Effect of Annealing on Microstructure and Mechanical Properties of V_0.3CoCr_1.2NiTi_1.1Al_0.2 EHEA

The evolution of microstructure and mechanical properties of V_0.3CoCr_1.2NiTi_1.1Al_0.2 eutectic high entropy alloy (EHEA) subjected to annealing heat treatments at different temperatures was investigated. The results reveal that the as cast V_0.3CoCr_1.2NiTi_1.1Al_0.2 EHEA exhibits irregular eutectic colonies with coupled growth of FCC and BCC phases and grain boundary region with two-phase non-coupled growth. In the grain boundary regions, the FCC matrix phase contains spherical BCC precipitates and needle-like HCP Ti(Ni,Co)₃ precipitates. Annealing at 500 ℃ refines the two types of nano- precipitates at the grain boundaries. Annealing at 700 ℃ results in slight coarsening of the nano-precipitates. Annealing at 900 ℃ further coarsens the precipitates and furthermore coarsens the grain boundary areas. Annealing at 1100 ℃ simultaneously coarsens the eutectic colonies, the grain boundary regions and the nano-precipitates. The fine nano-precipitates with a dispersed distribution combined with the stability of the other phases are attributed to the optimal compressive strength and compressive strain of the alloy annealed at 500 ℃. Increasing the annealing temperature promotes microstructure coarsening, which consequently leads to the deterioration of the mechanical properties.