Role of martensitic transformation sequences on deformation-induced martensitic transformation at high strain rates: A quasi in-situ study

Abstract

For the mechanical behavior of steels with metastable austenite, it was widely accepted that high strain rates could weaken deformation-induced martensitic transformation (DIMT) and reduce work-hardening behavior. This limits the application of these steels for energy-absorption applications, e.g., impact conditions. However, the mechanism of strain-rate dependent DIMT is still not well understood and it greatly limited the further alloy design. Thus, with the aim of revealing the intrinsic relations between strain rate and martensitic transformation, quasi-in-situ EBSD (Electron Backscatter Diffraction) tests were performed under different strain rates in this study. The influence of the martensitic transformation sequences on the DIMT volume fraction was investigated. With increasing strain rate, the γ→ε→α’ transformation was inhibited, and the γ→α’ transformation sequences dominated. The martensitic transformation sequences firstly affect the nucleation behavior. The α′-martensite nucleation sites changed from ε or twin to twin only, and thus, the α′-martensite nucleation barrier increased. In addition to nucleation, the variant selection phenomenon caused by the absence of ε-martensite under high strain rates increased the elastic strain energy, which inhibited the growth of α′-martensite. By affecting both nucleation and growth, high strain rates suppress the γ→ε→α’ transformation, and inhibit DIMT kinetics. Thus, by tailoring the martensitic transformation sequences, two methods are proposed to enhance DIMT behavior via suitable control of the stacking fault energy. This work provides a comprehensive explanation for the strain-rate dependence of DIMT, which can be used to tailor the mechanical properties of steels with metastable austenite.