high-entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.202002619
  • Dislocation plasticity and detwinning under thermal stresses in nanotwinned Ag thin films
    Kini, M.K. and Merola, C. and Breitbach, B. and Klapproth, D. and Philippi, B. and Molin, J.-B. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 198 (2020)
    Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars. © 2020
    view abstract10.1016/j.actamat.2020.07.056
  • Dislocation plasticity in FeCoCrMnNi high-entropy alloy: quantitative insights from in situ transmission electron microscopy deformation
    Lee, S. and Duarte, M.J. and Feuerbacher, M. and Soler, R. and Kirchlechner, C. and Liebscher, C.H. and Oh, S.H. and Dehm, G.
    Materials Research Letters 8 (2020)
    The mechanical properties of high-entropy alloys (HEAs) are still not deeply understood. Detailed knowledge of the strengthening mechanism, especially, the atomistic origin of solid solution hardening and its interplay with dislocation plasticity is needed. Here, we report on the dislocation glide behavior of a FeCoCrNiMn face-centered cubic (FCC) single crystal studied by in situ deformation in a transmission electron microscope (TEM). The threshold shear stress for dislocation glide in a thin foil is measured from dislocation curvature as exceeding 400 MPa. Interestingly, dislocations are prevented from straightening upon unloading due to high frictional stresses. IMPACT STATEMENT: The fiction stress for dislocation glide in a FeCoCrMnNi HEA is assessed by direct measurement of dislocation line curvature during in situ TEM deformation, which is higher compared to other FCC metals, explaining the outstanding yield and flow stress of the HEA. © 2020, © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/21663831.2020.1741469
  • Dislocation-induced breakthrough of strength and ductility trade-off in a non-equiatomic high-entropy alloy
    Guo, W. and Su, J. and Lu, W. and Liebscher, C.H. and Kirchlechner, C. and Ikeda, Y. and Körmann, F. and Liu, X. and Xue, Y. and Dehm, G.
    Acta Materialia 185 (2020)
    In conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5 ± 216.1 µm to 18.3 ± 9.3 µm. The ultimate tensile strength increases from 543 ± 4 MPa to 621 ± 8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7%) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.11.055
  • Early stage phase separation of AlCoCr0.75Cu0.5FeNi high-entropy powder at the nanoscale
    Peter, N.J. and Duarte, M.J. and Liebscher, C.H. and Srivastava, V.C. and Uhlenwinkel, V. and Jägle, E.A. and Dehm, G.
    Journal of Alloys and Compounds 820 (2020)
    High entropy alloys are generally considered to be single phase material. This state is, however, typically a non-equilibrium state after fabrication at high cooling rates. Phase constitution after fabrication or heat treatment is mostly known for isothermal annealing only and for casts as well as rapidly quenched alloys. Knowledge on early phase separation stages of high entropy alloys and their mechanisms are missing so far. Here, we present results on phase separation at intermediate cooling rates, by characterization of gas atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase nature of the powder particles, aberration-corrected scanning transmission electron microscopy and atom probe tomography reveal a nanoscale phase separation into Ni–Al-rich B2 and Fe–Cr-rich A2 regions as well as a high number density of 3.1 × 1024 Cu-rich clusters per m3 in the B2 matrix. The observed phase separation and cluster formation are linked to spinodal decomposition and nucleation processes, respectively. The study highlights that adequate characterization techniques need to be chosen when making statements about phase stability and structural evolution in compositionally complex alloys. © 2019 The Authors
    view abstract10.1016/j.jallcom.2019.153149
  • Effect of Oxygen on High-temperature Phase Equilibria in Ternary Ti-Al-Nb Alloys
    Distl, B. and Dehm, G. and Stein, F.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 (2020)
    Alloys based on titanium aluminides received a lot of attention because of their capability to substitute Ni-based superalloys in high-temperature applications. However, the phase equilibria between the main microstructure constituents (αTi), (βTi), γ (TiAl) and α2(Ti3Al) can be shifted significantly by impurities such as oxygen especially at high temperatures. This behavior is investigated on the tie-triangle (αTi) + (βTi) + γ (TiAl) in the ternary Ti-Al-Nb system at 1300 °C. An explanation for this behavior could be the occupation of octahedral voids by impurities in certain phases. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/zaac.202000098
  • Effect of size and domain orientation on strength of Barium Titanate
    Mathews, N.G. and Saxena, A.K. and Kirchlechner, C. and Dehm, G. and Jaya, B.N.
    Scripta Materialia 182 (2020)
    Microscale mechanical behaviour of single crystalline Barium Titanate (BaTiO3), a ferroelectric ceramic was studied by uniaxial in situ micropillar compression and nanoindentation. It was observed that pillars below 1 µm diameter reached the theoretical strength of BaTiO3 whereas larger pillars yielded at lower stress values with multiple stress drops confirming slip activity. A size scaling exponent of 0.96 ± 0.09 was estimated for BaTiO3 which is close to one of the soft fcc metals. The material's strength, hardness and deformation behaviour did not show any dependence on the character of the ferroelectric domain, within our error bars. © 2020
    view abstract10.1016/j.scriptamat.2020.02.039
  • Electronic structure based design of thin film metallic glasses with superior fracture toughness
    Evertz, S. and Kirchlechner, I. and Soler, R. and Kirchlechner, C. and Kontis, P. and Bednarcik, J. and Gault, B. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Materials and Design 186 (2020)
    High fracture toughness is crucial for the application of metallic glasses as structural materials to avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloying Pd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. © 2018 The Authors
    view abstract10.1016/j.matdes.2019.108327
  • Experimental conditions affecting the measured fracture toughness at the microscale: Notch geometry and crack extension measurement
    Saxena, A.K. and Brinckmann, S. and Völker, B. and Dehm, G. and Kirchlechner, C.
    Materials and Design 191 (2020)
    Experimental fracture mechanics at the microscale became an indispensable tool for understanding and developing advanced material systems. In case of linear elastic fracture mechanics, stringent requirements are typically only warranted for very brittle materials. The material properties of semi-brittle materials might be accessible by elasto-plastic fracture mechanics. However, challenges exist in determining the crack length, in producing geometry and notch geometry, in defining of the initiation toughness and in extracting the size independent crack resistance curves. In this study, we assess current approaches of measuring the fracture toughness of semi-brittle materials by elasto-plastic fracture mechanics. We investigate the notch geometry (through thickness notch and bridge notch), the notch depth and the method of determining in situ the crack length for ultrafine grained tungsten. Further challenges due to the overlap of sample size and crack process zone are identified. Finally, we propose a workflow for analyzing the elasto-plastic fracture toughness of material systems at the microscale. © 2020 The Authors
    view abstract10.1016/j.matdes.2020.108582
  • How tensile tests allow a screening of the fracture toughness of hard coatings
    Völker, B. and Du, C. and Fager, H. and Rueß, H. and Soler, R. and Kirchlechner, C. and Dehm, G. and Schneider, J.M.
    Surface and Coatings Technology 390 (2020)
    In hard coating development there is a need for fast and efficient screening methods to assess the influence of changes in composition and structure evolution on the mechanical behavior. Nanoindentation constitutes a method that allows fast and efficient probing of mechanical properties. For another important mechanical characteristic of hard coatings, the fracture toughness, no quick benchmark test to compare coatings with various compositions and/or morphologies, exist. Therefore, the goal of this investigation was to determine if a tensile test setup allows for fast and efficient qualitative screening of fracture toughness trends in hard coatings compared to accurate but time consuming micro-bending beam experiments. TiAlN and VAlN, each deposited on ductile Cu-substrates, were chosen for this investigation. In situ scanning electron microscopy tensile tests were performed. Here, the strain at crack initiation of the coating was utilized as the experimental parameter, which is most representative for the fracture toughness. The experiments indicate that TiAlN exhibits a higher fracture toughness than VAlN. This was confirmed using time demanding in situ micro-bending beam fracture experiments. Hence, it is established that under the given conditions macroscopic in situ SEM tensile tests can be used as a fast and efficient screening method for fracture toughness trends of hard coatings. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125645
  • Influence of Ti3Ni4 precipitates on the indentation-induced two-way shape-memory effect in Nickel-Titanium
    Laursen, C.M. and Peter, N.J. and Gerstein, G. and Maier, H.J. and Dehm, G. and Frick, C.P.
    Materials Science and Engineering A 792 (2020)
    Nickel Titanium (NiTi) alloys have been used for many years based on their unique ability to exhibit the shape-memory and pseudoelastic effects. The indentation-induced two-way shape memory effect (TWSME) is a specific sub-capability of this alloy such that a repeatably switchable surface can be created by “training” the material through mechanical indentation and activated through temperature transitions between the austenitic and martensitic phases. This study sought to observe the effect Ti3Ni4 precipitate aging would have on the indentation-induced TWSME. Ti3Ni4 has previously been shown as an effective method to alter NiTi transformation temperatures, yet it was unclear what effect localized stress fields around precipitates would have on the TWSME. The results presented here indicate that growth of precipitates in the alloy before training suppresses the resultant indentation-induced TWSME, and small precipitates, which cause minimal lattice mismatch to the matrix (i.e. highest coherency), have the strongest role in suppressing the effect. It is suggest that lattice coherency acts to inhibit plastic deformation, suppressing the creation of the preferred microstructure under the indent required to guide the TWSME. Therefore, precipitate aging is not a recommended alternative to precise alloying in order to alter transformation temperatures with the goal of maximizing the indentation-induced TWSME effect within a targeted temperate transformation regime. © 2020 Elsevier B.V.
    view abstract10.1016/j.msea.2020.139373
  • Insight into indentation-induced plastic flow in austenitic stainless steel
    Xia, W. and Dehm, G. and Brinckmann, S.
    Journal of Materials Science 55 (2020)
    The indentation-induced plasticity and roughness have been investigated intensively by experiments and simulations during the last decades. However, the precise mechanisms of how dislocation flow leads to pile-up formation are still not completely understood, although this is one of the initial steps causing surface roughening in tribological contacts at low loads. In this work, { 001 } -, { 101 } - and { 111 } -grain orientations in an austenite stainless steel [(face-centered cubic (FCC) phase]) are indented with varying load forces. By using scanning electron-based methods and slip plane analysis, we reveal: (1) how slip-steps show the change of pile-up formation, (2) how the slip-plane inclination determines the dislocation flow and (3) how slip-plane interactions result in the final pile-up shape during indentation. We find that the flow direction transforms from the forward flow to the sideway at a transition angle of 55 ∘- 58 ∘ between the slip-plane and the surface. We use large displacement finite element method simulations to validate an inversion of the resolved shear stress at this transition angle. We provide insights into the evolution of plasticity in dislocation-mediated FCC metal indentations, with the potential application of this information for indentation simulations and for understanding the initial stage of scratching during tribology in the future. © 2020, The Author(s).
    view abstract10.1007/s10853-020-04646-y
  • Interfacial fracture toughness of sintered hybrid silver interconnects
    Wang, S. and Kirchlechner, C. and Keer, L. and Dehm, G. and Yao, Y.
    Journal of Materials Science 55 (2020)
    The interfacial fracture toughness of sintered hybrid silver nanoparticles (AgNPs) on both Au and Cu substrates is studied as a function of sintering temperature. Interfacial microstructure and porosity evolution of Au/AgNPs and Cu/AgNPs are observed to impact the fracture toughness. An Au–Ag interfacial diffusion layer is resolved at the interface of Au/AgNPs interconnects, while an oxide layer is found at the interface of Cu/AgNPs interconnects. Both porosity and pore sizes of the sintered silver interconnects are analyzed across the micro- and macro-length scales and related to the interfacial fracture toughness. The experimental observations can be theoretically described, which permits to predict the fracture toughness of the sintered silver interconnects. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10853-019-04212-1
  • Interfacial nanophases stabilize nanotwins in high-entropy alloys
    Lu, W. and Liebscher, C.H. and Yan, F. and Fang, X. and Li, L. and Li, J. and Guo, W. and Dehm, G. and Raabe, D. and Li, Z.
    Acta Materialia 185 (2020)
    Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nanosized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.12.010
  • Interplay of Chemistry and Faceting at Grain Boundaries in a Model Al Alloy
    Zhao, H. and Huber, L. and Lu, W. and Peter, N.J. and An, D. and De Geuser, F. and Dehm, G. and Ponge, D. and Neugebauer, J. and Gault, B. and Raabe, D.
    Physical Review Letters 124 (2020)
    The boundary between two crystal grains can decompose into arrays of facets with distinct crystallographic character. Faceting occurs to minimize the system's free energy, i.e., when the total interfacial energy of all facets is below that of the topologically shortest interface plane. In a model Al-Zn-Mg-Cu alloy, we show that faceting occurs at investigated grain boundaries and that the local chemistry is strongly correlated with the facet character. The self-consistent coevolution of facet structure and chemistry leads to the formation of periodic segregation patterns of 5-10 nm, or to preferential precipitation. This study shows that segregation-faceting interplay is not limited to bicrystals but exists in bulk engineering Al alloys and hence affects their performance. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevLett.124.106102
  • Investigation of single asperity wear at the microscale in an austenitic steel
    Xia, W. and Dehm, G. and Brinckmann, S.
    Wear 452-453 (2020)
    Engineering surfaces consist of microasperities, which result in plasticity during the run-in of the tribological system. A fundamental insight of the plastic flow during single asperity wear at the microscale is required to thoroughly understand tribology induced microstructure at the macroscale and to tailor future metal surfaces. In this work, we indent and wear {001}-, {101}- and {111}-grains in an austenite stainless steel. The transition from indentation to ploughing and the evolution of plasticity during ploughing is addressed. We find that slip-step and pile-up evolution during the indentation segment of the wear experiment influence the plasticity during the ploughing segment that follows upon the indentation. We conclude that the pile-up evolution dominates the development of the friction force during ploughing at the microscale. © 2020 The Authors
    view abstract10.1016/j.wear.2020.203289
  • Mapping the mechanical properties in nitride coatings at the nanometer scale
    Zhang, Z. and Chen, Z. and Holec, D. and Liebscher, C.H. and Koutná, N. and Bartosik, M. and Zheng, Y. and Dehm, G. and Mayrhofer, P.H.
    Acta Materialia 194 (2020)
    We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. Gradient structures are also accompanied by a modulation of the chemical compositions. A combined experimental analysis based on valence electrons and inner shell electrons allowed mapping the mechanical properties of the multilayer at the nanometer scale and further unveiled the effect of oxygen impurities on the bulk modulus. We found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.04.024
  • Microscale plastic anisotropy of basal and pyramidal I slip in pure magnesium tested in shear
    Seok, M.-Y. and Gopalan, H. and Nandy, S. and Zaefferer, S. and Raabe, D. and Kirchlechner, C. and Dehm, G.
    Materialia 14 (2020)
    An optimised micro-shear testing protocol was adopted to measure the critical resolved shear stresses for basal and pyramidal I slip systems in pure magnesium. The micro-shear samples are carefully aligned for basal and pyramidal I slip by electron backscatter diffraction and fabricated by focussed ion beam milling. In situ scanning electron microscopy based shear testing identified that the two different sample orientations lead to activation of basal or 〈c+a〉pyramidal I slip, respectively. The critical resolved shear stress for basal slip was found to be 57 ± 19 MPa, and 371 ± 81 MPa for pyramidal I slip, albeit for slightly different geometric dimensions. Accounting for sample size-dependent flow stress for basal slip, we found that the plastic anisotropy with respect to pyramidal I slip is substantially reduced to a factor of 3 at the microscale compared to nearly a factor of 100 in the bulk. Multiple slip systems are therefore expected to operate in ultra-fine grain sized magnesium offering a pathway for improving ductility. © 2020
    view abstract10.1016/j.mtla.2020.100932
  • Microstructure evolution and thermal stability of equiatomic CoCrFeNi films on (0001) α-Al2O3
    Addab, Y. and Kini, M.K. and Courtois, B. and Savan, A. and Ludwig, Al. and Bozzolo, N. and Scheu, C. and Dehm, G. and Chatain, D.
    Acta Materialia 200 (2020)
    Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K. The fcc-phase persists while the original <111> texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[11¯0]//α-Al2O3(0001)[11¯00]) and OR2 (M(111)[11¯0]//α-Al2O3(0001)[112¯0])and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.09.064
  • Observations of grain-boundary phase transformations in an elemental metal
    Meiners, T. and Frolov, T. and Rudd, R.E. and Dehm, G. and Liebscher, C.H.
    Nature 579 (2020)
    The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4–6. The terminology ‘complexion’ was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8–19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20–24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [11 1 ¯] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27–29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41586-020-2082-6
  • On the commensuration of plastic plowing at the microscale
    Tsybenko, H. and Xia, W. and Dehm, G. and Brinckmann, S.
