Activation of multiple deformation mechanisms and HDI hardening devoting to significant work-hardening of gradient-dislocation structured TRIP steel

Abstract

Combining hetero-structure and TRIP effect can be a feasible approach to enhancing the strength-ductility synergy. Here, varied gradient microstructures with gradual increase in dislocation density with or without martensite content from the center to the surface were constructed in a commercial austenitic stainless steel with strong TRIP effect by cyclic torsion (CT) processes. These designed gradient microstructures resulted in excellent strength-ductility synergies, for example, after CT processing with 20° torsion angle, the yield strength was enhanced 2.7 times, while 78% of the uniform elongation and 121% of the static toughness was achieved, compared to its CG counterpart. The high yield strength was caused by the introduced dislocations and the additional hetero-deformation induced (HDI) strengthening (>350 MPa) due to the mechanical incompatibility between different zones. The high ductility and toughness originated from the unexpectedly continuous work hardening from the soft center to the hard surface. Amazingly, the work hardening provided by the hard surface zones outperformed that by the soft central zones during the middle and late tensile deformation. The multiple-stage activation of stack faults, continuous γ→α′ martensitic transformation (i.e., TRIP effect), mechanical nanotwinning and Lomer-Cottrell locks combined with the HDI work-hardening devoted to the significantly enhanced work hardening capacity of the hard surface zones. This study provides a strategy of combining the gradient dislocation microstructure and TRIP effect to develop advanced metallic materials with high strength and ductility/toughness.