Comparing the deformation-induced martensitic transformation with the athermal martensitic transformation in Fe-Cr-Ni alloys

Abstract

The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both cases, there is a significant chemical driving force for the transformation from γ to α′-martensite (α′), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α′ that forms within individual shear bands if the stability of γ is low. Neighbouring α′ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favorable oriented with respect to the applied stress/strain field. The formation of α′ at individual shear bands is also rare, since no ε is present and instead α′ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.