    Tribology International 151 (2020)
    Improving the wear properties requires an understanding of the mechanisms during the initial stages of wear. This paper focuses on the transition from static indentation to dynamic wear, i.e. the evolution from stationary to sliding contact. By using single micrometer-sized roughness peaks on copper, cementite and austenitic steel, we observe that the conservation of contact area, elastic recovery, and the front pile-up development are the dominant mechanisms. The elastic recovery leads to an additional contact area at the backside of the asperity. The influence of the crystallographic orientation was found to be negligible during the initial increase in wear depth but significant during the later stages. Moreover, the partial wear depth recovery is observed only in ductile materials. © 2020 Elsevier Ltd
    view abstract10.1016/j.triboint.2020.106477
  • Review on Quantum Mechanically Guided Design of Ultra-Strong Metallic Glasses
    Evertz, S. and Schnabel, V. and Köhler, M. and Kirchlechner, I. and Kontis, P. and Chen, Y.-T. and Soler, R. and Jaya, B.N. and Kirchlechner, C. and Music, D. and Gault, B. and Schneider, J.M. and Raabe, D. and Dehm, G.
    Frontiers in Materials 7 (2020)
    Quantum mechanically guided materials design has been used to predict the mechanical property trends in crystalline materials. Thereby, the identification of composition-structure-property relationships is enabled. However, quantum mechanics based design guidelines and material selection criteria for ultra-strong metallic glasses have been lacking. Hence, based on an ab initio model for metallic glasses in conjunction with an experimental high-throughput methodology geared toward revealing the relationship between chemistry, topology and mechanical properties, we propose principles for the design of tough as well as stiff metallic glasses. The main design notion is that a low fraction of hybridized bonds compared to the overall bonding in a metallic glass can be used as a criterion for the identification of damage-tolerant metallic glass systems. To enhance the stiffness of metallic glasses, the bond energy density must be increased as the bond energy density is the origin of stiffness in metallic glasses. The thermal expansion, which is an important glass-forming identifier, can be predicted based on the Debye-Grüneisen model. © Copyright © 2020 Evertz, Schnabel, Köhler, Kirchlechner, Kontis, Chen, Soler, Jaya, Kirchlechner, Music, Gault, Schneider, Raabe and Dehm.
    view abstract10.3389/fmats.2020.00089
  • Size dependent strength, slip transfer and slip compatibility in nanotwinned silver
    Kini, M.K. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 184 (2020)
    Perfect slip transfer through single coherent Σ3 twin boundaries is known to be a cross-slip-like mechanism occurring at low stresses, which is expected to strongly depend on material properties like stacking fault energy. In the present study, we extend the argument of perfect slip transfer to (i) multiple closely spaced coherent twin boundaries in a nanotwinned thin film and (ii) to materials with very low stacking fault energy. The slip transfer is indicated by the continuity of slip steps and observed across up to 100 coherent Σ3 boundaries. The study addresses size scaling due to multiple weak obstacles for dislocation motion and discusses the underlying deformation mechanisms. The importance of strain compatibility is further extended to incoherent twin boundaries. © 2019
    view abstract10.1016/j.actamat.2019.11.042
  • Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper
    Meiners, T. and Duarte, J.M. and Richter, G. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 190 (2020)
    Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.02.064
  • Thin-Film Microtensile-Test Structures for High-Throughput Characterization of Mechanical Properties
    Oellers, T. and Arigela, V.G. and Kirchlechner, C. and Dehm, G. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    A photolithographic process for the rapid fabrication of thin-film tensile-test structures is presented. The process is applicable to various physical vapor deposition techniques and can be used for the combinatorial fabrication of thin-film tensile-test structure materials libraries for the high-throughput characterization of mechanical properties. The functionality of the fabrication process and the feasibility of performing high-quality measurements with these structures are demonstrated with Cu tensile-test structures. In addition, the scalability from unary structures to libraries with compositional variations is demonstrated. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.9b00182
  • Time-dependent plasticity in silicon microbeams mediated by dislocation nucleation
    Elhebeary, M. and Harzer, T. and Dehm, G. and Saif, M.T.A.
    Proceedings of the National Academy of Sciences of the United States of America 117 (2020)
    Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temperatures. Bulk silicon is brittle, but it becomes ductile at about 540 °C. It creeps (deforms plastically with time) at high temperatures (∼800 °C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 °C) above a threshold stress. We achieve this stress by bending single-crystal silicon microbeams using an in situ thermomechanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron microscopy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon. © 2020 National Academy of Sciences. All rights reserved.
    view abstract10.1073/pnas.2002681117
  • Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstract10.1038/s41467-019-14062-9
  • Advances in in situ nanomechanical testing
    Minor, A.M. and Dehm, G.
    MRS Bulletin 44 (2019)
    In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed. Copyright © Materials Research Society 2019.
    view abstract10.1557/mrs.2019.127
  • Aggregation control of Ru and Ir nanoparticles by tunable aryl alkyl imidazolium ionic liquids
    Schmolke, L. and Lerch, S. and Bülow, M. and Siebels, M. and Schmitz, A. and Thomas, J. and Dehm, G. and Held, C. and Strassner, T. and Janiak, C.
    Nanoscale 11 (2019)
    Metal-nanoparticles (M-NPs) were synthesized in a wet-chemical synthesis route in tunable aryl alkyl ionic liquids (TAAILs) based on the 1-aryl-3-alkyl-substituted imidazolium motif from Ru3(CO)12 and Ir4(CO)12 by microwave-heating induced thermal decomposition. The size and size dispersion of the NPs were determined by transmission electron microscopy (TEM) to an average diameter of 2.2(±0.1) to 3.9(±0.3) nm for Ru-NPs and to an average diameter of 1.4(±0.1) to 2.4(±0.1) nm for Ir-NPs. The TAAILs used contain the same bis(trifluoromethylsulfonyl)imide anion but differ in the substituents on the 1-aryl ring, e.g. 2-methyl-, 4-methoxy- and 2,4-dimethyl groups and in the 3-alkyl chain lengths (C4H9, C5H11, C8H17, C9H19, C11H23). All used TAAILs are suitable for the stabilization of Ru- and Ir-NPs over months in the IL dispersion. Different from all other investigations on M-NP/IL systems which we are aware of the particle separation properties of the TAAILs vary strongly as a function of the aryl substituent. Good NP separation can be achieved with the 4-methoxyphenyl- and 2,4-dimethylphenyl-substituted ILs, irrespective of the 3-alkyl chain lengths. Significant aggregation can be observed for 2-methylphenyl-substituted ILs. The good NP separation can be correlated with a negative electrostatic potential at the 4-methoxyphenyl or 4-methylphenyl substituent that is in the para-position of the aryl ring, whereas the 2-(ortho-)methylphenyl group assumes no negative potential. ϵ-ePC-SAFT calculations were used to validate that the interactions between ILs and the washing agents (required for TEM analyses) do not cause the observed aggregation/separation behaviour of the M-NPs. Ru-NPs were investigated as catalysts for the solvent-free hydrogenation of benzene to cyclohexane under mild conditions (70 °C, 10 bar) with activities up to 760 (mol cyclohexane) (mol Ru)-1 h-1 and over 95% conversion in ten consecutive runs for Ru-NPs. No significant loss of catalytic activity could be observed. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c8nr10286d
  • Atomic level bonding mechanism in steel/aluminum joints produced by cold pressure welding
    Peter, N.J. and Gerlitzky, C. and Altin, A. and Wohletz, S. and Krieger, W. and Tran, T.H. and Liebscher, C.H. and Scheu, C. and Dehm, G. and Groche, P. and Erbe, A.
    Materialia 7 (2019)
    Cold pressure welding of aluminum alloys and steels offers an attractive, cost-effective opportunity of joining the two most important structural materials. Aim of this work is to investigate the bonding mechanism between aluminum alloy EN AW6082 and carbon steel C15, joint by cold extrusion welding after targeted heat treatment. Mechanical testing under tensile loads revealed site specific interface strengths between 30 and 60% of the constituent materials. However, investigation of fracture surfaces indicates that interface strengths can exceed the AW6082 strength in areas where the bonding process substantially enlarged the Fe-Al interface region, due to the welding geometry. Near atomic scale investigations of the interface using aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy disclose two different regions along the cold pressure welded interface. The majority of the interfacial area is oxygen free with Fe-based and Al-based crystals joining. No intermetallic phase was identified. Adhesion is attributed to a more covalent bond characteristic at the Fe-Al interface evidenced in the Al-L2,3 edge. A small fraction of the steel/aluminum interface was covered with oxidic regions of about 10 nm thickness. This oxide is predominantly an amorphous aluminum oxide, with only few Fe-O bonds. The observed metallurgical bond formation is explained by an extended Bay model through (i) cracking of the brittle, nanoscopic native oxide layer on the materials during forming, (ii) formation of metallic contact between Al and Fe, under reduction of iron oxides by aluminum, and (iii) formation of covalent-like Al-Fe bonds across the interface. © 2019 Elsevier Ltd
    view abstract10.1016/j.mtla.2019.100396
  • Au–Sn solders applied in transient liquid phase bonding: Microstructure and mechanical behavior
    Du, C. and Soeler, R. and Völker, B. and Matoy, K. and Zechner, J. and Langer, G. and Reisinger, M. and Todt, J. and Kirchlechner, C. and Dehm, G.
    Materialia 8 (2019)
    Transient liquid phase bonding offers one option to generate a robust lead-free die attach with sound thermal and electrical conductivity in microelectronic packages. However, it is a challenge to characterize the microstructure and mechanical behavior of the bonding layer because of its ultra-thin thickness and its nano-sized constituents. We show that microstructures and local mechanical properties of such a modern solder systems are accessible with state-of-the-art methods including transmission electron microscope, synchrotron X-ray nano-beam diffraction as well as micro-scale mechanical testing. Three sub-regions with different morphologies have been identified within the bonding area, and their contained phases have been mapped. Two of the sub-regions contain nano-sized intermetallic compounds (IMC) while the third one is mostly composed of an FCC Au–Cu solid solution with a Cu concentration gradient. On top of that, micro-cantilever bending testing has been conducted to investigate the mechanical behavior of the bonding region. The two IMC sub-regions show brittle behavior while the Au–Cu sub-region is ductile. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2019.100503
  • Development of a high-temperature micromechanics stage with a novel temperature measurement approach
    Arigela, V.G. and Oellers, T. and Ludwig, Al. and Kirchlechner, C. and Dehm, G.
    Review of Scientific Instruments 90 (2019)
    The study of mechanical properties of materials at high temperatures at the microstructural length regime requires dedicated setups for testing. Despite the advances in the instrumentation in these setups over the last decade, further optimization is required in order to extend the temperature range well-beyond 600 °C. Particularly, an improvement of the contact temperature measurement is required. A design with a novel approach of temperature measurement with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature microstraining rig with the addition of two miniature hot stages, one each carrying the sample and indenter tip. The sample reaches temperatures of >600 °C with a 50 W diode laser system. The stages have slots for the working sample as well as a reference sample on both ends for precise temperature measurements, relying on the symmetry of the stage toward the ends. The whole setup is placed inside a custom-made steel chamber, capable of attaining a vacuum of 10-4 Pa. Alternatively, the apparatus can be operated under environmental conditions by applying various gases. Here, the unique design and its high temperature capabilities will be presented together with the first results of microtension experiments on freestanding copper thin films at 400 °C. © 2019 Author(s).
    view abstract10.1063/1.5086261
  • How close can indents be placed without risking an erroneous pop-in statistics?
    Li, J. and Dehm, G. and Kirchlechner, C.
    Materialia 7 (2019)
    Load-displacement curves with spherical indenters often exhibit a so-called pop-in, which can be interpreted as elastic-plastic transition due to activation or even nucleation of dislocation sources. Due to the stochastic nature of deformation at the micron scale a wide distribution of pop-in loads (referred as pop-in statistics) can be observed. This work presents the critical distance at which a mutual interaction of dislocations from two indents can be excluded. It is found that the critical distance strongly depends on the material. A quantitative model for estimating the critical distance is provided, which might serve an experimentalist as a guideline for planning reliable experiments. © 2019
    view abstract10.1016/j.mtla.2019.100378
  • Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films
    Glushko, O. and Dehm, G.
    Acta Materialia 169 (2019)
    Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.03.004
  • Iron Aluminides
    Palm, M. and Stein, F. and Dehm, G.
    Annual Review of Materials Research 49 (2019)
    The iron aluminides discussed here are Fe-Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews. © 2019 by Annual Reviews. All rights reserved.
    view abstract10.1146/annurev-matsci-070218-125911
  • Micro fracture investigations of white etching layers
    Saxena, A.K. and Kumar, A. and Herbig, M. and Brinckmann, S. and Dehm, G. and Kirchlechner, C.
    Materials and Design 180 (2019)
    The fracture behavior of a white etching layer formed on the rail surface in pearlitic steels during the rail-wheel contact is investigated using indentation-based microcantilever fracture tests. The sample thickness is in the order of 5 μm. The local fracture toughness of the white etching layer, its neighboring brown etching layer, martensite and pearlite with similar chemical composition are determined and compared to ferritic steels. All samples show stable crack growth accompanied by significant plasticity at the crack tip. The toughnesses scale inversely with the microhardness. The white etching layer exhibits a toughness of 16.0 ± 1.2 MPa m1/2 which is in the same range as the fully martensitic steel. It is shown that the local fracture toughness can be roughly estimated based on the Vickers hardness of the white etching layer. Also, an estimation of a critical defect size in white etching layers which considerably furthers the understanding of crack initiation is made in this study. Furthermore, various criteria for analyzing the elasto plastic fracture toughness are compared. © 2019 The Authors
    view abstract10.1016/j.matdes.2019.107892
  • On pinning-depinning and microkink-flow in solid state dewetting: Insights by in-situ ESEM on Al thin films
    Hieke, S.W. and Willinger, M.-G. and Wang, Z.-J. and Richter, G. and Chatain, D. and Dehm, G. and Scheu, C.
    Acta Materialia 165 (2019)
    The dynamics of solid state dewetting phenomena of a 50 nm thick, mazed bicrystalline Al film on single crystalline α-Al2O3 (sapphire) substrates was studied in-situ using an environmental scanning electron microscope (ESEM). The bicrystalline Al thin films served as a model system where the influence of grain boundaries and texture effects are well determined compared to polycrystalline films. The experiments were performed in controlled oxidizing and reducing atmospheres at 773 K and 823 K, respectively, to shed light on the differences in dewetting mechanisms and dynamics. While the reducing atmosphere led to spontaneous dewetting at 823 K after an incubation time of a few minutes, a hierarchical dewetting process was observed for the sluggish dewetting under oxidizing conditions. Voids initiated at (substrate or surface) defects and expanded trying to maintain a hexagonal shape. Pinning and depinning processes led to a discontinuous void growth and irregular void shapes including finger instabilities. As a consequence, the void growth followed a variety of power law exponents between 0.10 and 0.55. A new microkink-flow mechanism was discovered at the terminating Al planes at the void. © 2018
    view abstract10.1016/j.actamat.2018.11.028
  • Oxygen-mediated deformation and grain refinement in Cu-Fe nanocrystalline alloys
    Guo, J. and Duarte, M.J. and Zhang, Y. and Bachmaier, A. and Gammer, C. and Dehm, G. and Pippan, R. and Zhang, Z.
    Acta Materialia 166 (2019)
    Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.040
  • Plastic deformation of tungsten due to deuterium plasma exposure: Insights from micro-compression tests
    Fang, X. and Rasinski, M. and Kreter, A. and Kirchlechner, C. and Linsmeier, C. and Dehm, G. and Brinckmann, S.
    Scripta Materialia 162 (2019)
    Nanoindentation tests have shown that exposure to deuterium plasma causes a decrease in pop-in load and an increase in hardness of tungsten. In this work, we use micro-compression tests to investigate the plastic deformation and apparent strain hardening of tungsten exposed to deuterium plasma. In comparison with the pillars tested at reference state, the pillars tested after exposure showed an increased apparent strain hardening rate as well as an increased multitude of slip traces, which is attributed to the presence of deuterium. The micro-compression results are in agreement with the nanoindentation study on the pop-in and hardness of tungsten. © 2018
    view abstract10.1016/j.scriptamat.2018.10.052
  • Synthesis and mechanical testing of grain boundaries at the micro and sub-micro scale
    Malyar, N.V. and Springer, H. and Wichert, J. and Dehm, G. and Kirchlechner, C.
    Materialpruefung/Materials Testing 61 (2019)
    The important role of grain boundaries for the mechanical properties of polycrystalline materials has been recognized for many decades. Up to now, the underlying deformation mechanisms at the nano- and micro scale are not understood quantitatively. An overview of the synthesis and subsequent mechanical testing of specific grain boundaries at the micro and sub-micro scale is discussed in the present contribution, including various methods for producing one or multiple specific, crystallographically well-defined grain boundaries. Furthermore, established micromachining methods for isolating and measuring local dislocation-grain boundary interactions are portrayed. Examples of the techniques described are shown with to the aid of copper grain boundaries. © Carl Hanser Verlag, München
    view abstract10.3139/120.111286
  • Synthesis, microstructure, and hardness of rapidly solidified Cu-Cr alloys
    Garzón-Manjón, A. and Christiansen, L. and Kirchlechner, I. and Breitbach, B. and Liebscher, C.H. and Springer, H. and Dehm, G.
