Isothermal Beta Grain Growth Kinetics of TC4-DT Titanium Alloy under Two Different Prior Processing Conditions: Deformed vs. Undeformed

摘要:

研究了具有初始等轴组织的TC4-DT钛合金在变形和未变形的状态下在β单相区等温加热时的晶粒长大动力学。变形在两相区进行，变形程度为60%，变形后空冷。加热温度分别选取相变点以上10、20、30 ℃， 保温时分别为2、5、10、30、60、120 min。利用金相分析软件对晶粒尺寸进行了定量测定并分析获得了两种条件下的晶粒生长的时间指数和激活能。未变形试样的晶粒生长的时间指数和激活的变化范围分别是0.34~0.35和86.8~130 kJ·mol-1, 在变形条件下变为0.36~0.39和76.6~110 kJ·mol-1。研究结果表明在相同的加热条件下，变形试样具有较高的晶粒生长指数和较低的生长激活能。这是因为晶粒生长时间指数随温度的变化和生长激活能随时间的变化与固溶原子的扩散和晶界的迁移的交互作用有关，而变形可以促进原子的扩散和晶界的迁移。另外还研究了在不同的加热温度、时间以及变形条件下晶粒尺寸的均匀度的变化情况，这种变化是β相的形核速率和生长速率在上述因素的影响下共同作用的结果。

Abstract:

β grain growth kinetics of TC4-DT titanium alloy under two different prior processing conditions (deformed vs. undeformed) was investigated during isothermal heat treatments. Deformation was carried out in α+β two-phase field with deformation degree of 60% and air cooling. Heat treatment scheme with the temperatures above β transus temperature 10, 20, 30 °C and soaking time of 2, 5, 10, 30, 60, 120 min was used. The grain size parameters have been obtained by means of the image analysis software. Growth time exponent (n) and activation energy (Q) have been established for both conditions. For the undeformed samples, n varied in the range of 0.34~0.35 and Q in the range of 86.8~130 kJ·mol-1, while after deformation, the values of n and Q changed in the domain of 0.36~0.39 and 76.6~110 kJ·mol-1, respectively. Results show that the samples subjected to deformation exhibit higher growth time exponent and lower activation energy compared to the samples without deformation under the same heat treatment conditions. The variations of the growth time exponent with temperature and the activation energy with soaking time are ascribed to the interaction between diffusion of solute atoms and migration of grain boundaries both of which can be promoted by deformation. The influences of heating temperature, time and deformation on the uniformity of grain size were also studied and the intrinsic mechanism is attributed to the combined effect of β phase nucleation rate and β phase growth velocity under the above factors.