Reconfiguring dual-phase structure with equal micro-hardness to achieve double strength with slight ductility sacrifice

Abstract

• After flash annealing, the hardness of ferrite and austenite is equal. • The steel obtains the doubling yield strength with slight elongation sacrifice. • Hall-Petch strengthening contributes 60% to yield strength, 40% by HDI hardening. • The high strain hardening rate originates from the multiple deformation mechanisms. • The multilevel SF networks and high-density L-C locks can improve strain hardening. The application of duplex stainless steel is severely restricted owing to its inherently low yield strength. A high yield strength of duplex stainless steel is required to address the lightweight issues and reduce the emission of polluting gases. Herein, excellent mechanical properties of duplex stainless steel were achieved using a flash annealing treatment to obtain austenite and ferrite phases with equal hardness, and the yield strength doubled without a substantial ductility sacrifice. The improvement in yield strength was caused by the grain boundary strengthening (a contribution of 60% to the yield strength) and hetero-deformation-induced strengthening (remaining 40% contribution to the yield strength) owing to the significant difference in the strain partitioning between the two phases. The high strain-hardening capability ensured ductility without a significant decrease; this correlated to the multiple microstructure deformation mechanisms. These mechanisms included the synergetic deformation of the ferrite with dislocation cell formation and austenite accompanied by the sequential appearance of dislocation accumulation, stacking faults, and deformation-induced nanoscale twins and martensite. Deformation-induced multilevel stacking fault networks and high-density Lomer-Cotterll locks further improved the strain hardening response by enhancing the hetero-deformation behavior of austenite and ferrite.