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

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}< 11 (2) over bar >(gamma) Shockley partial dislocations associated with a reversible martensitic transformation between the face-centered cubic gamma-austenite and hexagonal close-packed epsilon-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 (Delta G(gamma ->epsilon) = -250 J/mol), Fe-30Mn-55i-1Al (Delta G(gamma ->epsilon)( )= -128 J/mol), and Fe-30Mn-45i-2Al (Delta G(gamma ->epsilon )= -8.5 J/mol), were studied to unravel the effect of phase stability on the degree of reversibility. The reversible martensitic transformation between gamma-austenite and epsilon-martensite during tension-compression loading is demonstrated as bulk-averaged insights in the Fe-30Mn-45i-2Al alloy. The forward gamma -> epsilon transformation was induced by tensile loading, and the formed epsilon plates were reversed to gamma during unloading and subsequent compressive loading. Furthermore, successive compressive loading induced a different variant of the epsilon plates from the variant formed under tensile loading, which also reverted to gamma by subsequent tensile loading. Such repetition of the gamma <-> epsilon transformation in the Fe-30Mn-45i-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 gamma <-> epsilon 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 Delta G(gamma ->epsilon ) . (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.