Influence of DIMT on impact toughness: Relationship between crack propagation and the a'-martensite morphology in austenitic steel

The trade-off between transformation-induced plasticity (TRIP) strengthening and the intrinsically brittle nature of deformation-induced alpha '-martensite (DIM) has been a long-standing dilemma in optimizing the strength-toughness synergy of austenitic steels. This has limited their potential use, particularly in energy absorption applications. Here, we propose a new strategy to minimize the negative effect of intrinsically brittle alpha '-martensite without limiting the TRIP. In this study, austenitic 321 stainless steel samples with different austenite grain sizes (AGSs) were fabricated to tailor the DIM morphology. The effect of the DIM morphology on the crack propagation energy was simultaneously evaluated. Electron channeling contrast imaging (ECCI) and selected area electron backscattered diffraction (EBSD) of the Charpy fractures reveals that lath-like alpha '-martensite effectively deflects cracks and increases the crack propagation energy (Ep). As a consequence, coarse-grained and ultra-coarse-grained (CG/UCG) steels with lath-like alpha '-martensite can simultaneously achieve high strength and crack propagation resistance, while ultra-fine-grained (UFG) steel with blocky DIM exhibits the lowest Ep. Furthermore, the morphology of DIM is controlled by its nucleation sites and adjacent crystallographic/phase boundaries. The twins or e-gamma boundaries promote the formation of lath-like alpha '-martensite. The gamma -> e ->alpha ' transformation sequence can be explained by the change in stacking fault energy (SFE), which provides further guidance for the design of austenitic steels with high strength and high toughness.

Automated summary

The trade-off between TRIP strengthening and the brittleness of DIM has been a long-standing dilemma in austenitic steels. In this study, the researchers propose a new strategy to minimize the negative effect of brittle alpha'-martensite without limiting TRIP. By controlling the DIM morphology, coarse-grained and ultra-coarse-grained steels can achieve high strength and crack propagation resistance. Furthermore, the morphology of DIM is influenced by its nucleation sites and crystallographic/phase boundaries.