Simultaneous enhancement of strength and ductility in friction stir processed 2205 duplex stainless steel with a bimodal structure: experiments and crystal plasticity modeling

Achieving excellent strength-ductility synergy is a long-lasting research theme for structural materials. However, attempts to enhance strength usually induce a loss of ductility, i.e., the strength-ductility trade-off. In the present study, the strength-ductility trade-off in duplex stainless steel (DSS) was overcome by developing a bimodal structure using friction stir processing (FSP). The ultimate tensile strength and elongation were improved by 140% and 109%, respectively, compared with those of the as-received materials. Plastic deformation and concurrent dynamic recrystallization (DRX) during FSP were responsible for the formation of bimodal structure. Incompatible deformation resulted in the accumulation of dislocations at the phase boundaries, which triggered interpenetrating nucleation between the austenite and ferrite phases during DRX, leading to a bimodal structure. The in situ mechanical responses of the bimodal structure during tensile deformation were investigated by crystal plasticity finite element modeling (CPFEM). The stress field distribution obtained from CPFEM revealed that the simultaneous enhancement of strength and ductility in a bimodal structure could be attributed to the formation of a unique dispersion-strengthened system with the austenite and ferrite phases. It is indicated that the present design of alternating fine austenite and coarse ferrite layers is a promising strategy for optimizing the mechanical properties of DSSs.