Transformation-induced plasticity via γ → ε → α’ and γ → ε → γ martensitic transformations in Fe–15Mn–10Cr–8Ni–4Si alloy

Abstract

Transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) are key mechanisms for achieving excellent strength–ductility in advanced high-strength steels (AHSSs). The Fe–15Mn–10Cr–8Ni–4Si alloy is an austenitic steel that exhibits high plastic fatigue durability due to reversible bidirectional γ→ε→γ martensitic transformation under push–pull cyclic loadings. In addition to the fatigue property, the Fe–15Mn–10Cr–8Ni–4Si alloy exhibits good strength–ductility balance (where the product of strength and ductility is 51.4 GPa%). In particular, its total elongation of 77% is comparable to those of TWIP steels. To elucidate the underlying mechanism for the TRIP-enhanced ductility, this study reports on the tensile deformation microstructure. The post-fracture microstructure is a γ-ε-α′ triple-phase structure that includes a significant amount of deformation-induced ε-martensite, whereas the frequency of γ-twinning was very limited. In addition to sluggish strain-induced ε-martensitic transformation (ε-TRIP), two-stage γ→ε→α′ martensitic transformation (two-stage TRIP) and bidirectional γ→ε→γ transformation (B-TRIP) under monotonic tensile loading were uniquely observed at intersections of the ε-martensite variants. The tensile property and microstructure of the Fe–15Mn–10Cr–8Ni–4Si alloy (where the Gibbs free energy difference ΔGγ→ε between γ-austenite and ε-martensite is −65.0 J/mol) are compared with those of the Fe–30Mn–(6−x)Si–xAl (x = 0, 1, 2, 3, 4, 5, 6 wt%) alloy system, with different ΔGγ→ε (−276.5–417.5 J/mol) resulting in different plastic deformation mechanisms (martensitic transformations or twinning). The three TRIP mechanisms with different transformation paths, i.e., γ→ε, ε→α’, and the reverse ε→γ, in the Fe–15Mn–10Cr–8Ni–4Si alloy are then discussed in terms of their contributions to its excellent ductility, which is comparable to that produced by TWIP.