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Isochronal and isothermal phase transformation in β + αacicular Ti–55531
Journal of Materials Science ( IF 4.5 ) Pub Date : 2019-11-11 , DOI: 10.1007/s10853-019-04146-8
Fuwen Chen , Guanglong Xu , Kechao Zhou , Hui Chang , Yuwen Cui

Duplex aging is one of the common heat treatments in titanium alloys. The microstructure introduced in the first-step aging has an effect on the growth/dissolution of α in the second-step aging. In the present work, a β + αacicular microstructure is preset in Ti–55531 (Ti-5Al-5Mo-5V-3Cr-1Zr wt%) alloy. The isochronal and isothermal phase transformation kinetics in the second-step aging is studied by combining the dilatometer test with microstructure characterization and local composition mapping. The phase transformations and corresponding temperature ranges are determined as β → αacicular [643–845 K] and αacicular → β [845–1130 K] by isochronal annealing. A TTT diagram for isothermal transformation kinetics is plotted based on the transformed phase fraction and reproduced by Johnson–Mehl–Avrami theory. The calculated kinetic curves are in good agreement with experiment ones. The C-shaped TTT curves verify the classical nucleation and growth of α in the second-step aging. In comparison with Ti–55531 alloy with preset β + αlath microstructure (in authors’ previous work), the α precipitation exhibits prolonged incubation period and slowed average transformation rate, which is evidenced by a right shift of C-curves for the α precipitation portion along the time axis. However, the C-curves of α dissolution show a left shift on the TTT diagram. The precipitation kinetics of α aciculae from dilatometry is synchronous with that obtained from the diffusion of Al detected in STEM mapping, while the diffusion of slow-diffusion elements lags behind the structural transformation. The TTT diagram and the dataset of microstructure features obtained in the present work can be employed to optimize processing in duplex aging.

中文翻译:

β + α针状 Ti–55531 中的等时和等温相变

双相时效是钛合金中常见的热处理之一。第一步时效中引入的微观结构对第二步时效中α的生长/溶解有影响。在目前的工作中,在 Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr wt%) 合金中预设了 β + α 针状组织。通过将膨胀计测试与微观结构表征和局部成分映射相结合,研究了第二步时效中的等时和等温相变动力学。相变和相应的温度范围由等时退火确定为 β → α 针状 [643–845 K] 和 α 针状 → β [845–1130 K]。等温转化动力学的 TTT 图是基于转化相分数绘制的,并通过 Johnson-Mehl-Avrami 理论再现。计算的动力学曲线与实验曲线吻合良好。C 形 TTT 曲线验证了 α 在第二步时效中的经典成核和生长。与具有预设 β + αlath 显微组织的 Ti-55531 合金(在作者之前的工作中)相比,α 沉淀表现出较长的孵化期和较慢的平均转变速率,这可以通过 α 沉淀部分的 C 曲线右移来证明沿着时间轴。然而,α 溶解的 C 曲线在 TTT 图上显示左移。膨胀法中α针状体的析出动力学与STEM映射中检测到的Al扩散得到的动力学是同步的,而慢扩散元素的扩散滞后于结构转变。
更新日期:2019-11-11
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