Microstructure evolution and mechanical property of a precipitation-strengthened refractory high-entropy alloy HfNbTaTiZr

The mechanical properties and microstructure evolution of a precipitation-strengthened refractory high-entropy alloy, HfNbTaTiZr, with equiaxed grains are reported. The as-cast alloy shows a high yield strength of 356 MPa at 1200 °C, and 1597 MPa with good ductility at room temperature. The enhancement in strength is attributed to precipitation strengthening. Afterwards, microstructure evolution was investigated under several annealing treatment conditions: 1000 °C for 24 h, 1200 °C for 24 h, 1400 °C for 24 h, and 1450 °C for 168 h. The hcp-1 phase appears at 1000 °C and gradually dissolves up to 1200 °C. When the temperature rises to 1400 °C, the hcp-1 phase disappears, and fine nanoprecipitates begin to form. After annealing at 1450 °C for one week, however, this alloy exhibits phase decomposition. With the formation of two kinds of new fcc phase, fine nanoprecipitate only remains in bcc matrix, and another new hcp-2 phase forms in the grain.