Effects of Isothermal Solution Treatment on Microstructure Evolution and Tensile Properties of High Strength Near-Β Ti Alloy

The influence of slow cooling rate and the isothermal holding temperature in isothermal solution treatment on microstructure evolution and tensile properties of near-β Ti alloy was investigated in this work. The results showed that plenty of colonies consisting of parallel α needles were formed during the slow cooling and the isothermal holding stages. In comparison with the primary α developed during solutionizing, the secondary α precipitates formed during aging were more effective in strengthening the alloy. The hardness difference observed due to the cooling rate changes was reduced as the isothermal solutionizing temperature was increased. With the isothermal holding temperature decreasing, the volume fraction of primary α was gradually increased, and the volume fraction of secondary α precipitates dropped accordingly. When held below 750 °C, basketweave microstructures could be obtained after aging treatment, because most of α phase had been formed upon solution stage with plate-like shape. These α plates could accommodate larger degree of local strain than secondary α precipitates, resulting in higher fracture elongation of alloy. A balanced combination of strength and ductility was achieved under the cooling rate of 10 °C/min and isothermal holding temperature of 800 °C, and the yield strength, ultimate tensile strength, elongation and area reduction were 1470, 1497 MPa, 4 and 5.8%, respectively.