Long term stability of a high-entropy CoCrFeNiTi alloy fabricated by mechanical alloying

The stability of high-entropy alloys is of great importance for their fundamentals and practical applications. We experimentally studied the phase and microstructural evolution of near-equiatomic Co0.18Cr0.20Fe0.24Ni0.19Ti0.19 alloy during long-term (up to 204 days) isothermal annealing at 873, 1073, and 1273 K. The as-synthesized material fabricated by mechanical alloying was found to be metastable, i.e., after one day of annealing at 873 K, the concentration of Ti atoms in the multicomponent solid solution decreased by a factor of three. However, the resulting composition, Co0.22Cr0.23Fe0.29Ni0.20Ti0.06, persisted for more than 200 days of heat treatment and can be considered a stable phase. It consists of two FCC phases with essentially indistinguishable chemical compositions and similar crystal lattice parameters, i.e., 3.597 Å and 3.616 Å. The released Ti atoms work like an inner getter, extracting dissolved contaminating oxygen from the multicomponent phase and forming rounded TiOx sub-micron particles that may work as dispersion strengthening and hardening factors. Some plate-like Co0.2Ni0.5Ti0.3 intermetallic phases also precipitated. In addition to the phase transformations, coarsening of the microstructure continued throughout the annealing time. An ordered superstructure of rounded multicomponent clusters (∼ 0.5 nm) packed in a strained hexagonal structure with a periodicity of ∼1.1 nm was observed after annealing at 1273 K. The superlattice was formed as a result of the short-range ordering of the five-component matrix phase. Thus, after long-termed annealing high-entropy alloy Co-Cr-Fe-Ni-Ti attains stable, although not single-phase state.