Tension-compression asymmetry of metastable austenitic stainless steel studied by in-situ high-energy X-ray diffraction

Kurzfassung

This work studies the tension-compression asymmetry (TCA) of metastable austenitic stainless steel (MASS) in uniaxial loading depending on temperature. In-situ high-energy X-ray diffraction was used to simultaneously probe phase fractions, transformation kinetics, crystallographic texture, lattice strains, strain and stress partitioning between austenite and martensites during quasi-static tensile and compressive deformation at 24, 60 and 100 °C. Complementary relaxed-constraint crystal plasticity simulations and calculations of the mechanical driving force related to the formation of α’ and ε martensites were performed. At 24 °C, martensitic transformations (MTs) prevail, while at 100 °C dislocation slip is the dominant deformation mechanism for both load senses. Macroscopic stress-strain response and transformation behaviour exhibit TCA, with compression promoting the conversion of ε into α’. Transformation kinetics were analyzed in relation to shear banding and the geometric alignment of ε lamellas depending on load sense and temperature. A strong TCA was found for crystallographic texture, bearing signatures of grain rotation due to plastic slip and of MT in case of austenite (γ). For both load senses, the relative strengths of austenite and martensite texture fibres were related to the driving force anisotropy for α’ formation calculated based on the phenomenological theory of martensite crystallography. Texture evolution of α’ is largely controlled by the MT itself, not by grain rotation. Analysis of differently orientated austenite grain families revealed a pronounced TCA of the lattice strains, linked to the γ → ε MT. This was found to be a direct consequence of driving force and volume change related to ε formation. Furthermore, stress is shared differently between austenite and martensites in tension vs. in compression. γ hardens more and hence carries a larger portion of the total stress in compression than in tension. The origin for this TCA could be found in the elasto-plastic accommodation of the volume change related to α’ formation. These findings can aid the development of new material laws for MASSs that are sensitive to load-sense and temperature for advanced forming simulations.