Microscopic insights of the extraordinary work-hardening due to phase transformation

Commercial 316L stainless steel is known for its appreciable strength and ductility, as well as strong resistance against corrosion and radiation damage. Remarkably, upon cooling, 316L maintains high ductility while the strength increases significantly, making the alloy an excellent choice for applications at low temperatures. Despite these attractive properties, the physical mechanisms underlying the outstanding low-temperature mechanical properties have not been established. Here, we report an in situ neutron diffraction study of 316L that reveals an extraordinary work-hardening rate (WHR) of ∼7 GPa at 15 K. Detailed analyses show that the major contribution to the excellent strength and ductility comes from the transformation-induced plasticity (TRIP) effect, introduced by the austenite-to-martensite (γ-to-α′) phase transition. A dramatic increase in the WHR is observed along with the transformation; the WHR declined when the austenite phase is exhausted. During plastic deformation, the volume-fraction weighted phase stress and stress contribution from the α′-martensite increase significantly. The neutron diffraction data further suggest that the γ-to-α′ phase transformation was mediated by the ε-martensite, as evidenced by the concurrent decline of the ε phase with the γ phase. This study sheds light on the extraordinary work-hardening effect due to phase transformation, which will provide guidance in the design of complex alloys.