Achieving the strength and ductility synergy in a steel through nanoprecipitation and its induced grain refinement

Abstract

There is a constant demand in human society to develop steels with superior strength and ductility. The strength-ductility trade-off is hardly broken by using one of the four traditional strengthening methods solely, namely, strain hardening, grain refinement, precipitation and solid solution strengthening. The design and development of advanced steels need to introduce as many strengthening mechanisms as possible for achieving desirable strength. To achieve good ductility with high uniform elongation before fracture, necking need to be suppressed by continuous work hardening of the material. Here, we demonstrate that by adding Al (∼6%) and Ti (∼3%) into the Fe-26Ni-1.2C (at.%) steel, nanosized γ′-(Ni, Fe)₃(Al, Ti) (∼20 vol%) can precipitate out in the initial austenitic matrix after one-step aging. The formation of γ′ nanoprecipitates triggers γ → α phase transformation of the matrix during cooling, introducing strain hardening due to volume expansion, and simultaneously refines grains from 15 to 1–2 μm due to the hindering effect of nanoprecipitates on grain boundary migration. Moreover, the γ′ nanoprecipitates not only can hinder dislocation motion in the matrix, but also can plastically deform under high stress to enhance the ductility of the steel. Therefore, the nanoprecipitate-strengthening ultrafine-grained steel suppresses necking instability, which enhances the strength, uniform elongation and work hardening capability simultaneously.