Optimizing crack initiation energy in austenitic steel via controlled martensitic transformation

Although the seemingly negative effect of deformation-induced martensite (DIM) volume fraction on the impact toughness of austenitic steels has been well documented, it relies mostly on analyzing crack propagation without explicitly considering the crack initiation process which, however, plays a crucial role in these ductile alloys. The dependence of crack initiation energy (Ei) on martensitic transformation mechanisms is still ambiguous, inhibiting the precise design of damage-tolerant and ductile alloys. Here, we explore the temperature-dependent crack initiation energy of a SUS321 stainless steel at various temperatures (25, –50, and –196 °C). Contrary to the crack propagation energy (Ep), the Ei has a weak correlation with the volume fraction of α′-martensite but a strong correlation with the martensitic transformation rate. Also contrary to the traditional viewpoint of Ep considering ε-martensite as a detrimental phase, a high volume fraction of ε-martensite turns out to be beneficial to the increase of Ei, thereby enhancing impact toughness. As such, an optimal value (15 mJ/m2) for the stacking fault energy (SFE), which dictates the γ→ε→α′ transformation sequence, is given as a new design guideline for enhancing the Ei and consequently the impact toughness of ductile steels. The generality of this guideline is further validated in multiple austenitic steels with different compositions and grain sizes.