Abnormal twinning mechanisms and martensitic transformation in Fe60Mn12Cr12Ni8Si8 high entropy alloy under cyclic tension-compression loading

Abstract

Twinning and martensitic transformations are well-understood under monotonic loading, yet how stress reversal and the associated kinematic reversibility, inherent to cyclic deformation, affects these mechanisms remains insufficiently understood. Here, we systematically investigate the microstructural evolution of a metastable high entropy alloy Fe₆₀Mn₁₂Cr₁₂Ni₈Si₈ under cyclic tension-compression (CTC) loading. Multi-scale characterizations reveal that the cyclic stress reversal fundamentally alters the transformation pathway compared to the monotonic tension. The initial, undeformed material consists of a face-centered cubic γ phase. Monotonic tension primarily activates deformation-induced martensitic transformation, whereas CTC produces markedly different microstructural pathways depending on strain amplitude. At a low strain amplitude (0.5%), short-range glide of Shockley partial dislocations promotes extensive formation of HCP ε-martensite (a fraction of ∼68.3%). In contrast, high-strain-amplitude CTC loading (2.0%) activates an abnormal transformation-mediated twinning mechanism. This process, driven by the reversible motion of Shockley partial dislocation within confined ε-martensite, leads to a refined γ/γ_twin/ε nano-laminate structure with a spacing of ∼2.6 nm. Furthermore, we identify unconventional polymorphic transformation pathways accommodating the high local stress concentrations: (i) nucleation of body-centered cubic α′-martensite at a specific interface where the two γ phases maintain an 86° angle between their respective ${\left(11\overline{1}\right)}_{\gamma }$ planes, and (ii) a direct γ to body-centered tetragonal α-martensite transition via continuous lattice shearing along $\left(111\right){\left[11\overline{2}\right]}_{\gamma }$. These mechanisms are attributed to the unique stress accommodation requirements in the highly confined nano-laminates. The resulting hierarchical microstructure not only relieves local stress concentrations but also contributes to the good cyclic durability. Overall, these findings establish an atomistic mechanistic link between cyclic reversibility and transformation/twinning pathway selection, and suggest a processing-enabled route to engineer heterogeneous γ/γ_twin/ε nano-laminate structure in bulk metastable alloys at room temperature.