Martensitic transformation-governed Lüders deformation enables large ductility and late-stage strain hardening in ultrafine-grained austenitic stainless steel at low temperatures

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Deformation-induced martensitic transformation that rapidly occurs generally leads to a significant reduction in elongation of metastable austenitic steels, particularly at low temperatures. However, we proved that the ultrafine-grained 304 stainless steel is an exception. Using a hybrid method of in situ neutron diffraction and digital image correlation, we found that this material exhibits Lüders deformation after yielding, in which the deformation behavior changes from a cooperation mechanism involving dislocation slip and martensitic transformation to one primarily governed by martensitic transformation, as the temperature decreases from 295 K to 77 K. Such martensitic transformation-governed Lüders deformation delays the activation of plastic deformation in both the austenite parent and martensite product, resulting in delayed strain hardening. This preserves the strain-hardening capability for the later stage of deformation, thereby maintaining a remarkable elongation of 29 % while achieving a high tensile strength of 1.87 GPa at 77 K. Insights from this study point to a feasible pathway to achieve both high strength and high ductility by rearranging the deformation modes of ultrafine-grained materials.