    Journal of Alloys and Compounds 794 (2019)
    Cu-Cr alloys with ∼32 at.% Cr were rapidly solidified by splat quenching or laser melting techniques with the intention to form a very fine grained, non-equilibrium nanostructure similar to those obtained by severe plastic deformation or thin film deposition. The rapidly solidified Cu-Cr alloys were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Both synthesis techniques lead to a similar two-phase microstructure with a nearly pure fcc Cu matrix with μm grain sizes and bcc Cr particles highly supersaturated with Cu. Splat quenching provides finer bcc particles with dimensions of less than 50 nm compared to laser melting with particle sizes of 100–2000 nm. In case of laser melting, (14 ± 2) at.% Cu are contained in the bcc phase, while splat quenching freezes (20 ± 2) at.% Cu in the bcc particles. The microstructures are discussed and compared to the non-equilibrium microstructures reported in literature using severe plastic deformation and thin films deposition. © 2019 Elsevier B.V.
    view abstract10.1016/j.jallcom.2019.04.209
  • Towards quantifying the shear delamination of thin films
    Brinckmann, S. and Soler, R. and Dehm, G.
    Materialia 8 (2019)
    The design of multilayer stacks in the semiconductor industry requires knowledge of the delamination quantities. Previously, the determination of the mode-II failure has been problematic since the success-rate of current methods is low or the samples require superlayers that potentially alter the stack. We develop a method for mode-II testing that applies an additional normal force, which prevents mode-I opening and buckling. This method requires a nanoindenter with lateral force measurement capabilities and a FIB or lithographically structured sample. The experiments are fast, allowing for high-throughput testing. The method is applied to quantify the mode-II fracture of the Cu–SiO2 interface and we show the origins of the observed size-effect. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2019.100421
  • Tribolayer formation during macro- and microscale cyclic contact
    Brinckmann, S. and Stratmann, A. and Dehm, G. and Jacobs, G.
    Tribology International 129 (2019)
    The formation of tribological surface layers is essential for ultra-mild wear. These surface layers form during machine operation in lubricated conditions with additives. Zinc-dialkyldithiophosphates (ZDDP) are frequently used anti-wear additives, but their future use is limited due to environmental concerns. The design of next-generation additives requires a thorough understanding of the tribolayer formation mechanisms of current additives. The present work describes novel macro- and microscale experiments at room temperature that have a limited number of parameters and that use cyclic contact to form tribological surface layers. The new setup allows focusing on the influence of the mechanical stresses, which are very localized in this setup. This study shows that compression creates surface layers even with limited shear driven lubricant flow. © 2018 Elsevier Ltd
    view abstract10.1016/j.triboint.2018.08.029
  • Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials
    Šestan, A. and Zavašnik, J. and Kržmanc, M.M. and Kocen, M. and Jenuš, P. and Novak, S. and Čeh, M. and Dehm, G.
    Journal of Nuclear Materials 524 (2019)
    Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm
    view abstract10.1016/j.jnucmat.2019.06.030
  • Unraveling indentation-induced slip steps in austenitic stainless steel
    Xia, W. and Dehm, G. and Brinckmann, S.
    Materials and Design 183 (2019)
    Nanoindentation has been used intensively during the last decades to characterize experimentally the elastic and plastic material properties of phases at the microscale. Accompanying simulations have investigated the plastic mechanisms during nanoindentation. While experiments and simulations have led to a thorough understanding of most mechanisms during nanoindentation, the plasticity on positively and negatively inclined slip planes is still not completely clear. In this work, {1 0 0}-, {1 0 1}- and {1 1 1}-grains of an austenitic stainless steel are indented to better understand the dislocation mediated plasticity through slip step analysis. We observe that slip occurs on positively and negatively inclined slip planes during nanoindentation and we propose methods to differentiate between both types of planes. We find that slip steps on positively inclined slip planes form preferentially during the early stage as compared to the formation of slip steps on negative inclination, which occurs during the later deformation stage due to the change in surface topography. By calculating the resolved shear stress in the presence and absence of pile-ups, we reveal the origin of slip on positively and negatively inclined planes as well as the reason for the sequence of occurrences. We conclude that accounting for the surface topography evolution in experiments and simulations is essential in predicting the plastic slip activation during nanoindentation. © 2019 The Authors
    view abstract10.1016/j.matdes.2019.108169
  • Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys
    Lu, W. and Liebscher, C.H. and Dehm, G. and Raabe, D. and Li, Z.
    Advanced Materials 30 (2018)
    Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy alloy containing multiple principal elements is reported. This is achieved by triggering both, dynamic forward transformation from a faced-centered-cubic γ matrix into a hexagonal-close-packed ε nanolaminate structure and the dynamic reverse transformation from ε into γ. This new mechanism is referred to as the “bidirectional transformation induced plasticity” (B-TRIP) effect, which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.201804727
  • Dislocation slip transmission through a coherent Σ3{111} copper twin boundary: Strain rate sensitivity, activation volume and strength distribution function
    Malyar, N.V. and Grabowski, B. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 161 (2018)
    We present the first measurement of the strain rate sensitivity of the ideal dislocation slip transmission through a coherent Σ3{111} copper twin boundary. For this purpose we have deformed 129 geometrically identical samples at different strain rates. The micron-sized samples are either single crystalline (87 pillars) or contain one vertical Σ3{111} twin boundary (42 pillars). The strain rate sensitivity of the ideal slip transmission event is 0.015 ± 0.009. This value is considerably lower than the strain rate sensitivity observed for nano-twinned bulk materials, which is addressed to multiple simultaneously activated deformation processes present in the latter case. The activation volume of the ideal slip transmission points towards a cross-slip like transmission process of dislocations through the twin boundary. Furthermore, the high number of geometrically identical samples is used to discuss the ability to identify the strength distribution function of micropillars. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.09.045
  • Formation of eta carbide in ferrous martensite by room temperature aging
    Lu, W. and Herbig, M. and Liebscher, C.H. and Morsdorf, L. and Marceau, R.K.W. and Dehm, G. and Raabe, D.
    Acta Materialia 158 (2018)
    For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, (1¯10) α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years. © 2018
    view abstract10.1016/j.actamat.2018.07.071
  • Fracture toughness of Mo2BC thin films: Intrinsic toughness versus system toughening
    Soler, R. and Gleich, S. and Kirchlechner, C. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Materials and Design 154 (2018)
    The fracture behaviour and microstructure evolution of sputtered Mo2BC films as a function of their deposition temperature is studied. Bipolar pulsed direct current magnetron sputtering was used to deposit Mo2BC thin films onto Si (100) wafers at substrate temperatures ranging from 380 to 630 °C. Microstructural characterization by transmission electron microscopy revealed that increasing the deposition temperature induces larger and more elongated grains, and a higher degree of crystallinity, transitioning from a partially amorphous to a fully crystalline film. The intrinsic fracture toughness of the Mo2BC films was studied by focussed ion beam milled micro-cantilever bending tests. A mild dependency of the intrinsic fracture toughness on the substrate deposition temperature was found. Fractograph analysis showed that the fracture behaviour was dominated by intergranular fracture or by fracture within the amorphous regions. Additionally, nanoindentation based fracture toughness measurements were used to probe the fracture behaviour of the Mo2BC/Si system, where residual stresses define the ‘apparent’ fracture toughness of the system. Depending on the substrate deposition temperature either compressive or tensile residual stresses developed in the films. This causes a relative change in the system toughness by up to one order of magnitude. The fracture experiments clearly reveal that notched cantilevers provide intrinsic toughness values of a material, while nanoindentation probes the toughness of the entire coating-substrate system. The combination of both techniques provides valuable design information for enhancing fracture resistance of Mo2BC films. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.05.015
  • Hydrogen embrittlement of tungsten induced by deuterium plasma: Insights from nanoindentation tests
    Fang, X. and Kreter, A. and Rasinski, M. and Kirchlechner, C. and Brinckmann, S. and Linsmeier, C. and Dehm, G.
    Journal of Materials Research 33 (2018)
    Hydrogen exposure has been found to result in metal embrittlement. In this work, we use nanoindentation to study the mechanical properties of polycrystalline tungsten subjected to deuterium plasma exposure. For the purpose of comparison, nanoindentation tests on exposed and unexposed reference tungsten were carried out. The results exhibit a decrease in the pop-in load and an increase in hardness on the exposed tungsten sample after deuterium exposure. No significant influence of grain orientation on the pop-in load was observed. After a desorption time of td ≥ 168 h, both the pop-in load and hardness exhibit a recovering trend toward the reference state without deuterium exposure. The decrease of pop-in load is explained using the defactant theory, which suggests that the presence of deuterium facilitates the dislocation nucleation. The increase of hardness is discussed based on two possible mechanisms of the defactant theory and hydrogen pinning of dislocations. © 2018 Materials Research Society.
    view abstract10.1557/jmr.2018.305
  • In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys
    Guo, J. and Haberfehlner, G. and Rosalie, J. and Li, L. and Duarte, M.J. and Kothleitner, G. and Dehm, G. and He, Y. and Pippan, R. and Zhang, Z.
    Nature Communications 9 (2018)
    Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. © 2018 The Author(s).
    view abstract10.1038/s41467-018-03288-8
  • Influence of composition and crystal structure on the fracture toughness of NbCo2 Laves phase studied by micro-cantilever bending tests
    Luo, W. and Kirchlechner, C. and Fang, X. and Brinckmann, S. and Dehm, G. and Stein, F.
    Materials and Design 145 (2018)
    Cubic and hexagonal NbCo2 Laves phases are known to have composition dependent hardness and yield strength. However, it is unknown whether this dependence is also reflected in their fracture toughness values. In order to elucidate the fracture behavior, single-crystalline micro-cantilevers of the cubic and hexagonal NbCo2 Laves phases having different compositions were fabricated in the diffusion layers grown by the diffusion couple technique. Micro-cantilever bending tests were performed to study the composition- and crystal-structure-dependence of the fracture toughness. To exclude the influence of micro-cantilever geometry, pentagonal and rectangular beams were tested and found to result in the same fracture toughness value. The present results reveal that neither a change of the crystal structure nor a change in chemical composition has a significant influence on the fracture toughness of NbCo2 Laves phase. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.02.045
  • Microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure
    Soler, R. and Evirgen, A. and Yao, M. and Kirchlechner, C. and Stein, F. and Feuerbacher, M. and Raabe, D. and Dehm, G.
    Acta Materialia 156 (2018)
    The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ∼10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal <a> system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.06.010
  • Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstract10.1016/j.tsf.2017.10.030
  • Modifying the nanostructure and the mechanical properties of Mo2BC hard coatings: Influence of substrate temperature during magnetron sputtering
    Gleich, S. and Soler, R. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Hans, M. and Primetzhofer, D. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Materials and Design 142 (2018)
    A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings. © 2018 The Authors
    view abstract10.1016/j.matdes.2018.01.029
  • Nano-laminated thin film metallic glass design for outstanding mechanical properties
    Kontis, P. and Köhler, M. and Evertz, S. and Chen, Y.-T. and Schnabel, V. and Soler, R. and Bednarick, J. and Kirchlechner, C. and Dehm, G. and Raabe, D. and Schneider, J.M. and Gault, B.
    Scripta Materialia 155 (2018)
    We report the enhancement of fracture toughness and strength of a cobalt‑tantalum-based metallic glass thin film with increasing boron content. The improvement of the mechanical performance is attributed to the formation of a compositionally lamellar compared to uniform glass microstructure, which becomes thinner with increasing boron content as revealed by transmission electron microscopy. Compositional variations across the lamellar structure are revealed by atom probe tomography. Cobalt- and boron-rich regions alternate sequentially, whereas tantalum exhibits slight variations across the lamellae. Our results can be utilized in future design efforts for metallic glass thin films with outstanding mechanical performance. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2018.06.015
  • On the nature of twin boundary-associated strengthening in Fe-Mn-C steel
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 (2018)
    We unravel the nature of twin boundary-associated strengthening in Fe-Mn-C twinning-induced plasticity steel (TWIPs) by micro-pillar compression tests. Dislocation interactions with a coherent twin boundary and their role on strain hardening were investigated. The results indicate that twin-matrix bundles dynamically introduced by deformation twinning and their interaction with dislocations are required for strengthening Fe-Mn-C TWIPs, while single coherent twin boundaries enable dislocation transmission. Correlative studies on orientation dependent deformation mechanisms, detailed dislocation-twin boundary interactions, and the resulting local stress-strain responses suggest that twin boundary-associated strengthening is primarily caused by the reduction of the mean free dislocation path in nano-twinned microstructures. © 2018
    view abstract10.1016/j.scriptamat.2018.07.009
  • On the segregation of Re at dislocations in the γ’ phase of Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Kontis, P. and Dehm, G. and Raabe, D. and Gault, B. and Eggeler, G.
    Materialia 4 (2018)
    We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ’ precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750 °C under an applied stress of 800 MPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ’ interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2018.09.018
  • Overview on micro- and nanomechanical testing: New insights in interface plasticity and fracture at small length scales
    Dehm, G. and Jaya, B.N. and Raghavan, R. and Kirchlechner, C.
    Acta Materialia 142 (2018)
    Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.06.019
  • Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper
    Peter, N.J. and Frolov, T. and Duarte, M.J. and Hadian, R. and Ophus, C. and Kirchlechner, C. and Liebscher, C.H. and Dehm, G.
    Physical Review Letters 121 (2018)
    We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5{210} segments and Ag-depleted {230}/{100} asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag. © 2018 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevLett.121.255502
  • Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.015702
  • Sulfur – induced embrittlement in high-purity, polycrystalline copper
    Meiners, T. and Peng, Z. and Gault, B. and Liebscher, C.H. and Dehm, G.
    Acta Materialia 156 (2018)
    Tensile tests were carried out in high-purity, polycrystalline copper alloys with three concentrations of sulfur impurities (14, 27 and 7920 at ppm) at temperatures between 20 °C and 400 °C. The ductility drops with increasing sulfur concentration and temperature while the ultimate tensile strength increases. The alloys exhibit a grain size of several millimeters and contain mostly random grain boundaries (GBs). The microstructure and composition is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The microstructure of the samples with sulfur contents of 14 and 27 ppm consists of globular grains and neither of the microanalytical techniques employed reveals the formation of Cu-sulfides or sulfur segregation to GBs. Even after annealing at 500 °C, no sulfide formation or sulfur segregation to GBs was detected. In the alloy with a sulfur content of 7920 ppm, a dendritic structure is observed and in the interdendritic region monoclinic Cu2S precipitates with a size range from 5 nm to several μm are observed at GBs and also within the grains. The influence of S on the ductility is discussed considering the TEM and APT results. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.06.013
  • Tetragonal fcc-Fe induced by κ -carbide precipitates: Atomic scale insights from correlative electron microscopy, atom probe tomography, and density functional theory
    Liebscher, C.H. and Yao, M. and Dey, P. and Lipińska-Chwalek, M. and Berkels, B. and Gault, B. and Hickel, T. and Herbig, M. and Mayer, J. and Neugebauer, J. and Raabe, D. and Dehm, G. and Scheu, C.
    Physical Review Materials 2 (2018)
    Correlative scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations resolve the correlation between elastic strain fields and local impurity concentrations on the atomic scale. The correlative approach is applied to coherent interfaces in a κ-carbide strengthened low-density steel and establishes a tetragonal distortion of fcc-Fe. An interfacial roughness of ∼1nm and a localized carbon concentration gradient extending over ∼2-3nm is revealed, which originates from the mechano-chemical coupling between local strain and composition. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.023804
  • Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering
    Gleich, S. and Breitbach, B. and Peter, N.J. and Soler, R. and Bolvardi, H. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Surface and Coatings Technology 349 (2018)
    The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings. © 2018
    view abstract10.1016/j.surfcoat.2018.06.006
  • 100 years public-private partnership in metallurgical and materials science research
    Raabe, D. and Dehm, G. and Neugebauer, J. and Rohwerder, M.
    Materials Today 20 (2017)
    view abstract10.1016/j.mattod.2017.02.005
  • Annealing induced void formation in epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Dehm, G. and Scheu, C.
    Acta Materialia 140 (2017)
    In this work faceted voids are studied which were induced by solid state dewetting at 600 °C of tetracrystalline Al thin films covered with a native oxide layer. Hexagonally shaped voids are observed in a few locations where Al is uniformly redistributed to the surrounding thin film. Although faceted, the majority of the voids exhibit irregular shapes caused by pinning of distinct sides of the retracting Al thin film. The two different Al|void shapes (hexagonal or irregular) are investigated by site-specific cross-sectional transmission electron microscopy (TEM) analysis. The TEM studies reveal Al|void regions with and without rims and ridges. The presence of rims and ridges is explained by a discontinuous void formation process caused by pinning of the retracting Al film. During annealing, crystallization and a thickness increase of the surface oxide, which is still continuously covering the Al thin film as well as the void, occurs. The surface scale undergoes a phase transformation from the amorphous state to γ-Al2O3, which is confirmed locally on the nanometer scale using scanning TEM techniques including electron energy loss near-edge structure investigations. Spherical aberration corrected atomic column resolved scanning TEM revealed a cube-on-cube orientation relationship between the Al thin film and the γ-Al2O3 surface oxide. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.08.050
  • Compressed Bi-crystal micropillars showing a sigmoidal deformation state – A computational study
    Toth, F. and Kirchlechner, C. and Fischer, F.D. and Dehm, G. and Rammerstorfer, F.G.
