Effect of solid solution cooling rate on α phase morphology of full lamellar metastable β titanium alloy

The effect of solid solution cooling rate on mechanical properties and the morphology of strip α phase of TB17 titanium alloy was studied by Gleeble thermal simulation compression testing machine, microhardness tester, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that after solid solution of TB17 titanium alloy with basket-weave microstructure in α+β phase region, when the solid solution cooling rate is 200℃/min, the microhardness of the titanium alloy is 250HV. With the decrease of solid solution cooling rate, the microhardness increases gradually. When the solid solution cooling rate is reduced to 1℃/min (furnace cooling), the microhardness increases to 320HV. And TB17 titanium alloy occurs β→α phase transformation during cooling process, and secondary α phase precipitates on β matrix, and the strip-shaped α phase transforms into "fork-like" structure. As the solid solution cooling rate decreases, the "fork-like" structure gradually becomes thicker and larger. When the rate is 40℃/min, the width of the "fork-like" structure is about 14 nm. When the rate is 10℃/min, the width is about 100 nm. When the rate is 1℃/min (furnace cooling), the width is about 300 nm. When the solid solution cooling rate is greater than 10℃/min, the side and end faces of the strip-shaped α phase are wrapped with the rhombohedral martensite α″ phase, and the existence of the martensite phase promotes the transformation of α phase and the formation of the "fork-like" structure. When the solid solution cooling rate is gradually reduced to about 1℃/min which is equivalent to the furnace cooling rate, the "fork-like" structure becomes thicker, the orthorhombic martensite phase on the end and side faces of the strip-shaped α phase disappears, and α″→α phase transformation occurs.