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

Commercial 316 L stainless steel is known for its appreciable strength and ductility, as well as strong resistance against corrosion and radiation damage. Remarkably, upon cooling, 316 L 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 316 L 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.