Serration-induced plasticity in phase transformative stainless steel 316L upon ultracold deformation at 4.2 K

Abstract

Eras of hydrogen economy and aerospace have reignited interest in long-used austenitic stainless steel series owing to cost-effective and desirable mechanical properties for cryogenic applications. However, their mechanical response at liquid helium temperature (4.2 K), a critical temperature for liquid hydrogen and aerospace applications, is still arguable, and the associated underlying deformation mechanisms upon such ultracold deformation remain elusive. Here, we investigate the microstructural evolution of stainless steel 316L during ultracold tensile deformation at 4.2 K, showcasing microstructures about how tensile properties conducted at 4.2 K are distinguishable compared to the widely known ones at 77 K and about what deformation mechanism at 4.2 K prevails. We make clear that ultracold tensile deformation of stainless steel 316L enables a large ductility, increasing from 52 % at 77 K to 60 % at 4.2 K, even a drop of loading temperature from 77 K to 4.2 K, and even severe discontinuous plastic serration flow in the strain-stress curves at 4.2 K. Microstructural analysis of 4.2 K-ultracold deformation structure elucidates that simultaneous enhancement in strength and ductility is the consequence of both effects, i.e., the first-order solid-state transformation hardening effect by phase transformation of austenite to highly dislocated ά-martensite microstructure in the earlier plastic strain regime, followed by twinning-induced plasticity effect within the deformation-induced ά-martensitic body-centered cubic (BCC) grains in the later stages of deformation. The advent of deformation twin boundaries in ά-martensite grains acts as robust impediments to dislocation accumulation, resulting in substantial work hardening. Further, our high-resolution transmission electron microscopy analysis, taken from the regions of stress-induced adiabatic shear bands, reveals two significant features: (i) the intersection of plentiful coherent twins imposes atomic-scale stacking faults, nano-incoherent twins, and disconnections into BCC ά-martensite; (ii) at these localized temperature-rise regions, the ordered domain is often observed in the BCC martensite. Our findings can provide both a new paradigm of serration impact on mechanical properties and the origin of serrated yielding in the plastic strain regime at 4.2 K, guiding the twinning-utilized alloy design conformable for ultracold applications. © 2024 Elsevier B.V.