Continuous Cooling Transformation Diagram of TC17 Titanium Alloy

The effects of continuous cooling rates on the microstructure and mechanical properties of TC17 titanium alloy were investigated. Dilatometric curves of TC17 titanium alloy under different cooling rates were measured by high differential dilatometry Bähr DIL805A/D. Using the derivative method to obtain the beginning and end points of TC17 titanium alloy at different cooling rates, to establish a continuous cooling transformation (CCT) diagram of TC17 titanium alloy. The effect of cooling rate on microstructure evolution and mechanical properties of TC17 alloy was studied by scanning electric microscopy (SEM), X-ray diffraction (XRD) and micro-hardness analysis. The results showed that the microstructure of TC17 was the α+β structure at the low cooling rate, When the cooling rate was less than 0.50℃•s-1, all the lamellar α phase and β phase structure were obtained, when the cooling rates was from 0.35 to 3.00℃•s-1, the β→α+β+βm phase transformation and lamellar structure and untransformed metastable β phase were observed in the alloy, when the cooling rate exceeded 3.50℃•s-1, the alloy was only composed of a single metastable β phase. Thus, a rate of 3.50℃•s-1 was considered as critical cooling rate of the alloy under continuous cooling condition. The micro-hardness of TC17 alloy initially increased and then decreased with increasing of the cooling rate, and at cooling rate of 0.50℃•s-1, the micro-hardness reached a maximum of HV 436. The activation energy for the β→α+β phase transformation in TC17 alloy was evaluated on the basis of Kissinger equation, and the value was 218.447 kJ•mol-1. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.