Multiscale in-situ studies of strain-induced martensite formation in inter-critically annealed extra-low-carbon martensitic stainless steel

An extra low carbon martensitic stainless steel with 16% ultrafine grained metastable reverted austenite was subjected to uniaxial tensile testing and investigated with in-situ energy-dispersive synchrotron X-ray diffraction (XRD) and in-situ electron backscatter diffraction (EBSD) to reveal the complex interplay between stress, strain and martensitic transformation. In-situ XRD demonstrated that, upon surpassing the yield strength, the fraction of reverted austenite declined linearly with increasing true stress, which was associated with transformation-induced plasticity (TRIP). EBSD and XRD consistently showed that the texture of martensite evolved from an initially weak texture towards a strong 110α′ fiber parallel to the tensile axis. For the first time, stress partitioning between (remaining) reverted austenite and the martensite matrix was determined quantitatively during in-situ XRD by averaging over the stress values obtained from lattice strains for multiple reflections. Martensite accommodates the majority of the applied load while reverted austenite is severely plastically deformed. XRD shows strong plastic anisotropy in austenite. In-situ forward-scatter electron imaging and advanced variant analysis of the EBSD data indicate that plastic deformation and strain-induced austenite-to-martensite transformation is concentrated along boundaries between martensite blocks and packets which are inclined up to ∼55° with respect to the tensile direction. These regions were preferred sites for strain-induced martensite formation.