Intermediate stacking fault and twinning induced cooperative strain evolution of dual phase in lean duplex stainless steels with excellent cryogenic strength-ductility combinations

Abstract

The effects of intermediate substructures on the strain evolution in dual phase and the correlated mechanical properties at different temperatures were investigated in two lean duplex stainless steels (LDSSs) with different stacking fault energy (SFE). At room temperature (RT), the 1#-LDSS shows a lower yield strength (YS) but high tensile elongation (TEL) values than those of the 2#-LDSS due to a more significant TRIP effect, resulting from a lower SFE and mechanical stability of austenite (γ). Meanwhile, the plasticity of the ferrite (α) strip is also accommodated by the grain rotation process and texture transformation from component ⟨123‾⟩{111} to ⟨112‾⟩{111}, preventing a drastic dislocation generation in the α/γ interfaces. However at liquid nitrogen temperature (LNT), although a strong dislocation hardening of dual phase accompanied with a two-stage γ→ε→α’ transformation sequence occurs at low strains of 1#-LDSS, the severe dislocation accumulation in ferrite becomes the potential site of crack nucleation, leading to a high ductility loss compared to the RT. In contrast, a simultaneous enhancement of strength and ductility can be obtained in the 2#-LDSS at LNT. The introduction of intermediate stacking faults (SFs) and twins due to a lower critical resolved shear stress (CRSS) can not only provide additional work hardening capacity in the early deformation stage, but also lead to a gentle dislocation increment in ferrite. The cooperative co-deformation contributes to a sustainable high strain hardening until large strains. Therefore, a high YS over 1.3 GPa and a large TEL of about 50% can be obtained in the 2#-LDSS