Low-cycle fatigue study of oligocrystalline austenitic materials

Bojan Mitevski, University of Duisburg-Essen, Duisburg, Germany
Kelvin Harryadi, University of Duisburg-Essen, Duisburg, Germany
Alfons Fischer, University of Duisburg-Essen, Duisburg, Germany
Sabine Weiß, BTU Cottbus-Senftenberg, Cottbus, Germany

The aim of this study is to present the microstructural behaviour analysis of structures consisting of few crystals or oligocrystalline structure in low cycle fatigue (LCF) experiments. Oligocrystalline structures are often found in micro-scale components such as cardiovascular stents or microelectromechanical systems (MEMS).

Research on micro-scale components material behaviour has been mainly driven by the unique properties of oligocrystalline structures, which differ from either mono- or polycrystalline structures. There are several studies in which lower or higher average (tensile) strength of wires compared to larger bulk specimens have been observed depending on the material and the number of crystals in the specimens’ cross-section. The total ductility of a material determined with oligocrystalline specimens is lower than the one of its larger bulk samples.

This study presents the fatigue analysis of austenitic stainless steel X2CrNiMo18-15-3 (1.4441; AISI 316L). This steel is one of the preferable materials used for implants in the biomedical field due to its excellent corrosion resistance and its tendency to undergo work hardening, which implies a good strength to ductility ratio, on top of its relatively economic processing cost and its general availability. Alongside the 316L, a CoCrFe-alloy with 56% Co, 32% Cr and 0,1% C) is used for comparison.

The experiments are conducted as total stress controlled low cycle fatigue tests inside a scanning-electron-microscope (SEM) for the first 1000 cycles and were extended outside by means of a special servohydraulic test rig. The tests are done up to 100.000 cycles and stopped for SEM-analyzes at each order of magnitude of cycles. The analysis is done primarily by electron backscatter diffraction (EBSD), including standard crystallographic analyses such as Inverse Pole Figure (IPF) mapping, Kernel Average Misorientation mapping, and Schmid-Factor mapping. The results show a high dependency of the local crystallographic orientation on the local deformation.

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