Phase transformations and mechanical behavior in a non-equiatomic Ti10Fe30Co30Ni30 medium-entropy alloy

Phase transformation, microstructure and mechanical behavior of a non-equiatomic Ti10Fe30Co30Ni30 medium entropy alloy (MEA) were systematically studied in as-cast and annealing states. The as-cast MEA was composed of gamma-fcc matrix, spherical ordered gamma'-L1(2) precipitates (with an average size of similar to 118 nm) and plate-like D0(24)-structured eta phase in the interdendritic (ID) regions, along with gamma-fcc matrix and numerous gamma' nano precipitates (with an average size of similar to 20 nm) in the dendrites (DR). The as-cast MEA followed by annealing at 670 degrees C for 8 h (denoted as AT-670 MEA) comprised irregular rod-like gamma' phase, B2-structured Fe-Co phase and lamellar-like structure consisting of parallelly elongated gamma' and B2 phase in the ID regions. In the DR regions, the AT-670 MEA also consisted of gamma-fcc matrix and numerous gamma' nanoprecipitates, whereas the gamma' nanoprecipitates were noticeably coarser than those of the as-cast counterpart. The complicated phases and complex microstructures of the AT-670 MEA were principally caused by phase decompositions at intermediate temperature. Only gamma-fcc matrix and needle-like gamma' precipitates could be observed after the as-cast MEA annealing at 1050 degrees C for 8 h (denoted as AT-1050 MEA). The formation of needle-like gamma' precipitates was attributed to the coalescence of spherical gamma' phase in the ID regions, driven by the increase of elastic stress between the gamma-fcc/gamma'-L1(2) interface during the annealing process. Among these three alloys, the AT-1050 MEA exhibits the best combination of strength and ductility, with a tensile yield strength of similar to 669 MPa, an ultimate strength of similar to 987 MPa and a total elongation of similar to 10.9%. The present study provides an effective pathway to improve mechanical properties by tuning microstructures and phase compositions.