Characterization of deformation-induced martensite with various AGSs upon Charpy impact loading and correlation with transformation mechanisms

Abstract

Deformation-induced martensitic transformation (DIMT) during quasi-static loading has drawn much attention in recent years since it is considered as one of the key strengthening mechanisms in advanced high-strength steels (AHSS). However, systematic investigations on martensitic transformation at high strain-rates are scarce due to difficulties in experimental designs. In this contribution, interrupted Charpy tests were used to study the transformation sequence of DIMT under high strain-rates (~10 2 –10 3 s −1 ). Austenitic 321 stainless steel samples with increasing austenite grain sizes (AGSs) were designed to demonstrate the change in transformation sequence of α'-martensite and its morphology. Quasi-in-situ electron backscattered diffraction (EBSD) and selected transmission electron microscopy (TEM) experiments have revealed that, as the AGSs increases, the nucleation sites of α'-martensite change from austenite grain boundaries in ultra-fine-grained (UFG) samples to additional twin boundaries in fine-grained (FG) samples, and finally to ε-martensite in coarse-grained (CG) samples. Due to the spatial constraints imposed by different crystallographic boundaries, the morphology of transformed α'-martensite shifts from blocky in UFG samples to film-like in CG samples, with FG samples having a mixed morphology. It is found that the changed transformation sequence can be understood by considering the combined effect of stacking fault energy and twinning nucleation stress of austenite. • Quasi-in-situ observation of DIMT under dynamic loading conditions. • TRIP (ε + α'), TWIP+TRIP (α'), and TRIP (α') occurred with decreasing austenite grain sizes. • The morphology of DIM is controlled by intermediate deformation products. • The absence of twins and ε-martensite resulted in blocky α'-martensite in UFG steels. • High SFE and twinning nucleation stress suppressed the formation of ε-martensite and twins.