    Materials Science and Engineering A 700 (2017)
    It is the aim of this paper to show the mechanisms behind the experimental observations of rather smooth sigmoidal deformations in bi-crystal micropillar tests (in contrast to single crystal micro-compression tests) and to point out that the appearance of such deformation modes are a further reason for being careful when interpreting the force-axial displacement behavior in terms of stress-strain curves. Instabilities, i.e., buckling and subsequent post-buckling deformations, inhomogeneous strain fields and substantial deformations of the base as well as pronounced free surface effects are considered. The influences of imperfections and of friction as well as a possible clearance in the guidance of the loading device are taken into account, too. From these studies, the experimenter may get information how and with which limitations material parameters can be obtained from such compression tests in combination with simulations. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.05.098
  • Dislocation interaction and twinning-induced plasticity in face-centered cubic Fe-Mn-C micro-pillars
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and De Cooman, B.C. and Raabe, D.
    Acta Materialia 132 (2017)
    Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in polycrystalline steel. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interactions of dislocations with twin boundaries. The material investigated is an austenitic Fe-22 wt%Mn-0.6 wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that deformation twinning induces higher flow stresses and a more unstable work-hardening behavior than dislocation glide, while dislocation glide dominated deformation results in a stable work-hardening behavior. The higher flow stresses and unstable work-hardening behavior in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.04.043
  • Dislocation-twin boundary interaction in small scale Cu bi-crystals loaded in different crystallographic directions
    Malyar, N.V. and Micha, J.-S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 (2017)
    The mechanical behavior of several 5×5 micron sized Cu bi-crystals with a single coherent Σ3{111} twin boundary (TB) is studied by in situ Laue microdiffraction (μLaue) compression with the aim to unravel the slip transfer mechanisms through TBs. Single crystalline pillars (SCP) are additionally tested and used as reference samples. Engineering stress-strain curves and post mortem scanning electron microscopy (SEM) images were correlated to the local evolution of the TB angle, the storage of geometrically necessary dislocations and crystal orientations investigated by in situ X-ray Laue microdiffraction (μLaue). Both μLaue and post mortem SEM demonstrate multiple transmission events through the TB without significant storage of geometrically necessary dislocations in the crystals or at the boundary, independent on the compression direction. Nevertheless, at engineering strains larger than 5% a small dislocation pile-up was once observed temporarily at the boundary. Upper and lower bounds for the transmission stress are discussed based on the current experimental results. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.02.067
  • Effect of annealing on the size dependent deformation behavior of thin cobalt films on flexible substrates
    Marx, V.M. and Cordill, M.J. and Többens, D.M. and Kirchlechner, C. and Dehm, G.
    Thin Solid Films 624 (2017)
    The effect of film thickness as well as the influence of heat treatment on the deformation behavior of thin cobalt films (50–2000 nm) on polyimide substrates was investigated using various tensile tests. Straining under an optical light microscope provides information about the fracture strain and cracking behavior. The annealed films exhibit enhanced crack onset strains between 4 and 7% compared to the as-deposited films with fracture strains of 1–2%. This is partly achieved by a mechanically induced martensitic phase transformation of cobalt from the face-centered cubic (FCC) to the hexagonal-closed packed (HCP) phase. Thereby, it was shown that the heat treatment can be used to increase the amount of metastable FCC phase. Complementary synchrotron diffraction experiments were used to determine the lattice strains which initially increase during straining. After reaching a maximum, the lattice strains decrease in the case of the as-deposited films due to crack formation and in the case of the annealed films due the strain-induced phase transformation and localized plastic deformation in the form of necks. At higher engineering strains, the formation of cracks is also observed in the heat treated samples. Additionally, a decrease of the maximum lattice strain could be found for the HCP phase below a film thickness of 200 nm and grain size of 50 nm in the as-deposited films which is caused by cracking. © 2017
    view abstract10.1016/j.tsf.2017.01.011
  • Electronic structure of metastable bcc Cu-Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations
    Liebscher, C.H. and Freysoldt, C. and Dennenwaldt, T. and Harzer, T.P. and Dehm, G.
    Ultramicroscopy 178 (2017)
    Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for (110)-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr. © 2016 Elsevier B.V.
    view abstract10.1016/j.ultramic.2016.07.011
  • Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing
    Jaya, B.N. and Goto, S. and Richter, G. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 707 (2017)
    In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4 MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.09.010
  • Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity
    Pizzutilo, E. and Freakley, S.J. and Cherevko, S. and Venkatesan, S. and Hutchings, G.J. and Liebscher, C.H. and Dehm, G. and Mayrhofer, K.J.J.
    ACS Catalysis 7 (2017)
    During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b01447
  • In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys
    Zhang, Z. and Guo, J. and Dehm, G. and Pippan, R.
    Acta Materialia 138 (2017)
    We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time. © 2017
    view abstract10.1016/j.actamat.2017.07.039
  • In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstract10.1016/j.jallcom.2016.10.302
  • Kinetics and crystallization path of a Fe-based metallic glass alloy
    Duarte, M.J. and Kostka, A. and Crespo, D. and Jimenez, J.A. and Dippel, A.-C. and Renner, F.U. and Dehm, G.
    Acta Materialia 127 (2017)
    The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe50Cr15Mo14C15B6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr6Fe18Mo5, M23(C,B)6, M7C3, η-Fe3Mo3C and FeMo2B2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.01.031
  • Maintaining strength in supersaturated copper–chromium thin films annealed at 0.5 of the melting temperature of Cu
    Raghavan, R. and Harzer, T.P. and Djaziri, S. and Hieke, S.W. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Science 52 (2017)
    The thermal stability of evaporated copper–chromium alloy films was studied by correlating hardness trends from nanoindentation to nanostructural–compositional changes from transmission electron microscopy. In particular, the hardness evolution with ageing time at ambient and elevated temperatures of two compositions, dilute (Cu96Cr4) and chromium-rich (Cu67Cr33) solutions, was studied. Due to the negligible mutual miscibility of copper and chromium, the chosen solid solutions are trapped in metastable states as supersaturated solid solutions with face-centred cubic and body-centred cubic phases. Nano-mechanical probing of the nanostructural evolution as a function of temperature provided interesting insights into the phase separation of these systems. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s10853-016-0386-6
  • Mechanical size effects in a single crystalline equiatomic FeCrCoMnNi high entropy alloy
    Raghavan, R. and Kirchlechner, C. and Jaya, B.N. and Feuerbacher, M. and Dehm, G.
    Scripta Materialia 129 (2017)
    The size dependence of the mechanical behavior of a single crystalline equiatomic FeCrCoMnNi single phase high entropy alloy was studied using in situ SEM microcompression. Electron back-scattered diffraction was used in conjunction with high-resolution scanning electron microscopy to identify the dominant slip system activated for accommodating plastic flow. The scaling of the yield strength with the size of the micropillar is discussed in comparison with the size dependence observed in face-centered and body-centered cubic single crystalline metals. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2016.10.026
  • Microcantilever Fracture Testing of Intermetallic Cu3Sn in Lead-Free Solder Interconnects
    Philippi, B. and Matoy, K. and Zechner, J. and Kirchlechner, C. and Dehm, G.
    Journal of Electronic Materials 46 (2017)
    Driven by legislation and the abolishment of harmful and hazardous lead-containing solders, lead-free replacement materials are in continuous development. Assessment of the mechanical properties of intermetallic phases such as Cu3Sn that evolve at the interface between solder and copper metalization is crucial to predict performance and meet the high reliability demands in typical application fields of microelectronics. While representative material parameters and fracture properties are required to assess mechanical behavior, indentation-based testing produces different results depending on the sample type. In this work, focused ion beam machined cantilevers were used to unravel the impact of microstructure on the fracture behavior of Sn-Ag-Cu lead-free solder joints. Fracture testing on notched cantilevers showed brittle fracture for Cu3Sn. Unnotched samples allowed measurement of the fracture stress, to estimate the critical defect size in unnotched Cu3Sn microcantilevers. © 2016, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11664-016-5203-0
  • Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Breitbach, B. and Dehm, G. and Scheu, C.
    Acta Materialia 133 (2017)
    Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.05.026
  • Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography
    Gleich, S. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Soler, R. and Pradeep, K.G. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 122 (2017)
    In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects. © 2017 Author(s).
    view abstract10.1063/1.4999304
  • On the influence of microcantilever pre-crack geometries on the apparent fracture toughness of brittle materials
    Brinckmann, S. and Matoy, K. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 136 (2017)
    Focused ion beam machined microcantilevers are frequently used for fracture mechanics analysis of inhomogeneous solids at the micrometer scale. A finite element method study about the influence of the pre-crack geometry on the apparent fracture toughness is provided. We discuss the influence of material bridges and the effect of rounded pre-crack corners when two dimensional models are employed to evaluate the fracture toughness. We conclude with a guideline for introducing an optimized pre-crack. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.07.014
  • Pre- and post-buckling behavior of bi-crystalline micropillars: Origin and consequences
    Kirchlechner, C. and Toth, F. and Rammerstorfer, F.G. and Fischer, F.D. and Dehm, G.
    Acta Materialia 124 (2017)
    Compression of micropillars is routinely used to measure the material response under uniaxial load. In bi-crystalline pillars an S-shaped grain-boundary together with an S-shaped pillar is often observed after deformation raising the question of its origin and consequences for stress-strain materials data. In addition to dislocation and grain-boundary based mechanisms, this observation can be caused by buckling and subsequent post-buckling deformation. Deviations from the classical pre- and post-buckling deformation behavior are assigned to imperfections, which are categorized in extrinsic and intrinsic imperfections in this work. In the present paper, the S-shaped actual deformation state is particularly promoted by an intrinsic imperfection, caused by a material heterogeneity (due to the bi-crystal arrangement). This kind of deformation behavior is investigated by micro-compression experiments on 7 × 7 × 21 μm3 sized bi-crystal copper pillars with nearly elastic (axial Young's modulus) homogeneity and identical Schmid factors for both grain orientations. Complementary finite element simulations are performed, in which also the role of friction and of an extrinsic imperfection in the form of initial misalignment of the loading on the S-shape are considered. There, a material model describing the flow stress distribution caused by a dislocation pile-up at the grain-boundary is applied. Finally, suggestions to prevent buckling and, thus, transversal post-buckling displacements during micropillar compression tests are given with the goal to extract engineering stress-strain curves. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.10.075
  • Size effect in bi-crystalline micropillars with a penetrable high angle grain boundary
    Malyar, N.V. and Micha, J.S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 (2017)
    The implications of various size effects on the deformation behavior of and near grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle grain boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-grain-boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the grain size being the characteristic length scale. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.03.003
  • Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstract10.1016/j.tsf.2016.12.048
  • Strain rate dependence of the slip transfer through a penetrable high angle grain boundary in copper
    Malyar, N.V. and Dehm, G. and Kirchlechner, C.
    Scripta Materialia 138 (2017)
    Micro pillar compression is used to analyze the strain rate dependence of copper pillars containing a penetrable high-angle grain boundary via in situ compression tests at strain rates ranging from  10− 1 to 10− 4 s− 1. While the grain-boundary containing pillars exhibit a clear strain-rate dependence of m = 0.04 ± 0.02, their single crystal counterparts seem to have a weak strain rate dependence of m = 0.01 ± 0.01. The results strongly suggest that the movement of the dislocation line in the grain boundary, required to change its orientation from the incoming to the outgoing slip plane, is the critical process in deforming this kind of grain-boundary containing pillars. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2017.05.042
  • Stress intensity factor dependence on anisotropy and geometry during micro-fracture experiments
    Brinckmann, S. and Kirchlechner, C. and Dehm, G.
    Scripta Materialia 127 (2017)
    Miniaturized fracture beam experiments are often used to identify the fracture toughness of single phases and particular grain boundaries because large-scale experiments reveal only homogenized material properties. The evaluation of the microscale toughness is based on isotropic 2D models although the majority of materials are anisotropic. Moreover, the thickness influences the fracture toughness because the crack driving force is maximum in the beam center. This study quantifies the influence of anisotropy, Poisson's ratio and beam geometry using thousands of 3D simulations. We give guidelines for micro-cantilever design and quantify the changes in fracture toughness, if the guidelines cannot be fulfilled. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2016.08.027
  • Surface optical phonon propagation in defect modulated nanowires
    Venkatesan, S. and Mancabelli, T. and Krogstrup, P. and Hartschuh, A. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 121 (2017)
    Planar defects, such as stacking faults and twins, are the most common defects in III-V semiconductor nanowires. Here we report on the effect of surface perturbation caused by twin planes on surface optical (SO) phonon modes. Self-catalyzed GaAs nanowires with varying planar defect density were grown by molecular beam epitaxy and investigated by Raman spectroscopy and transmission electron microscopy (TEM). SO phonon peaks have been detected, and the corresponding spatial period along the nanowire axis were measured to be 1.47 μm (±0.47 μm) and 446 nm (±35 nm) for wires with twin densities of about 0.6 (±0.2) and 2.2 (±0.18) per micron. For the wires with extremely high density of twins, no SO phonon peaks were detected. TEM analysis of the wires reveal that the average distance between the defects are in good agreement with the SO phonon spatial period determined by Raman spectroscopy. © 2017 Author(s).
    view abstract10.1063/1.4976564
  • Are Mo2BC nanocrystalline coatings damage resistant? Insights from comparative tension experiments
    Djaziri, S. and Gleich, S. and Bolvardi, H. and Kirchlechner, C. and Hans, M. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Surface and Coatings Technology 289 (2016)
    Mo2BC nanocrystalline coatings were deposited on Cu substrates to compare their mechanical performance with bench-mark TiAlN, and pure Mo, Al and Al2O3 reference coatings. The Mo2BC coatings were characterized by X-ray diffraction and transmission electron microscopy to analyze the microstructure. In order to study the damage behavior, the coatings were subjected to uniaxial tensile loading and the crack spacing with increasing strain was monitored using optical and scanning electron microscopy. Based on crack density measurements, the Mo2BC coatings were found to be significantly less prone to cracking than the bench-mark TiAlN coatings. The higher resistance to cracking arises from the electronic structure of the Mo2BC nanolaminates, which imparts moderate ductility to the deformation behavior. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.02.010
  • Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
    Peter, N.J. and Liebscher, C.H. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 32 (2016)
    Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. Copyright © Materials Research Society 2016
    view abstract10.1557/jmr.2016.398
  • Coccospheres confer mechanical protection: New evidence for an old hypothesis
    Jaya, B.N. and Hoffmann, R. and Kirchlechner, C. and Dehm, G. and Scheu, C. and Langer, G.
    Acta Biomaterialia 42 (2016)
    Emiliania huxleyi has evolved an extremely intricate coccosphere architecture. The coccosphere is comprised of interlocking coccoliths embedded in a polysaccharide matrix. In this work, we performed in-situ scanning electron microscopy based compression tests and conclude that coccospheres have a mechanical protection function. The coccosphere exhibits exceptional damage tolerance in terms of inelastic deformation, recovery and stable crack growth before catastrophic fracture, a feature, which is not found in monolithic ceramic structures. Some of the mechanical features of the coccospheres are due to their architecture, especially polysaccharide matrix that acts as a kind of bio-adhesive. Our data provide strong evidence for the mechanical protection-hypothesis of coccolithophore calcification, without excluding other functions of calcification such as various biochemical roles discussed in the literature. Statement of Significance Although bio-mechanics of shell structures like nacre have been studied over the past decade, coccospheres present an architecture that is quite distinct and complex. It is a porous cell structure evolved to protect the living algae cell inside it in the oceans, subjected to significant hydrostatic pressure. Despite being made of extremely brittle constituents like calcium carbonate, our study finds that coccospheres possess significant damage tolerance especially due to their interlocking coccolith architecture. This will have consequences in bio-mimetic design, especially relating to high pressure applications. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actbio.2016.07.036
  • Deformation-Induced Martensite: A New Paradigm for Exceptional Steels
    Djaziri, S. and Li, Y. and Nematollahi, G.A. and Grabowski, B. and Goto, S. and Kirchlechner, C. and Kostka, A. and Doyle, S. and Neugebauer, J. and Raabe, D. and Dehm, G.
