Latent Heat of TB18 Titanium Alloy during β to α Phase Transition by DSC and First-Principles Methods

The phase transition of titanium alloys is sensitive to the heat-treatment procedure, accompanied with the latent heat induced by phase transition. However, the latent heat during phase transition of titanium alloy has not been systematically studied, which may result in the gap between designed and actual temperature of the sample and affect the final mechanical properties. In this work, DSC (differential scanning calorimetry) and first-principles simulate methods were used to study the β → α phase transition process of TB18 (Ti–Al-Mo-V-Cr-Nb-Fe system) metastable β titanium alloy, especially to reveal the influence of the heating rate on latent heat. The ratio of latent heat to endothermic heat of the sample during temperature rising was introduced to interpret the effect of latent heat to actual temperature. The ratio of latent heat to endothermic heat at 1 ℃/min is about 15 to 20 times higher than that at 10 ℃/min. The higher ratio indicates that the latent heat of phase transition has a more significant effect on the temperature, which is related to the temperature range of phase transition and the α volume fraction. Compared with the heating rate of 1 ℃/min, the β → α phase transition takes place at higher temperature and the volume fraction of α is smaller at 10 ℃/min. Meanwhile, there is a precipitation free zone between grain boundary α and intragranular α and the distribution of α lamellae is heterogeneous when the heating rate is 10 ℃/min. Both of the experimental and theoretical results suggest that the latent heat of phase transition is the main cause of the temperature fluctuation during heat-treatment process. This work has guiding significance for microstructure optimization affected by temperature, to achieve the desired mechanical properties.