Cooperative roles of twinning, transformation and strain partitioning on the excellent cryogenic strength-toughness synergy of dual-heterogeneous metastable high entropy alloys

In a metastable Fe50Mn30Co10Cr10 high entropy alloy (HEA), a dual-heterogeneous structure, encompassing both grain size and dislocation heterogeneity, has been constructed to obtain excellent cryogenic mechanical performance by varying the annealing temperature (630 and 740 degrees C) following severe deformation. The HS-630 sample, consisting of recovered coarse-grains (CG), recrystallized fine-grains (FGs) and un-recrystallized ultrafine-grains (UFGs), possesses higher yield strength (YS) of -965 MPa, ultimate tensile strength (UTS) of -1680 MPa and uniform elongation (UEL) of -45 % when compared to the fully-recrystallized HS-740 sample at liquid nitrogen temperature (LNT). The difference in YS is primarily caused by the grain boundary and dislocation strengthening of UFGs, whereas the higher ductility of the HS-630 sample is associated with the hierarchical activation of transformation- and twinning-induced plasticity (TRIP and TWIP) effects, and compatible strain partitioning between heterogeneous domains. The dual-heterogeneity results in different critical stresses of e-phase nucleation in heterogeneous domains, and promotes sustainable TRIP effect in CGs and the emergence of TWIP effect in UFGs with strain progressing. Moreover, the strain of the ductile transformed e-phase is mainly accommodated by non-basal slip, which contributes to the strain hardening in the later deformation stage. Furthermore, the higher cryogenic impact toughness of the HS-630 sample can be attributed to the synergistic effects of the fracture mode, the transformation characteristics, and the heterogeneous strain partitioning.