    Advanced Materials 28 (2016)
    view abstract10.1002/adma.201601526
  • Electronic hybridisation implications for the damage-tolerance of thin film metallic glasses
    Schnabel, V. and Jaya, B.N. and Köhler, M. and Music, D. and Kirchlechner, C. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Scientific Reports 6 (2016)
    A paramount challenge in materials science is to design damage-tolerant glasses. Poisson's ratio is commonly used as a criterion to gauge the brittle-ductile transition in glasses. However, our data, as well as results in the literature, are in conflict with the concept of Poisson's ratio serving as a universal parameter for fracture energy. Here, we identify the electronic structure fingerprint associated with damage tolerance in thin film metallic glasses. Our correlative theoretical and experimental data reveal that the fraction of bonds stemming from hybridised states compared to the overall bonding can be associated with damage tolerance in thin film metallic glasses. © The Author(s) 2016.
    view abstract10.1038/srep36556
  • Fracture toughness of intermetallic Cu6Sn5 in lead-free solder microelectronics
    Philippi, B. and Matoy, K. and Zechner, J. and Kirchlechner, C. and Dehm, G.
    Scripta Materialia 123 (2016)
    Even though lead-free Sn-Ag-Cu based solder alloys are emerging as promising candidates to replace well-established but hazardous lead-containing solders, they suffer from limited knowledge about their mechanical performance. To ensure the high reliability demands in microelectronics, fracture properties need to be quantified with respect to crucial microstructural elements like Cu6Sn5 intermetallic compounds formed at common Cu metallization interfaces. While indentation fracture testing methods have shown varying fracture toughness values, micro-cantilever fracture testing is applied in this work on FIB-prepared single-crystalline Cu6Sn5 cantilevers to clarify on the microstructure-fracture relationship. © 2016 Elsevier B.V.
    view abstract10.1016/j.scriptamat.2016.05.039
  • Heat-Induced Phase Transformation of Three-Dimensional Nb3O7(OH) Superstructures: Effect of Atmosphere and Electron Beam
    Betzler, S.B. and Harzer, T. and Ciston, J. and Dahmen, U. and Dehm, G. and Scheu, C.
    Crystal Growth and Design 16 (2016)
    Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb3O7(OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H2O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb2O5. In addition to the high thermal stability of Nb3O7(OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases. © 2016 American Chemical Society.
    view abstract10.1021/acs.cgd.6b00386
  • Importance and Challenges of Electrochemical in Situ Liquid Cell Electron Microscopy for Energy Conversion Research
    Hodnik, N. and Dehm, G. and Mayrhofer, K.J.J.
    Accounts of Chemical Research 49 (2016)
    ConspectusThe foreseeable worldwide energy and environmental challenges demand renewable alternative sources, energy conversion, and storage technologies. Therefore, electrochemical energy conversion devices like fuel cells, electrolyzes, and supercapacitors along with photoelectrochemical devices and batteries have high potential to become increasingly important in the near future. Catalytic performance in electrochemical energy conversion results from the tailored properties of complex nanometer-sized metal and metal oxide particles, as well as support nanostructures. Exposed facets, surface defects, and other structural and compositional features of the catalyst nanoparticles affect the electrocatalytic performance to varying degrees. The characterization of the nanometer-size and atomic regime of electrocatalysts and its evolution over time are therefore paramount for an improved understanding and significant optimization of such important technologies like electrolyzers or fuel cells. Transmission electron microscopy (TEM) and scanning transmission electron microscope (STEM) are to a great extent nondestructive characterization tools that provide structural, morphological, and compositional information with nanoscale or even atomic resolution. Due to recent marked advancement in electron microscopy equipment such as aberration corrections and monochromators, such insightful information is now accessible in many institutions around the world and provides huge benefit to everyone using electron microscopy characterization in general.Classical ex situ TEM characterization of random catalyst locations however suffers from two limitations regarding catalysis. First, the necessary low operation pressures in the range of 10-6 to 10-9 mbar for TEM are not in line with typical reaction conditions, especially considering electrocatalytic solid-liquid interfaces, so that the active state cannot be assessed. Second, and somewhat related, is the lack of time resolution for the evaluation of alterations of the usually highly heterogeneous nanomaterials. Two methods offer a solution to these shortcomings, namely, identical location TEM (IL-TEM) and electrochemical in situ liquid TEM. The former is already well established and has delivered novel insights particularly into degradation processes; however, characterization is still performed in vacuum. The latter circumvents this issue by using dedicated in situ TEM holders but introduces extremely demanding technical challenges. Although the introduction of revolutionizing thin SiN window cells, which elegantly confine the specimen from vacuum, has allowed demonstration of the potential of the in situ approach, the reproducibility and data interpretation is still limited predominately due to the strong interaction of the electron beam with the supporting electrolyte and electrode material. Because of the importance of understanding the nanoelectrochemical structure-function relationship, this Account aims to convey a timely perspective on the opportunities and particularly the challenges in electrochemical identical location TEM and in situ liquid cell TEM with a focus on electrochemical energy conversion. © 2016 American Chemical Society.
    view abstract10.1021/acs.accounts.6b00330
  • Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures
    Jaya, B.N. and Wheeler, J.M. and Wehrs, J. and Best, J.P. and Soler, R. and Michler, J. and Kirchlechner, C. and Dehm, G.
    Nano Letters 16 (2016)
    The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in KC was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si. © 2016 American Chemical Society.
    view abstract10.1021/acs.nanolett.6b03461
  • Size and orientation dependent mechanical behavior of body-centered tetragonal Sn at 0.6 of the melting temperature
    Philippi, B. and Kirchlechner, C. and Micha, J.S. and Dehm, G.
    Acta Materialia 115 (2016)
    Although, tin is one of the most prominent metals in soldering, very little is known about its mechanical behavior. In addition, possible size-effects of tin can become restricting for the ongoing miniaturization of microelectronic devices. Due to the low melting temperature of 505.15 K and the body-centered tetragonal crystal structure, differences in the mechanical behavior compared to face-centered cubic and body-centered cubic metals can be expected. Since Tin is especially interesting because of its multiple different slip systems, post mortem slip step analysis allowed to determine the activated slip systems and thus, to calculate size dependent critical resolved shear stresses. The measured size scaling exponent (-1.07 ± 0.06) is close to model-predictions of -1, irrespective of the activated families of slip systems in different orientations. Furthermore, an exceptional low scatter of the flow stress in various samples and no apparent hardening is found. It is concluded, that the activation of dislocation sources instead of dislocation-dislocation interactions are responsible for the observed behavior. This is in line with complementary μLaue diffraction experiments which indicate an unresolvable low density of geometrical necessary dislocations. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.05.055
  • Strain-induced phase transformation of a thin Co film on flexible substrates
    Marx, V.M. and Kirchlechner, C. and Breitbach, B. and Cordill, M.J. and Többens, D.M. and Waitz, T. and Dehm, G.
    Acta Materialia 121 (2016)
    The strain-induced FCC to HCP phase transformation of a two phase Co film on polyimide was investigated by performing a tensile test in an X-ray diffractometer. During straining of the 2 μm thick film, the intensity of the (002)FCC peak continuously decreases at engineering strains between 2 and 8% and remains constant at higher strains. Complementary in situ tensile tests under an optical light microscope showed crack formation at 6.7% and crack saturation at around 10% engineering strain. The strain-induced phase transformation starts before the first cracks form leading to a maximum lattice strain of approximately 0.9% as initiation strain measured from the (101¯1)HCP peak with the sin2ψ method which converts to a film stress of approximately 1270 ± 150 MPa. It could be revealed that a strain-induced phase transformation can enhance the ductility and therefore delay the crack onset of a thin cobalt film. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.09.015
  • Superlattice effect for enhanced fracture toughness of hard coatings
    Hahn, R. and Bartosik, M. and Soler, R. and Kirchlechner, C. and Dehm, G. and Mayrhofer, P.H.
    Scripta Materialia 124 (2016)
    Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found –by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films– that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ ~ 6 nm. For ultrathin layers (Λ ~ 2 nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices. © 2016 Elsevier B.V.
    view abstract10.1016/j.scriptamat.2016.06.030
  • Adhesion measurement of a buried Cr interlayer on polyimide
    Marx, V.M. and Kirchlechner, C. and Zizak, I. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 95 (2015)
    A fundamental knowledge and understanding of the adhesion behaviour of metal-polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50-200 nm Cu with a 10 nm Cr adhesion layer on 50 μm thick polyimide. If the Cu film thickness is decreased to 50 nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy. © 2015 Taylor & Francis.
    view abstract10.1080/14786435.2014.920543
  • Can microscale fracture tests provide reliable fracture toughness values? A case study in silicon
    Jaya, B.N. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 30 (2015)
    Fracture toughness testing of materials at the micrometer scale has become essential due to the continuing miniaturization of devices accompanied by findings of size effects in fracture behavior. Many techniques have emerged in the recent past to carry out fracture toughness measurements at the relevant micro and nanolength scales, but they lack ASTM standards that are prescribed for bulk scale tests. Also, differences in reported values arise at the microscale due to the sample preparation technique, test method, geometry, and investigator. To correct for such discrepancies, we chose four different fracture toughness test geometries in practice, all of them micromachined in the focused ion beam (FIB), to investigate the fracture toughness of Si(100) at the micrometer scale. The average KIC that emerges from all four cases is a constant (0.8 MPa m1/2). The advantages and limitations of each of these geometries in terms of test parameters and the range of materials that can be tested are discussed. © Materials Research Society 2015.
    view abstract10.1557/jmr.2015.2
  • Comparing small scale plasticity of copper-chromium nanolayered and alloyed thin films at elevated temperatures
    Raghavan, R. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Phillipi, B. and Wehrs, J. and Wheeler, J.M. and Michler, J. and Dehm, G.
    Acta Materialia 93 (2015)
    Abstract The yield strengths and deformation mechanisms of Cu-Cr nanolayered and alloyed thin films were studied by microcompression testing at elevated temperatures. The mechanical response of the films with alternating layers of Cu and Cr with sub-100 nm interlayer thicknesses and alloyed films of the same average composition was compared to determine the role of the interfaces on deformation. Higher resistance to plastic flow at elevated temperatures was exhibited by the nanolayered films with smaller interlayer thickness among the layered films, while the alloyed film revealed an anomalous increase in strength with temperature exhibiting a deformation mechanism similar to the pure Cr film. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.04.008
  • Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction
    Wimmer, A. and Heinz, W. and Detzel, T. and Robl, W. and Nellessen, M. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 83 (2015)
    Polycrystalline Cu samples 20 × 20 μm2 in size were cyclically bent inside a scanning electron microscope until fracture occurred. The microstructural changes were investigated by secondary electron imaging and electron backscatter diffraction. The in situ experiments revealed that, for the coarse-grained samples, it is not the external stress that dominates the cyclic deformation, but the local internal strength. This is in strong contrast to macroscopic bending samples, where deformation always happens near the fixed end of the bending beam and decreases constantly with increasing distance from the fixation. For micron-sized polycrystalline samples, the grain dimensions, dislocation density evolution and grain orientation (Taylor factor) can define the location of failure if the grain size and sample diameter become similar in size. A comparison with cyclic in situ tension-tension experiments (ratio of minimum stress to maximum stress R ≈ 0) reveals that cyclic bending experiments (R ≈ -1) undergo bulk-like fatigue deformation with extrusions/intrusions, in contrast to the experiments with R ≈ 0. Both the cyclic tension-tension and bending experiments can be described by a Basquin equation, although different mechanisms lead to failure of the samples. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.10.012
  • Downscaling metal-dielectric interface fracture experiments to sub-micron dimensions: A feasibility study using TEM
    Völker, B. and Heinz, W. and Roth, R. and Batke, J.M. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 270 (2015)
    In this study a bending beam approach is miniaturized to the sub-micrometer size regime for in-situ transmission electron microscopy (TEM) investigations. The purpose of this study was to evaluate the feasibility of this method for determining interface fracture energies for metal-dielectric systems, which are comparable to other methods. The interfaces under investigation are Cu/borophosphosilicate glass (BPSG) and W(20. at.% Ti)/BPSG as examples for a weak and a strong interface, respectively. It was possible to fracture the Cu/BPSG interface during in-situ TEM experiments and to obtain estimates for the interface fracture energy comparable to literature values. However, the TEM approach imposes challenges which can complicate the determination of an interface energy release rate; these influences are discussed. The miniaturized TEM approach failed for W(20. at.% Ti)/BPSG because no interface fracture occurred in this case. Thus it is concluded, that the experimental setup of the in-situ TEM bending beam method utilized in this study is useful for weak interfaces but for strongly adhering interfaces it is inappropriate. © 2015 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2015.03.027
  • Following crack path selection in multifilm structures with weak and strong interfaces by in situ 4-point-bending
    Völker, B. and Venkatesan, S. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.-M. and Cordill, M.J. and Dehm, G.
    Journal of Materials Research 30 (2015)
    In this study, the interfacial adhesion of Cu and TiN on an annealed borophosphosilicate glass (BPSG) in a multilayer material stack was investigated. The two material systems, Cu/BPSG and TiN/BPSG, are representatives for weak and strong interfaces, respectively. A weak and a strong interface was chosen to identify possible differences in the fracture path selection for the multilayer material systems. To investigate this, in situ 4-point-bending experiments were performed under an optical microscope and in a scanning electron microscope. Complementary ex situ 4-point-bending experiments were carried out on the identical material systems. These tests revealed that for the two analyzed systems there is a large discrepancy in the success rate of failure along the interface of interest, which is a prerequisite for determining the corresponding interface energy release rate. This phenomenon can be understood by using theoretical findings of earlier studies reported in the literature, which are in agreement with the experimental outcome of the in situ 4-point-bending measurements presented here. © 2015 Materials Research Society.
    view abstract10.1557/jmr.2015.88
  • Formation of dislocation networks in a coherent Cu Σ3(1 1 1) twin boundary
    Jeon, J.B. and Dehm, G.
    Scripta Materialia 102 (2015)
    Molecular dynamics simulations were performed to investigate dislocation network formations in a coherent twin boundary in Cu. Depending on the activated glide system, the initial flawless twin boundary can be heavily or sparsely decorated by a dislocation network. The dislocation mechanism leading to a heavy dislocation network at the twin boundary and its consequence on mechanical properties will be discussed. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2015.02.016
  • Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
    Kapp, M.W. and Kirchlechner, C. and Pippan, R. and Dehm, G.
    Journal of Materials Research 30 (2015)
    Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.
    view abstract10.1557/jmr.2015.49
  • In Situ TEM Microcompression of Single and Bicrystalline Samples: Insights and Limitations
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    JOM 67 (2015)
    In situ micromechanical compression experiments in a transmission electron microscope enable the study and analysis of small-scale deformation behavior. The implementation of instrumented indenter systems allows measuring the force and displacement, providing additionally insights on sample strength and flow behavior. Using focused ion beam sample preparation, single- and bicrystalline specimens can be fabricated to study the influence of individual grain boundaries on the mechanical behavior. Taperless single crystalline and bicrystalline Cu compression pillars including a coherent twin boundary were deformed in scanning and conventional transmission electron microscopy mode to study the applicability of both techniques for examining dislocation dynamics and interaction with the boundary. Based on experimental results, possibilities and limitations of such experiments are critically discussed, including sample preparation, in situ annealing to remove ion beam-induced defects, imaging of dislocations, and acquisition of stress–strain data. Finally, an outlook is given on the potential of micromechanical in situ transmission electron microscopic experiments for analyzing the influence of grain boundaries on mechanical behavior. © 2015, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-015-1440-6
  • Influence of inclined twin boundaries on the deformation behavior of Cu micropillars
    Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 642 (2015)
    In situ micromechanical compression tests on Cu pillars were performed to evaluate the influence of twin boundaries on the mechanical behavior. The 1. μm sized Cu samples on a Si substrate prepared by focused ion beam milling were either single crystalline or contained 2-5 twin boundaries that were inclined to the compression direction. The strengths of the pillars vary, depending on the crystal orientation, associated twin boundary inclination and orientation of slip systems. Results show, that multiple slip systems are activated in each pillar. However, slip parallel to the twin boundaries prevails due to the long mean free path for dislocation movement. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.06.064
  • Influence of initial microstructure on thermomechanical fatigue behavior of Cu films on substrates
    Heinz, W. and Robl, W. and Dehm, G.
    Microelectronic Engineering 137 (2015)
    During a switch event in a power semiconductor device temperature changes of up to 300 K can occur in the Cu layer. Repeated switching operations causes cyclic thermal cycling which may finally lead to thermomechanical fatigue with severe microstructural changes. In this study, the influence of the starting microstructure and film thickness (600 nm and 5000 nm) on thermomechanical fatigue was investigated for epitaxial and polycrystalline Cu films for up to 1000 thermal cycles. Severe surface roughening and a texture change (crystal rotation) are detected during thermal cycling for the polycrystalline Cu films, while the epitaxial films maintain their microstructure. Controlling the initial microstructure of a Cu layer in a device exposed to cyclic thermomechanical straining is a route to delay surface damage. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2014.10.024
  • Interface fracture and chemistry of a tungsten-based metallization on borophosphosilicate glass
    Völker, B. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.M. and Schöberl, T. and Scheu, C. and Dehm, G.
