Abstract:Microstructural evolution of Ti-1300 alloys was investigated using an optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction during continuous cooling. Then, a continuous cooling transformation diagram was established using dilatometric method, and the different kinds of β phase decomposition modes in the alloy under continuous cooling were investigated in detail. Moreover, the β → α + β phase transformation and colony structures are observed for low cooling rates. When the cooling rate is from 0.3 °C/s to 1.5 °C/s, the β → α + β + βm phase transformation and needle-like structures are observed in the alloy. However, when the cooling rate exceeds 3 °C/s, the alloy is only composed of a single metastable β phase. Thus, a rate of 3 °C/s is considered as critical cooling rate of the alloy under continuous cooling condition. The concentration gradient of molybdenum equivalent is considered as a driving force of α phase growth in the alloy.The microhardness of the alloy initially increases and then decreases with increasing of the cooling rate. When the cooling rate is 0.3 °C/s, the microhardness of the alloy reaches its maximum.