Suppressed martensitic transformation under biaxial loading in low stacking fault energy metastable austenitic steels

The effect of uniaxial/biaxial loading on the martensitic transformation of low stacking fault energy, metastable austenitic stainless steel was studied by in-situ neutron diffraction on cruciform-shaped/dogbone samples. Uniaxial loading favors the martensitic transformation following the sequence γ → ε → α′ (i.e. fcc→hcp→bcc/bct crystal structure), where at low strains ε-martensite is the precursor of α′. During equibiaxial-loading, the formation of ε-martensite is suppressed and considerably less α′-martensite is observed at high strains.

Post mortem EBSD investigation reveal that the different deformation textures formed during uniaxial and equibiaxial loading play a significant role on the appearance/absence of ε-martensite which is a precursor for the martensitic transformation. The uniaxial-deformation texture facilitates the formation stacking faults as the majority of the grains have orientations (with respect to the loading direction) that the leading partial dislocations (LPDs) have higher Schmid factor than the trailing partial dislocations (TPDs), see Fig. 1. The appearance of stacking faults facilitates the local appearance of ε-martensite with an hcp crystal structure. In the opposite case, upon biaxial loading, the majority of the grains have orientations (with respect to the loading direction) that the LPDs have lower Schmid factor than the TPDs (see Fig. 1); this results in suppression of the formation of ε-martensite. In both loading states, the grains that contain martensite belong to orientations for which the LPDs have higher Schmid factor than the TPDs (see Figure).