Phase Transformation Mechanism and Microstructure Evolution of TC4 Alloy during Continuous Cooling

The effects of cooling rate on the microstructure and phase transformation mechanism of TC4 alloy during the continuous cooling of β phase region were studied by non-isothermal expansion method combined with microstructure observation. The phase transformation feature points at different cooling rates were obtained by the expansion curve and its first derivative curve. The characteristics of the phase transformation products in the corresponding phase transformation feature points were observed by optical microscopy (OM). The results show that with the increase of cooling rate (excluding water cooling), the nonlinearity of the expansion curve caused by phase transformation changes from one place to two, and the temperature corresponding to the peak value of the phase transformation rate curve also showed different changes. As the cooling rate increases, the microstructure sequentially underwent lamellar α, massive αm, and fine-needle α′ martensite. The phase transformation conditions of massive αm and α′ martensite are significantly different, and showed a competition trend between them. According to the phase transformation feature points and their corresponding microstructure, the continuous cooling diagram for TC4 β solution treatment was plotted.