Mechanical behavior and microstructural evolution of high-interstitial austenitic steels under high-pressure-torsion-fatigue

Stefanie Hanke, University Duisburg-Essen, Duisburg, Germany
Paul Breininger, University Duisburg-Essen, ,
Alfons Fischer, University Duisburg-Essen, ,

Techniques of Severe Plastic Deformation (SPD) employ the fact that in the presence of high compressive stresses crack formation is delayed or even prohibited. Therefore, material volumes may be plastically deformed to very high strains introducing high amounts of lattice defects, which leads to dynamic recrystallization and grain refinement. Under SPD typically monotonous loads are applied to relatively small material volumes. In the present study, a cyclic test (High-Pressure-Torsion-Fatigue, HPTF), developed at University Duisburg-Essen, is employed. Plastic strains are accumulated in the microstructure by cyclic torsion under the superposition of a constant axial pressure in unconstrained, cylindrical samples. The tested materials are high interstitial stainless austenites, which provide a favorable combination of high strength and ductility. These alloys undergo planar slip under uniaxial cyclic loading, accommodating the introduced strain by dislocation sliding localized on discrete sliding planes. In such standard tests, failure occurs by growth of multiple cracks initiating on persistent slip bands. The behavior of these steels under more complex multiaxial loading conditions, in particular the combination of cyclic torsion and monotonous compression, and the differences in the mechanisms of strain accommodation in the microstructure are investigated in this work. The steels X13CrMnMoN18-14-3 (1.4425) and CrMnCN0.85 are subjected to cyclic torsional loading (0.5° to 5.0°) and axial loads of 250 or 450 MPa. The mechanical material response is evaluated and the microstructural evolution is investigated using Electron-Backscatter-Diffraction.

Crack initiation limits the test duration, and dynamic recrystallization is not achieved. Nevertheless, the torsional cycling significantly enhances axial creep of the samples, reaching axial strains up to 20%, although pressures are below the yield strength. Thereby large plastic strains are accumulated throughout the samples’ cross sections. These first results imply that by adjusting the loading conditions and delaying crack initiation still higher strains, and possibly eventually dynamic recrystallization, may be achieved with this method.

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