Significant evolutions of microstructure and mechanical properties of Al0.3CoCrFeNiTix high-entropy alloys under annealing treatment

The effect of annealing treatment on microstructure and mechanical properties of Al 0.3 CoCrFeNiTi x high entropy alloys were investigated. The results indicated that addition of Ti in as-cast alloys caused the formation of L1 2 , Laves, γ and σ phases to increase the hardness and strength of the alloys. The microstructure remained stable below annealing at 700 °C. However, the (Fe, Cr)-rich phase was found in the interdendritic (ID) structure. Simultaneously, annealing at 700 °C increased the volume fraction of L1 2 , γ and σ phases and decreased the volume fraction of FCC and Laves phases. The (Al, Ni, Ti)-rich rod-shaped precipitated (RSP) phase was found in the FCC matrix after annealing at 800 °C. The hardness and yield strength showed a significant enhancement after annealing at 700–800 °C. The coarsening phenomenon of (Fe, Cr)-rich and RSP phases could be observed after annealing at 900–1000 °C. Simultaneously, annealing at 900–1000 °C caused the further decrease in the volume fraction of FCC and Laves, causing the decrease in yield strength and to improve plasticity of the alloys. The main strengthening mechanisms of as-cast and annealed Ti x alloys were discussed, confirming that the precipitation strengthening mechanism was the main strengthening mechanism. • Addition of Ti in Al 0.3 CoCrFeNiTi x HEAs promotes the formationof L1 2 , Laves, γ and σ phases. Annealing treatment increases the volume fraction of L1 2 , γ, σ phases and decreases the volume fraction of FCC and Laves phases. • Annealing at 700 °C promotes the formation of the rod-shaped (Fe, Cr)-rich precipitated phase with BCC phase structure. • Annealing at 800 °C promotes the formation of the rod-shaped (Al, Ni, Ti)-rich precipitated phase with the same as the L1 2 phase. • The precipitation and precipitated particle strengthening mechanisms are enhanced after annealing at 700–800 °C to increase the hardness and strength. • The solid-solution, precipitation and precipitated particle strengthening mechanisms are weakened after 900–1000 °C to improve plasticity.