Characteristic of phase precipitation and its role in grain evolution and mechanical properties of high entropy alloy

High entropy alloys have attracted great interests in research due to the novel designing concept. In this work, the microstructural evolution of Al_0.3CoCrFeNi high entropy alloy after cold rolling and annealing treatment (at 750 °C, 850 °C and 950 °C) was investigated by scanning electron microscopy and electron backscatter diffraction. It was found that this alloy was subject to recrystallization, grain growth and abnormal grain growth, accompanied by second phase precipitation with increasing annealing time. Precipitates were formed in non-recrystallized areas at low temperatures, which delayed recrystallization and subsequent grain growth. On the other hand, precipitates were detected in both recrystallized grain interiors and boundaries at all temperatures after recrystallization, and with increasing annealing time, their total volume fractions firstly increased, then decreased and finally remained nearly unchanged. The grain growth was highly impeded by precipitates, e.g., the grain size was only ∼1.41 μm after 150 h annealing at 750 °C, which was reflected by the higher activation energy for grain growth (Q) of ∼1730 kJ/mol compared with other reported HEAs and conventional alloys. In addition, the resistance to grain growth and appearance time of abnormal grain growth (detected simultaneously with recrystallization completion at 950 °C) decreased with increasing temperature, which was explained by the variation of Zener effect. Considering the above results, the 750 °C/96 h annealed specimen with the homogeneous microstructure of micron-sized (∼1.13 μm) grains and abundant submicron-sized (∼0.40 μm) precipitates was selected and verified to possess a good combination of tensile and fatigue properties.