Twin density gradient induces enhanced yield strength-and-ductility synergy in a S31254 super austenitic stainless steel

Gradient structure (GS), as a typical heterostructure, is arousing great interest for an improved synergy between the strength and ductility which are mutually conflicting. Recently, a novel design of GS is proposed by taking the density of twins in grains, instead of common grain size, as a gradient variable, showing the key role in strain hardening by the nano-scale twin boundaries. Following this idea, here, a deformation twin-density GS was produced by means of the technique of surface mechanical attrition treatment in a S31254 super austenitic stainless steel. To be specific, the GS consisted of a central coarse-grained (CG) core, with two sides sandwiched by the gradient-structured layer (GL), where the density of deformation twins appears gradient in grains along the depth towards the CG core. The tensile tests show that as compared to CG counterpart, yield strength in GS increases 80% to 0.5 GPa, along with comparable ductility of 36%. The interrupted tensile tests show the presence of mechanical hysteresis loops during each unload-reload cycle, indicative of the generation of hetero-deformation-induced (HDI) stress during tensile deformation. Furthermore, both the HDI stress and HDI strain hardening account for a large proportion of global flow stress and forest hardening. The deformation twins and their evolutions, with the emphasis on their interaction with the dislocations, are investigated in detail by means of EBSD and TEM observations to correlate the mechanical properties. The present results shed light on the crucial role of deformation twins in the twin-density gradient for the synergistic enhancement of both strength and ductility.