    Philosophical Magazine 95 (2015)
    In microelectronic devices, the interface between barrier metal and dielectric is of particular interest for a reliable electronic functionality. However, it is frequently observed that this interface is prone to failure. In this work, the strength of interfaces between an as-deposited borophosphosilicate dielectric glass (BPSG) layer and a W(Ti) metallization with and without Ti interlayer was the centre of interest. Four-point-bending tests were used for the mechanical characterization combined with a topological and chemical analysis of the fracture surfaces. In addition, the interface chemistry was studied locally prior to the testing to search for a possible Ti enrichment at the interface. The fracture results will be discussed taking the chemical and topological information into account. © 2015 Taylor & Francis.
    view abstract10.1080/14786435.2014.913108
  • Internal and external stresses: In situ TEM compression of Cu bicrystals containing a twin boundary
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    Scripta Materialia 100 (2015)
    Uniaxial compression experiments on single- and twinned bicrystalline Cu samples using conventional and scanning in situ transmission electron microscopy reveal no increase in flow stress for the bicrystals. Dislocation curvature and dislocation source size analysis combined with indenter force measurements show agreement between local internal stresses acting on the dislocations and external stresses imposed by the indenter, indicating no stress concentrations due to the twin boundary. Furthermore, the dislocation density evolution shows stochastic variations but never a complete dislocation starvation. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2014.12.023
  • Mechanical and chemical investigation of the interface between tungsten-based metallizations and annealed borophosphosilicate glass
    Völker, B. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.M. and Schöberl, T. and Cordill, M.J. and Dehm, G.
    Thin Solid Films 583 (2015)
    The focus of this study was on the interface between W-based metallizations and an annealed borophosphosilicate glass (BPSG) dielectric. W-based metallizations are often used in semiconductor devices because of their favourable properties as a diffusion barrier. The interface was characterized mechanically and chemically. For the determination of the interface energy release rate the 4-point-bending method was used. The fracture surfaces resulting from the 4-point-bending experiments were examined to determine the failing interface and the topography of the fracture surfaces. Chemical characterizations of intact interfaces were performed using an electron dispersive X-ray approach in a scanning transmission electron microscope to provide information why Ti incorporated in a W-layer improves the adhesion on annealed BPSG significantly compared to a pureW-layer. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2015.03.047
  • Micro-tension study of miniaturized cu lines at variable temperatures
    Wimmer, A. and Heinz, W. and Leitner, A. and Detzel, T. and Robl, W. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 92 (2015)
    In this study, tension experiments on Cu micro-samples at temperatures between 143 and 873 K were performed in order to analyze the influence of grain size, temperature and strain rate on the mechanical properties and fracture mode. The activation energy and evolution of the dislocation density have been analyzed to identify the deformation mechanisms. A transition from bulk-like to stochastic, small-scale behavior has been found with increasing grain size. Furthermore, dependent on the grain size and temperature a change from dislocation based plasticity to diffusion controlled deformation was observed. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.03.056
  • Nanostructure and mechanical behavior of metastable Cu-Cr thin films grown by molecular beam epitaxy
    Harzer, T.P. and Djaziri, S. and Raghavan, R. and Dehm, G.
    Acta Materialia 83 (2015)
    Metastable binary Cu-Cr alloys with nominal film thicknesses of 300 nm and chemical compositions in the range of 4-93 at.% Cr were synthesized via co-evaporation using molecular beam epitaxy. All films were grown onto Si (1 0 0) substrates at deposition rates of ∼0.05 nm s-1. The film microstructures were studied by using transmission electron microscopy and X-ray diffraction. The mechanical properties were investigated by the nanoindentation technique. Depending on the chemical composition, three types of thin film microstructures are observed, which all exhibit tensile residual stresses. For a Cr concentration of 4 at.% a single-phase face-centered cubic (fcc) film was formed, whereas single-phase body-centered cubic (bcc) films were observed for Cr concentrations ≥ 33 at.%. In the case of a Cu85Cr15 film, a two-phase fcc and bcc structure is formed. An increase of the atomic fraction of Cr induces a significant refinement of grain sizes, from ∼100 nm for a Cu96Cr4 alloy to ∼30 nm in the case of a Cu20Cr80 alloy. Consequently, the hardness of the alloy films increased from 5.5 to 11.8 GPa. In addition, the formation of supersaturated solid solutions of Cr in single-phase fcc Cu films and Cu in single-phase bcc films was observed. Predictions of existing models of solid solution and grain boundary strengthening were evaluated and correlated with the mechanical properties of the films with respect to film compositions and grain sizes. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.10.013
  • Nanotribology in austenite: Normal force dependence
    Brinckmann, S. and Fink, C.A.C. and Dehm, G.
    Wear 338-339 (2015)
    The friction of materials has long been understood as the collective contact and interaction of microasperities. The rise of micro- and nanomechanical instruments allows the study of single microasperity contact to understand fundamentally friction and wear. This study investigates the deformation due to a single microasperity, which performs a single stroke scratch in austenite. We find that the elastic and plastic equations for static indentation also apply for the dynamic scratching. Additionally, the friction coefficient is found to be normal force dependent and we observe three domains: microstructure dominated friction, plastic plowing dominated wear and wear particle dominated tribology. © 2015 Elsevier Ltd.
    view abstract10.1016/j.wear.2015.04.023
  • Nanotribology in austenite: Plastic plowing and crack formation
    Brinckmann, S. and Dehm, G.
    Wear 338-339 (2015)
    Especially during the run-in phase of metal friction, plasticity develops in the contact zone. If well controlled, the plasticity will lead to an improved microstructure that exhibits low wear rates during machine operation. We investigate the plasticity due to a single stroke of a micrometer-sized asperity to understand fundamentally tribology induced plasticity and microstructure formation. We find that the local crystal orientation has a significant influence on the development and spread of plasticity. Additionally, the complex three-dimensional stress state results in the formation of non-obvious plastic slip patterns. Finally, we observe crack formation in the scratch track even during the single stroke experiments. © 2015 Elsevier Ltd.
    view abstract10.1016/j.wear.2015.05.001
  • The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model
    Marx, V.M. and Toth, F. and Wiesinger, A. and Berger, J. and Kirchlechner, C. and Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 89 (2015)
    Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50-200 nm thick), bonded to polyimide directly or via a 10 nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50 nm than in the 200 nm copper films. © Acta Materialia Inc. Published by Elsevier Ltd.
    view abstract10.1016/j.actamat.2015.01.047
  • The peculiarity of the metal-ceramic interface
    Zhang, Z. and Long, Y. and Cazottes, S. and Daniel, R. and Mitterer, C. and Dehm, G.
    Scientific Reports 5 (2015)
    Important properties of materials are strongly influenced or controlled by the presence of solid interfaces, i.e. from the atomic arrangement in a region which is a few atomic spacing wide. Using the quantitative analysis of atom column positions enabled by C<inf>S</inf>-corrected transmission electron microscopy and theoretical calculations, atom behaviors at and adjacent to the interface was carefully explored. A regular variation of Cu interplanar spacing at a representative metal-ceramic interface was experimentally revealed, i.e. Cu-MgO (001). We also found the periodic fluctuations of the Cu and Mg atomic positions triggered by the interfacial geometrical misfit dislocations, which are partially verified by theoretical calculations using empirical potential approach. Direct measurements of the bond length of Cu-O at the coherent regions of the interface showed close correspondence with theoretical results. By successively imaging of geometrical misfit dislocations at different crystallographic directions, the strain fields around the interfacial geometrical misfit dislocation are quantitatively demonstrated at a nearly three-dimensional view. A quantitative evaluation between the measured and calculated strain fields using simplified model around the geometrical misfit dislocation is shown.
    view abstract10.1038/srep11460
  • Transition from shear to stress-assisted diffusion of copper-chromium nanolayered thin films at elevated temperatures
    Raghavan, R. and Wheeler, J.M. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Thomas, K. and Philippi, B. and Kirchlechner, C. and Jaya, B.N. and Wehrs, J. and Michler, J. and Dehm, G.
    Acta Materialia 100 (2015)
    The mechanical behavior of Cu-Cr nanolayered films and an alloy film of nominal composition Cu<inf>20</inf>Cr<inf>80</inf> at.% was studied by microcompression testing at temperatures from 25 °C to 300 °C. Comparing nanolayered films, plastic deformation and failure occurred at consistently higher stress levels in the film with the smaller layer thicknesses. Plasticity in the nanolayered films always initiated in the softer Cu layers followed by a finite strain-hardening response in the stress-strain curves. Failure indicated by a strain-softening response following the higher peak strength due to shearing and tearing at columnar boundaries of Cr was observed in the nanolayered films at 25 °C and 100 °C. A transition from shearing and crack formation across the Cu-Cr interfaces leading to anomalous grain growth or beading of the nanocrystalline Cu layers was observed at elevated temperatures of 200 °C and 300 °C. On the other hand, the Cu<inf>20</inf>Cr<inf>80</inf> at.% alloy film exhibited failure by columnar buckling consistently at elevated temperatures, but shearing promoted by buckling at the highest strengths among the films at ambient temperature. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.08.016
  • Crack deflection in multi-layered four-point bending samples
    Brinckmann, S. and Völker, B. and Dehm, G.
    International Journal of Fracture 190 (2014)
    Four-point bending experiments are conceptually the method of choice when investigating the delamination strength of multi-layered components, which are of particular interest for semiconductor applications. However, experimental studies have shown that the crack continues as mode-I crack in most cases while delamination is rarely observed, thus making the four-point bending method useless. This study uses the finite element method with cohesive zones to study crack propagation and the likelihood of turning the initial mode-I crack into a delamination crack in a multi-layered structure. We close with a conclusion which can help to increase the delamination probability and thereby help to determine the delamination strengths of layered structures. © 2014, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10704-014-9981-1
  • Damage evolution during cyclic tension-tension loading of micron-sized Cu lines
    Wimmer, A. and Leitner, A. and Detzel, T. and Robl, W. and Heinz, W. and Pippan, R. and Dehm, G.
    Acta Materialia 67 (2014)
    In this study, the low-cycle fatigue properties (1-15,000 cycles) of electrodeposited Cu, which is frequently used as metallization in the semiconductor industry, is analyzed with respect to its microstructure. Freestanding Cu tensile samples 20 μm × 20 μm × 130 μm were fabricated by a lithographic process. The grain size of the samples was modified by using three different process conditions for electrochemical Cu deposition. All samples were subjected to cyclic tension-tension testing performed with a miniaturized stress-controlled stage in situ in a scanning electron microscope until failure occurred. The number of cycles sustained prior to failure depends on the accumulated creep strain and can be related to the failure strain in a tensile test. It will be shown that the microstructure influences the number of cycles to failure and the failure mode. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.12.006
  • Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary
    Imrich, P.J. and Kirchlechner, C. and Motz, C. and Dehm, G.
    Acta Materialia 73 (2014)
    Micrometer-sized compression pillars containing a grain boundary are investigated to better understand under which conditions grain boundaries have a strengthening effect. The compression experiments were performed on focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples. Pillars containing a LAGB show increased strength, stronger hardening and smaller load drops compared to single crystals and exhibit a bent boundary and pillar shape. Samples with a CTB show no major difference in stress-strain data compared to the corresponding single-crystalline samples. This is due to the special orientation and symmetry of the twin boundary and is reflected in a characteristic pillar shape after deformation. The experimental findings can be related to the dislocation-boundary interactions at the different grain boundaries and are compared with three-dimensional discrete dislocation dynamics simulations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.04.022
  • Growth mechanism of Al2Cu precipitates during in situ TEM heating of a HPT deformed Al-3wt.%Cu alloy
    Rashkova, B. and Faller, M. and Pippan, R. and Dehm, G.
    Journal of Alloys and Compounds 600 (2014)
    The microstructural evolution of Al2Cu precipitates in an ultrafine-grained Al-3wt.% Cu model alloy produced by high-pressure torsion (HPT) was studied by in situ transmission electron microscopy (TEM). The precipitation growth was systematically investigated by isothermal heating experiments in the temperature range of 120 C to 170 C. The experimental data is analysed with respect of the diffusion kinetics and activation energy to determine the most prominent diffusion path: lattice or grain boundary diffusion. The results imply that grain boundary diffusion is the relevant mechanism for Al2Cu growth in the HPT deformed material. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2014.02.090
  • Intermetallic phase selection during homogenization for AA6082 alloy
    Li, J.H. and Wimmer, A. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 94 (2014)
    The intermetallic phase selection during homogenization in a AA6082 alloy has been investigated. A short homogenization treatment at 580 C for 4 h and a subsequent slow cooling with a rate of about 8 C/min, rather than a conventional quick quenching into water, results in the formation of the rounded discrete Al15(FeMnCr)3Si2 phase, which greatly suppressed the formation of the plate-shaped β-Al5FeSi phase. Most of the Al15(FeMnCr)3Si2 particles were observed to be very fine (about 200 nm) and homogeneously distributed in the Al matrix. Fine Mg2Si particles were also observed to be located in the vicinity of Al15(FeMnCr)3Si2 particles. Additionally, the β series precipitates (most likely β″-Mg 2Si), large-scale U2-AlMgSi and B′ (type C) phases were also observed within the Al matrix. This investigation demonstrates that the size and distribution of the desired intermetallic phases can be influenced by suitable homogenization treatments. © 2014 Taylor and Francis.
    view abstract10.1080/14786435.2013.868943
  • Temperature dependent transition of intragranular plastic to intergranular brittle failure in electrodeposited Cu micro-tensile samples
    Wimmer, A. and Smolka, M. and Heinz, W. and Detzel, T. and Robl, W. and Motz, C. and Eyert, V. and Wimmer, E. and Jahnel, F. and Treichler, R. and Dehm, G.
    Materials Science and Engineering A 618 (2014)
    Smaller grain sizes are known to improve the strength and ductility of metals by the Hall-Petch effect. Consequently, metallic thin films and structures which must sustain mechanical loads in service are deposited under processing conditions that lead to a fine grain size. In this study, we reveal that at temperatures as low as 473. K the failure mode of 99.99. at% pure electro-deposited Cu can change from ductile intragranular to brittle intergranular fracture. The embrittlement is accompanied by a decrease in strength and elongation to fracture. Chemical analyses indicate that the embrittlement is caused by impurities detected at grain boundaries. In situ micromechanical experiments in the scanning electron microscope and atomistic simulations are performed to study the underlying mechanisms. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.09.029
  • An elevated temperature study of a Ti adhesion layer on polyimide
    Taylor, A.A. and Cordill, M.J. and Bowles, L. and Schalko, J. and Dehm, G.
    Thin Solid Films 531 (2013)
    Titanium layers are used to promote adhesion between polymer substrates for flexible electronics and the Cu or Au conducting lines. Good adhesion of conducting lines in flexible circuits is critical in improving circuit performance and increasingcircuit lifetime. Nominally 50 nm thick Ti films on polyimide (PI) are investigated by fragmentation testing under uniaxial tensile load in the as-deposited state, at 350 C, and after annealing. The cracking and buckling of the films show clear differences between the as-deposited and the thermally treated samples, cracks are much straighter and buckles are smaller following heat treatment. These changes are correlated to a drop in adhesion of the samples following heat treatment. Adhesion values are determined from the buckle dimensions using a total energy approach as described in the work of Cordill et al. (Acta Mater. 2010). Cross-sectional transmission electron microscopy of the Ti/PI interface found evidence of a ~ 5 nm thick interlayer between the largely columnar Ti and the amorphous PI. This interlayer is amorphous in the as-deposited state but nano-crystalline in those coatings tested at elevated temperature or annealed. It is put forward that this alteration of the interfacial structure causes the reduced adhesion. © 2013 Elsevier B.V.
    view abstract10.1016/j.tsf.2013.01.016
  • Diffusive and massive phase transformations in Ti-Al-Nb alloys-Modelling and experiments
    Gamsjäger, E. and Liu, Y. and Rester, M. and Puschnig, P. and Draxl, C. and Clemens, H. and Dehm, G. and Fischer, F.D.
    Intermetallics 38 (2013)
    The thermodynamic properties of the Ti-Al-Nb system are obtained from recently published thermodynamic assessments. Based on these data the phase boundaries of the (α-Ti + γ-TiAl) two phase region are calculated by utilizing the CALPHAD approach and are compared to those, obtained by ab-initio calculations. It is found that the ab-initio phase boundaries deviate significantly from those based on the CALPHAD fit to experimental data which can be rationalized by the lack of vibrational entropy contributions in the present approach. Consequently a thermodynamic description based on the CALPHAD approach is used to further investigate the kinetics of the massive α → γm phase transformation in the Ti-Al-Nb system by means of a recently developed thick-interface model. Simulation of the transformation kinetics results in a massive transformation in the single-phase region only. However, very thin mole fraction spikes are obtained due to comparatively high interface velocities. It is likely that these spikes cannot be fully developed in experiments meaning that diffusion processes are partly suppressed (quasi-diffusionless transformation). A massive transformation in the two-phase region would then be possible. The theoretical predictions are compared to experimental studies performed on a Ti-45Al-5Nb alloy (composition in atomic percent). The alloy is heat treated slightly above the α-transus temperature and subsequently oil quenched to room temperature to generate γm-α2 interfaces. Energy-dispersive X-ray spectroscopy measurements were performed across γm- α2 interfaces in a scanning transmission electron microscope to search for chemical spikes. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2013.03.001
  • Elastoplastic buckling as source of misinterpretation of micropillar tests
    Daum, B. and Dehm, G. and Clemens, H. and Rester, M. and Fischer, F.D. and Rammerstorfer, F.G.
