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

Abstract

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 γ-fcc matrix, spherical ordered γ′-L12 precipitates (with an average size of ∼118 nm) and plate-like D024-structured η phase in the interdendritic (ID) regions, along with γ-fcc matrix and numerous γ′ nanoprecipitates (with an average size of ∼20 nm) in the dendrites (DR). The as-cast MEA followed by annealing at 670 °C for 8 h (denoted as AT-670 MEA) comprised irregular rod-like γ′ phase, B2-structured Fe–Co phase and lamellar-like structure consisting of parallelly elongated γ′ and B2 phase in the ID regions. In the DR regions, the AT-670 MEA also consisted of γ-fcc matrix and numerous γ′ nanoprecipitates, whereas the γ′ 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 γ-fcc matrix and needle-like γ′ precipitates could be observed after the as-cast MEA annealing at 1050 °C for 8 h (denoted as AT-1050 MEA). The formation of needle-like γ′ precipitates was attributed to the coalescence of spherical γ′ phase in the ID regions, driven by the increase of elastic stress between the γ-fcc/γ′-L12 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 ∼669 MPa, an ultimate strength of ∼987 MPa and a total elongation of ∼10.9%. The present study provides an effective pathway to improve mechanical properties by tuning microstructures and phase compositions.