Tailoring martensitic transformation for strength-ductility synergy in Co_36.8Ni_39.2Al₂₄ hypereutectic medium-entropy alloy

Tailoring the deformation-induced martensitic transformation (DIMT) presents an effective strategy to overcome the strength-ductility trade-off in high-entropy alloys. The characteristics of precipitates play a critical role in governing DIMT behavior. In this study, a Co_36.8Ni_39.2Al₂₄ hypereutectic medium-entropy alloy (HMEA) with a high fraction of primary B2 phase was selected to elucidate the contribution of DIMT. The precipitation behavior and the impact on mechanical properties in the Co_36.8Ni_39.2Al₂₄ HMEA were investigated through isothermal annealing at 650–950 °C. Annealing at temperatures above 650 °C fully reverted pre-existing martensite within the B2 matrix. Specifically, annealing at 800 °C produced a high density of nanoscale, ordered L1₂ precipitates with Kurdjumov-Sachs (K-S) orientation relationship with the B2 matrix, while higher temperatures led to coarsened, disordered face-centered cubic (FCC) precipitates with a loss of the K-S relationship. The 800 °C-annealed sample exhibited an optimal strength-ductility synergy, which originated from both the complete elimination of pre-existing martensite and the presence of low-misfit L1₂ precipitates that effectively retarded DIMT kinetics, thereby sustaining a high work-hardening rate. In contrast, the high-misfit FCC precipitates formed at 950 °C acted as high-strain nucleation sites, accelerating DIMT and leading to rapid transformation saturation, which weakened the transformation-induced plasticity (TRIP) effect. This work provides fundamental insight into precipitate-mediated control of DIMT and proposes a practical strategy for designing TRIP-assisted high-performance high-entropy alloys.