    Acta Materialia 61 (2013)
    Microscopic compression tests (micropillar tests) are typically used to obtain stiffness and strength properties of materials at small length scales. In this work it is shown that structural effects, in particular instabilities, have implications on the resulting load-displacement diagram. Care has to be taken when the measured load-displacement path of a micropillar is interpreted as a stress-strain path of the material. Several structural effects are discussed by means of computational analysis. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.04.046
  • Insights into the atomic and electronic structure triggered by ordered nitrogen vacancies in CrN
    Zhang, Z. and Li, H. and Daniel, R. and Mitterer, C. and Dehm, G.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    We report on the atomic and electronic structure of ordered nitrogen vacancies in CrN by using spherical aberration-corrected high-resolution transmission electron microscopy, electron energy-loss spectra, and ab initio calculations. The ordered nitrogen vacancies are identified to be distributed on {111} atomic planes. The vacancy concentrations were evaluated by quantitative high-resolution transmission electron microscopy. A direct consequence of the ordered nitrogen vacancies is a lattice shrinking leading to a reduced lattice constant, displaying a distorted CrN. The experimental measured lattice constant is quantitatively compared to the ab initio calculations. A relationship between the lattice constant and nitrogen vacancy concentration is theoretically and experimentally established, and quantitatively compared. The presence of the ordered N vacancies further induces the electronic changes as reflected in a very small core-level shift as well as a shift of the volume plasmon energy. Moreover, the change of the ionicity in CrN with nitrogen vacancy concentration is revealed. A direct relation between the covalent-ionic level of the bonding and the nitrogen vacancy concentration is shown. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.014104
  • Microcompression and cyclic deformation behaviors of coaxial copper bicrystals with a single twin boundary
    Li, L.L. and An, X.H. and Imrich, P.J. and Zhang, P. and Zhang, Z.J. and Dehm, G. and Zhang, Z.F.
    Scripta Materialia 69 (2013)
    This study reveals the extraordinary effect of an individual twin boundary (TB) on the mechanical behaviors of Cu bicrystals with a TB parallel to the loading axis whose primary slip vectors are always parallel to the TB plane. With direct experimental evidence, it is shown that there is no strengthening effect of the TB on the bicrystal under microcompression, and the TB is intrinsically strong enough to resist fatigue cracking with the slip bands becoming the preferential cracking sites finally. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.04.004
  • Transmission electron microscopy characterization of CrN films on MgO(001)
    Harzer, T.P. and Daniel, R. and Mitterer, C. and Dehm, G. and Zhang, Z.L.
    Thin Solid Films 545 (2013)
    Two CrN(001) films with different thickness were grown on MgO(001) substrates using unbalanced d.c.magnetron sputtering.The morphology and interfacial structure of the films are characterized by using conventional transmission electron microscopy, weak-beam dark-field microscopy and spherical aberration (CS)-corrected high-resolution transmission electron microscopy.The microscopy studies revealed the well-known cube-on-cube orientation relationship.While an interface dislocation network with b→=1/2aCrN< 100&gt; edge dislocations was identified, only part of the lattice mismatch is relaxed.The misfit dislocation structure and growth defects are analyzed and discussed based on the weak-beam dark-field and high-resolution transmission electron microscopy results.© 2013 Elsevier B.V.All rights reserved.
    view abstract10.1016/j.tsf.2013.07.064
  • Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples
    Kiener, D. and Zhang, Z. and Šturm, S. and Cazottes, S. and Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Philosophical Magazine 92 (2012)
    The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength. © 2012 Taylor & Francis.
    view abstract10.1080/14786435.2012.685966
  • Expected and unexpected plastic behavior at the micron scale: An in situ μlaue tensile study
    Kirchlechner, C. and Imrich, P.J. and Grosinger, W. and Kapp, M.W. and Keckes, J. and Micha, J.S. and Ulrich, O. and Thomas, O. and Labat, S. and Motz, C. and Dehm, G.
    Acta Materialia 60 (2012)
    The study of mechanical properties in micron- and submicron-sized metal crystals raises fundamental questions about the influence of size on different aspects of plasticity. In situ characterization of the microstructure evolution during loading is necessary to understand the physics underlying crystal deformation. In situ μLaue diffraction is able to provide unique statistical information on the evolution of type and density of stored dislocations. Here we show macroscopically expected and unexpected plastic behavior at low strains, observed during in situ μLaue tensile tests on micron-sized, single slip oriented Cu samples. Regardless of the initial behavior, a steady state is reached which qualifies a technical yield criterion at the micron scale. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.10.058
  • In situ study of γ-TiAl lamellae formation in supersaturated α 2-Ti 3Al grains
    Cha, L. and Schmoelzer, T. and Zhang, Z. and Mayer, S. and Clemens, H. and Staron, P. and Dehm, G.
    Advanced Engineering Materials 14 (2012)
    In situ heating transmission electron microscopy (TEM) was used to investigate the initial stage of γ-TiAl lamellae formation in an intermetallic Ti-45Al-7.5Nb alloy (in at.%). The material was heat treated and quenched in a non-equilibrium state to consist mainly of supersaturated, ordered α 2-Ti 3Al grains. Subsequently, specimens were annealed inside a TEM up to 750 °C. The in situ TEM study revealed that ultra-fine γ-TiAl laths precipitate in the α 2-matrix at ≈730 °C which exhibit the classical Blackburn orientation relationship, i.e. (0001)α 2//(111)γ and [$112̄0] α 2//< 110]γ. The microstructural development observed in the in situ TEM experiment is compared to results from conventional ex situ TEM studies. In order to investigate the precipitation behavior of the γ-phase with a complementary method, in situ high energy X-ray diffraction experiments were performed which confirmed the finding that γ-laths start to precipitate at ≈730 °C from the supersaturated α 2- matrix. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201100272
  • In-Situ Electron Microscopy: Applications in Physics, Chemistry and Meterials Science
    Dehm, G. and Howe, J.M. and Zweck, J.
    In-Situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science (2012)
    Adopting a didactical approach from fundamentals to actual experiments and applications, this handbook and ready reference covers real-time observations using modern scanning electron microscopy and transmission electron microscopy, while also providing information on the required stages and samples. The text begins with introductory material and the basics, before describing advancements and applications in dynamic transmission electron microscopy and reflection electron microscopy. Subsequently, the techniques needed to determine growth processes, chemical reactions and oxidation, irradiation effects, mechanical, magnetic, and ferroelectric properties as well as cathodoluminiscence and electromigration are discussed. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/9783527652167
  • In-Situ TEM Straining Experiments: Recent Progress in Stages and Small-Scale Mechanics
    Dehm, G. and Legros, M. and Kiener, D.
    In-Situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science (2012)
    view abstract10.1002/9783527652167.ch10
  • Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys
    Li, J.H. and Zarif, M.Z. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 92 (2012)
    The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5 FeSi intermetallic was also observed, depending on subsequent controlled cooling. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/14786435.2012.687840
  • Investigation of reversible plasticity in a micron-sized, single crystalline copper bending beam by X-ray μlaue diffraction
    Kirchlechner, C. and Grosinger, W. and Kapp, M.W. and Imrich, P.J. and Micha, J.-S. and Ulrich, O. and Keckes, J. and Dehm, G. and Motz, C.
    Philosophical Magazine 92 (2012)
    The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ μLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ Laue experiments are useable. A critical size is evidenced below which, depending on Young's modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample. © 2012 Taylor & Francis.
    view abstract10.1080/14786435.2012.669067
  • Kinetics and driving forces of abnormal grain growth in thin Cu films
    Sonnweber-Ribic, P. and Gruber, P.A. and Dehm, G. and Strunk, H.P. and Arzt, E.
    Acta Materialia 60 (2012)
    The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus "overrun" the (1 1 1) grains in size. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.12.030
  • Nanomechanical testing in materials research and development III
    Dehm, G. and Pharr, G.M. and Michler, J.
    Philosophical Magazine 92 (2012)
    view abstract10.1080/14786435.2012.727584
  • Novel temperature dependent tensile test of freestanding copper thin film structures
    Smolka, M. and Motz, C. and Detzel, T. and Robl, W. and Griesser, T. and Wimmer, A. and Dehm, G.
    Review of Scientific Instruments 83 (2012)
    The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures. © 2012 American Institute of Physics.
    view abstract10.1063/1.4725529
  • On the limits of the interfacial yield model for fragmentation testing of brittle films on polymer substrates
    Taylor, A.A. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 92 (2012)
    Fragmentation testing is frequently used to probe film fracture strain and the interfacial properties of thin brittle films on compliant substrates. A model based upon complete yield of the film/substrate interface is frequently used to analyse data after cracking has saturated. Additionally, the film is either assumed to have a single-valued failure stress or a distribution of strengths described by Weibull statistics. Recent work by the authors showed that consideration of film thickness variations and the application of neighbour ratio analysis brought 96% of the data for an Al x O y /Cu film/substrate system into compliance with the predictions for a film with a single-valued failure stress. In the present work Cr/PI (polyimide) and Cr/PET (polyethylene teraphthalate) systems are analysed according to the same methodology. The Cr films on polymer substrates crack such that the neighbour ratios considerably exceed the predicted limit of 2. The influence of the relative thickness of the film and substrate and the strain rate of the test is investigated. A deviation from the idealised mechanical model due to the large difference in elastic moduli of film and substrate is put forward as a possible cause of the observed behaviour. The importance of these results to the application of the interfacial yield model is discussed. © 2012 Taylor & Francis.
    view abstract10.1080/14786435.2012.723145
  • Quantitative Approaches for in situ SEM and TEM Deformation Studies
    Dehm, G. and Kiener, D. and Motz, C. and Smolka, M. and Pippan, R.
    Microscopy and Microanalysis 18 (2012)
    Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012. © 2012, Microscopy Society of America. All rights reserved.
    view abstract10.1017/S1431927612005533
  • Robust mechanical performance of chromium-coated polyethylene terephthalate over a broad range of conditions
    Cordill, M.J. and Taylor, A.A. and Berger, J. and Schmidegg, K. and Dehm, G.
    Philosophical Magazine 92 (2012)
    Mechanical properties of metal films on polymer substrates are normally studied in terms of the fracture and adhesion of the film, while the properties of the polymer substrate and testing conditions are overlooked. Substrate orientation and thickness, as well as strain rate and temperature effects, are examined using Cr films deposited onto polyethylene terephthalate substrates. A faster strain rate affects only the initial fracture strain of the Cr film and not the crack and buckle spacings in the high strain condition. The substrate orientation slightly changes the average crack spacing while the substrate thickness has little effect on the cracking and buckling behaviour. Straining experiments at high temperature increased the average crack spacing and led to a change in buckling mode. The lack of sizeable changes in the mechanical behaviour over the large range of testing procedures leads to a resilient material system for flexible applications. © 2012 Taylor & Francis.
    view abstract10.1080/14786435.2012.700418
  • Study of the human tooth using a low–voltage CS-corrected TEM
    Zhang, Z.L. and Dehm, G.
    Microscopy and Microanalysis 18 (2012)
    view abstract10.1017/S1431927612003121
  • Yield stress influenced by the ratio of wire diameter to grain size - A competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires
    Yang, B. and Motz, C. and Rester, M. and Dehm, G.
    Philosophical Magazine 92 (2012)
    Polycrystalline copper wires with diameters of 25, 30 and 50m were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the smaller is softer phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface. © 2012 Taylor & Francis.
    view abstract10.1080/14786435.2012.693215
  • Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model
    Rester, M. and Fischer, F.D. and Kirchlechner, C. and Schmoelzer, T. and Clemens, H. and Dehm, G.
    Acta Materialia 59 (2011)
    Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α2-Ti3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.02.016
  • Dislocation plasticity of Al film on polyimide investigated by cross-sectional in situ transmission electron microscopy straining
    Oh, S.H. and Rentenberger, C. and Im, J. and Motz, C. and Kiener, D. and Karnthaler, H.-P. and Dehm, G.
    Scripta Materialia 65 (2011)
    Tensile straining of a cross-sectional Al/polyimide was performed in a transmission electron microscope. The tensile deformation of Al was accomplished mainly by dislocations emitted from the film surface, which glide and impinge on the Al/polyimide interface. During further straining the interfacial dislocations disappeared, indicating dislocation core spreading, whereas threading dislocations moved towards the film surface. While the Al/polyimide interface remains flat and becomes depleted of dislocations, the Al surface becomes increasingly rough accompanied by a noticeable increase in dislocation density. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2011.06.001
  • Dislocation storage in single slip-oriented Cu micro-tensile samples: New insights via X-ray microdiffraction
    Kirchlechner, C. and Kiener, D. and Motz, C. and Labat, S. and Vaxelaire, N. and Perroud, O. and Micha, J.-S. and Ulrich, O. and Thomas, O. and Dehm, G. and Keckes, J.
    Philosophical Magazine 91 (2011)
    Synchrotron X-ray microdiffraction was used to characterize the deformation structure of single crystalline Cu micro-tensile specimens which were oriented for single slip. The 3-m thick samples were strained in situ in a scanning electron microscope (SEM). Electron microscopy observations revealed glide steps at the surface indicating single slip. While the slip steps at the surface must have formed by the predominant activation of the primary glide system, analysis of Laue peak streaking directions revealed that, even at low strains, dislocations had been activated and stored on an unpredicted slip system. Furthermore, the Laue scans showed that multiple slip takes over at a later state of deformation. © 2011 Taylor & Francis.
    view abstract10.1080/14786431003785639
  • Electrical properties and structure of grain boundaries in n-conducting BaTiO3 ceramics
    Hou, J. and Zhang, Z. and Preis, W. and Sitte, W. and Dehm, G.
    Journal of the European Ceramic Society 31 (2011)
    The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO3 ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed. © 2010 Elsevier Ltd.
    view abstract10.1016/j.jeurceramsoc.2010.11.016
  • Grain resolved orientation changes and texture evolution in a thermally strained Al film on Si substrate
    Heinz, W. and Dehm, G.
    Surface and Coatings Technology 206 (2011)
    Temperature changes induce thermal stresses in thin films on substrates due to differences in the thermal expansion coefficients. Repeated thermal cycling may finally lead to severe surface roughening and a change in film texture. In this study we investigate the orientation changes for a 600. nm thick Al film during subsequent thermal cycles between 25 °C and 450 °C by analyzing individual grains. The results reveal orientation changes by up to 3° after one thermal cycles and unexpected large orientation gradients within individual grains. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.07.046
  • Impact of instrumental constraints and imperfections on the dislocation structure in micron-sized Cu compression pillars
    Kirchlechner, C. and Keckes, J. and Motz, C. and Grosinger, W. and Kapp, M.W. and Micha, J.S. and Ulrich, O. and Dehm, G.
    Acta Materialia 59 (2011)
    In situ micro-Laue diffraction was used to study the plasticity in three 7 μm, initially identical, single-crystalline Cu pillars during compression. Movements of the Laue spot as well as Laue spot streaking were analyzed to obtain real-time insights into the storage of excess dislocations and the possible formation of dislocation cell structures. The results reveal that instrumental constraints lead to dislocation storage at the sample base and top, but will not affect the storage of excess dislocations in the sample center in case of an ideal alignment. In contrast, misaligned samples show early yielding due to the activation of an unpredicted slip system, storage of excess dislocations also in the sample center and, at a later stage, the formation of a complex dislocation substructure. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.05.037
  • In situ μlaue: Instrumental setup for the deformation of micron sized samples
    Kirchlechner, C. and Keckes, J. and Micha, J.-S. and Dehm, G.
    Advanced Engineering Materials 13 (2011)
    μLaue diffraction sheds light onto the deformation behavior of miniaturized samples. Here we present a new instrumental setup for the in situ deformation of micron sized specimens at BM32 of the ESRF synchrotron source. Furthermore, a compression test of a 7 μm sized single slip oriented copper pillar is presented, showing the activation of an unpredicted slip system due to misalignment and the formation of several sub-grains. The results of the compressed pillar as well as possibilities and crucial points for measuring and data evaluation are discussed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000286
  • In-situ TEM heating study of the γ lamellae formation inside the α2 matrix of a Ti-45Al-7.5Nb alloy
    Cha, L. and Clemens, H. and Dehm, G. and Zhang, Z.
