Evidence supporting reversible martensitic transformation under cyclic loading on Fe–Mn–Si–Al alloys using in situ neutron diffraction

Abstract

Fe–Mn–Si-based alloys, such as Fe–30Mn–4Si–2Al and Fe–15Mn–10Cr–8Ni–4Si (in mass%), show superior resistance to plastic fatigue compared to the conventional steels, which is ascribed to the reversible back-and-forth movement of {111}〈112¯〉γ Shockley partial dislocations associated with a reversible martensitic transformation between the face-centered cubic γ-austenite and hexagonal close-packed ε-martensite. The purpose of this study was to gather evidence of the reversible martensitic transformation using in situ neutron diffraction under cyclic loading. Three Fe–30Mn–Si–Al alloys with different Gibbs free energy differences at 298 K: Fe–30Mn–6Si (ΔGγ→ε = −250 J/mol), Fe–30Mn–5Si–1Al (ΔGγ→ε = −128 J/mol), and Fe–30Mn–4Si–2Al (ΔGγ→ε = −8.5 J/mol), were studied to unravel the effect of phase stability on the degree of reversibility. The reversible martensitic transformation between γ-austenite and ε-martensite during tension–compression loading is demonstrated as bulk-averaged insights in the Fe–30Mn–4Si–2Al alloy. The forward γ → ε transformation was induced by tensile loading, and the formed ε plates were reversed to γ during unloading and subsequent compressive loading. Furthermore, successive compressive loading induced a different variant of the ε plates from the variant formed under tensile loading, which also reverted to γ by subsequent tensile loading. Such repetition of the γ ↔ ε transformation in the Fe–30Mn-4Si–2Al alloy is thought to increase the plastic fatigue life compared to the Fe–30Mn–6Si and Fe–30Mn–Si–1Al alloys, in which the γ ↔ ε transformation is rarely reversible. Herein, we discuss the mechanism of the deformation-induced reverse transformation and propose an optimum thermodynamic condition: a negative close-to-zero ΔGγ→ε.