Multi-Phase Evolution and Surface Degradation Kinetics of a Non-Equiatomic (FeCoNiCr)85Ga15 High Entropy Alloy: The Role of Low-Temperature Thermal Activation

This study provides a rigorous analysis of the phase stability, mechanical behavior, and surface integrity of a non-equiatomic (FeCoNiCr)85Ga15 high-entropy alloy (HEA). By transitioning from the conventional equiatomic design to a gallium-doped 3d-transition metal matrix, we explore the interplay between lattice distortion and phase separation. Synthesized via vacuum arc melting, the as-cast alloy exhibits a non-homogeneous dendritic morphology consisting of a Cr-Fe-Co rich face-centered cubic (FCC) matrix and Ni-Ga rich body-centered cubic (BCC) interdendritic regions. While global thermodynamic criteria (δ = 3.65, ΔHmix = −9.28 kJ/mol, and Ω = 2.23) favor single-phase solid solution stability, the Valence Electron Concentration (VEC = 7.46) precisely forecasts this dual-phase structure. Following low-temperature annealing at 250 °C for 24 h, high lattice strain energy drives a significant morphological transformation where the continuous interdendritic network resolves into discrete, phase-separated B2/BCC “islands”. Mechanical and tribological characterizations reveal that this low-temperature thermal activation triggers precipitate hardening; the macro-hardness increases from 146 ± 11 HB to 153 ± 7.5 HB and the micro-hardness rises from 186 ± 4 HV0.5 to 206 ± 17.5 HV0.5, yielding enhanced resistance to oxidation-delamination wear. However, electrochemical evaluation in a 3.5 wt.% NaCl solution highlights a fundamental trade-off: the formation of localized galvanic micro-cells between the phase-separated islands and the matrix causes the corrosion current density (icorr) to increase from ≈10−9 A/cm2 in the as-cast state to ≈10−6 A/cm2 post-heat treatment, accompanied by a heightened susceptibility to localized pitting. These findings elucidate the primary role of electronic structure and minor p-block additions in regulating the lifecycle performance of transition metal HEAs under extreme conditions.