    Advanced Materials Research 146-147 (2011)
    In-situ heating transmission electron microscopy (TEM) was employed to investigate the initial stage of lamellae formation in a high Nb containing γ-TiAl based alloy. A Ti-45Al-7.5Nb alloy (at %), which was heat treated and quenched in a non-equilibrium state such that the matrix consists of ordered α2 grains, was annealed inside a TEM up to 750 °C. The in-situ TEM study reveals that γ laths precipitate in the α2 matrix at ∼750 °C possessing the classical Blackburn orientation relationship, i.e. (0001)α2 // (111)γ and [11-20]α2 // < 110]γ. The microstructure of the in-situ TEM experiment is compared to results from ex-situ heating and subsequent TEM studies.
    view abstract10.4028/
  • Interface failure and adhesion measured by focused ion beam cutting of metal-polymer interfaces
    Cordill, M.J. and Schmidegg, K. and Dehm, G.
    Philosophical Magazine Letters 91 (2011)
    New developments in flexible electronics require metal films to adhere to polymer substrates. Measuring the interfacial adhesion of these systems is challenging, requiring the formulation of new techniques and models. A strategy to measure the adhesion of Cr-polyethylene terephthalate (PET) interfaces using tensile straining and buckle formation is presented in this article. Focused ion beam cross-sectioning of the buckles reveals that the polymer substrate can locally fail, which may lead to an overestimate of adhesion. Cr-PET adhesion energy of 9.4±1.6 J/m2 is determined with the present approach. © 2011 Taylor &Francis.
    view abstract10.1080/09500839.2011.593575
  • Microstructure and adhesion of as-deposited and annealed Cu/Ti films on polyimide
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    International Journal of Materials Research 102 (2011)
    The ability to measure the adhesion energy of metal thin films on polymer substrates is important for the design of reliable flexible electronic devices. One technique is to create well-defined areas of delamination (buckles) as a consequence of lateral compressive stresses induced by tensile straining of the film-substrate system. The adhesion energy is calculated from the buckle dimensions. In order to improve the adhesion between the metal film and polymer substrate, thin adhesion layers can be incorporated. However, interdiffusion and reactions can occur between the adhesion layer and the metal film when subjected to elevated temperatures. This is detrimental for the interfacial adhesion, as will be discussed for Cu films on polyimide with a Ti interlayer subjected to annealing at 350°C. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110513
  • Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature
    Cha, L. and Clemens, H. and Dehm, G.
    International Journal of Materials Research 102 (2011)
    Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110526
  • Physical metallurgy and properties of β-solidifying TiAl based alloys
    Clemens, H. and Schmoelzer, T. and Schloffer, M. and Schwaighofer, E. and Mayer, S. and Dehm, G.
    Materials Research Society Symposium Proceedings 1295 (2011)
    In this paper, the physical metallurgy and properties of a novel family of high-strength γ-TiAl-based alloys is reviewed succinctly. These so-called TNM™ alloys contain Nb and Mo additions in the range of 3-7 atomic percent as well as small additions of B and C. For the definition of the alloy composition thermodynamic calculations using the CALPHAD method were conducted. The predicted phase transformation and ordering temperatures were verified by differential scanning calorimetry and in situ high-energy X-ray diffraction. TNM alloys solidify via the β-phase and exhibit an adjustable β-phase volume fraction at temperatures, where hot-working processes are performed. Due to the high volume fraction of β-phase these alloys can be processed isothermally as well as under near conventional conditions. In order to study the occurring deformation and recrystallization processes during hot-working, in situ diffraction experiments were conducted during compression tests at elevated temperatures. With subsequent heat-treatments a significant reduction of the β-phase is achieved. These outstanding features of TNM alloys distinguish them from other TiAl alloys which must exclusively be processed under isothermal conditions and/or which always exhibit a high fraction of β-phase at service temperature. After hot-working and multi-step heat-treatments, these alloys show yield strength levels > 800 MPa at room temperature and also good creep resistance at elevated temperatures. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.29
  • The effect of film thickness variations in periodic cracking: Analysis and experiments
    Taylor, A.A. and Edlmayr, V. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 206 (2011)
    Periodic cracking experiments are frequently used in the assessment of interface quality in brittle film/compliant substrate systems. Through these techniques it is possible to extract a quantitative measure of interface shear strength and therefore assess the mechanical suitability of the films for application. The influence of film thickness inhomogeneities on the crack spacing is assessed in this study. While film thickness inhomogeneities are always present in thin film systems, only nominal thickness values have been considered up to this point. By defining two separate regimes of film thickness variation, roughness and unevenness, in relation to the crack spacing, the influence of such variation on the data is analysed. The results of this analysis are then considered in reference to a model system of an amorphous alumina film on a copper substrate (AlxOy/Cu), the limits of this analysis are then discussed. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.07.047
  • The effect of temperature and strain rate on the periodic cracking of amorphous AlxOy films on Cu
    Taylor, A.A. and Edlmayr, V. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 206 (2011)
    The high temperature properties of metal/ceramic interfaces play an important role in the operation of microprocessors and coated products. Determination of the interface properties over a range of testing conditions is critical in understanding and improving the performance of such systems. Periodic cracking of ceramic films on metal substrates provides a direct measure of the interface strength. A model AlxOy/Cu system is investigated over temperatures of 25-650°C and at strain rates of 3.5{dot operator}10-2s-1 and 1.7{dot operator}10-5s-1. For this system it is found that temperature does not significantly affect the spacing of film cracks in the steady state, at a given strain rate. However, the high strain rate tests broaden the measured crack spacing distributions compared to the low strain rate tests. This broadening causes an increase in the average crack spacing at high strain rate. The minimum crack spacing was increased 15-20% by both increasing strain rate and increasing temperature. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.08.011
  • Work hardening in micropillar compression: In situ experiments and modeling
    Kiener, D. and Guruprasad, P.J. and Keralavarma, S.M. and Dehm, G. and Benzerga, A.A.
    Acta Materialia 59 (2011)
    Experimental measurements and simulation results for the evolution of plastic deformation and hardening in micropillars are compared. The stress-strain response of high-symmetry Cu single crystals is experimentally determined using in situ micropillar compression. Discrete dislocation simulations are conducted within a two-dimensional plane-strain framework with the dislocations modeled as line singularities in an isotropic elastic medium. Physics-based constitutive rules are employed for an adequate representation of hardening. The numerical parameters entering the simulations are directly identified from a subset of experimental data. The experimental measurements and simulation results for the flow stress at various strain levels and the hardening rates are in good quantitative agreement. Both flow strength and hardening rate are size-dependent and increase with decreasing pillar size. The size effect in hardening is mainly caused by the build-up of geometrically necessary dislocations. Their evolution is observed to be size-dependent and more localized for smaller sample volumes, which is also reflected in local crystal misorientations. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.03.003
  • Adhesion energies of Cr thin films on polyimide determined from buckling: Experiment and model
    Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 58 (2010)
    For the realization of flexible electronic devices, the metal-polymer interfaces upon which they are based need to be optimized. These interfaces are prone to fracture in such systems and hence form a weak point. In order to quantify the interfacial adhesion, novel mechanical tests and modeling approaches are required. In this study, a tensile testing approach that induces buckling of films by lateral contraction of the substrate is employed to cause delamination of the film. Based on a newly developed energy balance model, the adhesion energy of Cr films on polyimide substrates is determined by measuring the buckle geometry induced by the tensile test. The obtained minimum values for the adhesion energy (about 4.5 J m-2) of 50-190 nm thick films compare well to those found in the literature for metal films on polymer substrates. © 2010 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstract10.1016/j.actamat.2010.06.032
  • Can local hot spots induce α2/γ lamellae during incomplete massive transformation of γ-TiAl alloys?
    Fischer, F.D. and Cha, L. and Dehm, G. and Clemens, H.
    Intermetallics 18 (2010)
    Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local "hot spots" during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α2-Ti3Al grains and a small volume fraction of massively transformed γm-TiAl particles. The majority of the γm-particles show a featureless α2/γm interface, whereas in the immediate vicinity of some γm-particles lamellar α2/γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α2-parent grain is in the range of 1-10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γm transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1-10 μm. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2010.01.017
  • Cyclic loading behavior of micro-sized polycrystalline copper wires
    Yang, B. and Motz, C. and Grosinger, W. and Dehm, G.
    Procedia Engineering 2 (2010)
    Micro-sized polycrystalline copper wires of diameters ranging from 5 μm to 36 μm were cyclically loaded under stress-control in many steps with increasing the applied stress amplitude after a saturation state in plastic strain is reached. It is observed that the thicker wires show smaller plastic strain at saturation as well as smaller creep strain compared to the thinner wires. The results were discussed combined with the monotonic tensile results of these micro-sized wires. © 2010 Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2010.03.100
  • Disparate tendency of stress evolution of thin and thick electroplated cu films at room temperature
    Huang, R. and Robl, W. and Dehm, G. and Ceric, H. and Detzel, T.
    Proceedings of the International Symposium on the Physical and Failure Analysis of Integrated Circuits, IPFA (2010)
    The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress for different film thicknesses between 1.5μm and 20μm. This recrystallization process results in significant microstructure change and a film-thickness-dependent stress evolution at room temperature. This behavior can be explained by the superposition of grain growth and grain size dependent yielding. Existing models have been used and improved to describe the mechanisms related to stress evolution. © 2010 IEEE.
    view abstract10.1109/IPFA.2010.5532222
  • Fracture and delamination of chromium thin films on polymer substrates
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 (2010)
    New emerging technologies in the field of flexible electronic devices require that metal films adhere well and flex with polymer substrates. Common thin film materials used for these applications include copper (Cu) with an adhesion interlayer of chromium (Cr). Copper can be quite ductile and easily move with the polymer substrate. However, Cr is more brittle and fractures at lower strains than Cu. This study aims to examine the fracture and subsequent buckling and delamination of strained Cr films on polyimide (PI). In-situ scanning electron microscope (SEM) straining is used to systematically study the influence of film thickness on fracture and buckling strains. Film fracture and delamination depend on film thickness, and increases in crack and buckle density with decreasing thickness are explored by a shear lag model. © 2009 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-009-9988-9
  • In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals
    Rajagopalan, J. and Rentenberger, C. and Peter Karnthaler, H. and Dehm, G. and Saif, M.T.A.
    Acta Materialia 58 (2010)
    In situ transmission electron microscopy straining experiments with concurrent macroscopic stress-strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size dm = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (dm = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (dm = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals. © 2010 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2010.05.013
  • Investigation of the fatigue behavior of Al thin films with different microstructure
    Heinz, W. and Pippan, R. and Dehm, G.
    Materials Science and Engineering A 527 (2010)
    Cyclic compressive and tensile stresses occur in metallic films and interconnects applied in sensors and microelectronic devices when exposed to temperature changes. The stresses are induced by differences in the thermal expansion coefficients of the adjacent materials. Repeated cycling leads to damage evolution and, eventually, to failure. In this study we report on a successful strategy how to avoid thermal stress induced fatigue damage. We analysed the deformation structures of 0.2-2μm thick Al films subjected to thermal cycling between 100°C and 450°C up to 10,000 times. The investigations reveal that a reduction in film thickness or controlling the Al texture and the Al/substrate interface structure can be used to prevent thermo-mechanical fatigue damage. The findings are explained by orientation dependent plasticity and differences in dislocation mechanisms for different interface structures, and less accumulated plastic strain for thinner films. The approach is expected to apply in general for metallic films on substrates. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.08.046
  • Micron-sized fracture experiments on amorphous SiOx films and SiOx/SiNx multi-layers
    Matoy, K. and Schönherr, H. and Detzel, T. and Dehm, G.
    Thin Solid Films 518 (2010)
    In this study miniaturized monolithic cantilevers of thermally grown silicon oxide and multi-layer cantilevers of plasma enhanced chemical vapor deposited silicon oxide and nitride were mechanically characterized. In order to determine the fracture stress as well as the fracture toughness, un-notched and focused ion beam pre-notched cantilevers were tested. While the thickness of the monolithic cantilevers was varied from 280 nm to 2380 nm, the individual sub-layer thickness of the multi-layer cantilevers was adjusted to 50 nm. Bending experiments reveal a small increase of the fracture stresses with decreasing cantilever thicknesses. For the multi-layer stacks the tensile stress at fracture slightly exceeds the strength values of the corresponding monolithic materials. Furthermore, it is demonstrated that the specimens pre-notched by focused ion beam do not show significant changes in fracture toughness with varying pre-notch size. This makes the applied test a reproducible technique to determine fracture toughness of brittle films. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2010.05.114
  • Microplasticity phenomena in thermomechanically strained nickel thin films
    Taylor, A.A. and Oh, S.H. and Dehm, G.
    Journal of Materials Science 45 (2010)
    Magnetron sputtered Ni thin films on both oxidised Si (100) and α-Al2O3 (0001) substrates of thickness 150-1000 nm were tested thermomechanically with a wafer curvature system, as well as in situ in a transmission electron microscope. The films on oxidised Si have a {111}-textured columnar microstructure with a mean grain size similar to the film thickness. On (0001) α-Al2O3 a near single crystal epitaxy with two growth variants is achieved leading to a significantly larger grain size. The thermomechanical testing was analysed in terms of the room temperature/high temperature flow stresses in the films and the observed thermoelastic slopes. It was found that the room temperature flow stresses increased with decreasing film thickness until a plateau of ∼1100 MPa was reached for films thinner than 400 nm. This plateau is attributed to the present experiments exerting insufficient thermal strain to induce yielding in these thinner films. At 500 °C the compressive flow stresses of the films show a competition between dislocation and diffusion mediated plasticity. A size effect is also observed in the thermoelastic slopes of the films, with thinner films coming closer to the slope predicted by mismatch in thermal expansion coefficients. It is put forward here that this is due to a highly inhomogeneous stress distribution in the films arising from the grain size distribution. © 2010 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-010-4445-0
  • Stress, sheet resistance, and microstructure evolution of electroplated cu films during self-annealing
    Huang, R. and Robl, W. and Ceric, H. and Detzel, T. and Dehm, G.
    IEEE Transactions on Device and Materials Reliability 10 (2010)
    Electroplated copper films are known to change their microstructure due to the self-annealing effect. The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress and sheet resistance for different layer thicknesses between 1.5 and 20 μm. While the sheet resistance was found to decrease as time elapsed, a size-dependent change in film stress was observed. Films with the thickness of 5 μm; and below decrease in stress, while thicker films initially reveal an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain-size-dependent yielding. © 2010 IEEE.
    view abstract10.1109/TDMR.2009.2032768
  • Structural characterization of a Cu/MgO(001) interface using C S-corrected HRTEM
    Cazottes, S. and Zhang, Z.L. and Daniel, R. and Chawla, J.S. and Gall, D. and Dehm, G.
    Thin Solid Films 519 (2010)
    Epitaxial Cu(001) layers were deposited on MgO(001) substrates by magnetron sputtering and the atomic structure of the Cu-MgO interface was characterized by spherical aberration (CS)-corrected high-resolution transmission electron microscopy (HRTEM). The interface structure and the misfit dislocation network were determined by imaging in both the < 100&gt; and < 110&gt; directions. The dislocation network was found to lie along the < 100&gt; directions with a Burgers vector of 1/2 aCu < 100&gt; deduced from HRTEM images and geometrical phase analysis. The dislocations do not fully accommodate the lattice mismatch, yielding residual stress at the interface and an elongation of the Cu lattice along the [001] direction. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2010.09.017
  • Study of nanometer-scaled lamellar microstructure in a Ti-45Al-7.5Nb alloy - Experiments and modeling
    Fischer, F.D. and Waitz, T. and Scheu, Ch. and Cha, L. and Dehm, G. and Antretter, T. and Clemens, H.
    Intermetallics 18 (2010)
    Quenching of Ti-45 at%Al-7.5 at%Nb from the single α-phase region to room temperature followed by aging below the eutectoid temperature leads to the precipitation of ultra-fine γ-TiAl lamellae. In addition to an extensive experimental program, reported by Cha et al. in Intermetallics 16 (2008) 868-875, in this work a micromechanical and thermodynamical model is presented for the formation of γ-TiAl lamellae within the α2-Ti3Al parent phase. A global transformation condition allows to predict a thickness to length ratio in accordance with experimental observations. Furthermore, a local transformation condition offers the basis for a kinetic law. The modeling concept can be applied to similar problems of combined diffusive and displacive phase transformations. © 2009 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2009.09.012
  • Unveiling the atomic and electronic structure of the VN/MgO interface
    Zhang, Z. and Rashkova, B. and Dehm, G. and Lazar, P. and Redinger, J. and Podloucky, R.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    We report a quantitative comparison of the interface structure of VN/MgO(001) using ab initio density-functional theory (DFT), aberration-corrected high-resolution transmission electron microscopy (HRTEM), and electron energy-loss spectroscopy (EELS). By HRTEM, we show an atomic resolution structure of epitaxially grown VN film on MgO with a clearly resolved oxygen and nitrogen sublattice across the interface. As revealed by DFT, the (002) interplanar spacing oscillates in the first several VN layers across the interface. Interfacial chemistry determined by EELS analysis shows the preponderance of O and V atom at the interface, resulting in a small detectable core-level shift. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.060103
  • defects

  • in-situ electron microscopy

  • interfaces

  • mechanical properties

  • micromechanics

  • thin films

  • transmission electron microscopy

  • x-ray diffraction

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