Influence of Holding Time on Hot Deformation Behavior of Hard-Deformed Superalloy U720Li

保温时间对难变形高温合金U720Li热变形行为的影响

To improve the hot workability of hard-deformed superalloy U720Li,the effect of holding time before deformation(5 and 10 min)on hot deformation behavior was investigated by hot compression tests.Results show that the flow stress increases with increase in strain rate,while decreases with increase in deformation temperature and holding time.Based on the obtained Arrhenius-type constitutive models,the calculated peak stresses are in good agreement with experimental values,indicating that this model can accurately predict the hot deformation behavior of U720Li alloy,and the deformation activation energies for the holding time of 5 and 10 min were calculated to be 992.006 and 850.996 kJ·mol^(-1),respectively.Moreover,processing maps of U720Li alloy with these two holding durations were constructed.Through observation of deformation microstructures in each domain of the processing maps,the optimal hot working conditions for the holding time of 5 min are determined to be 1090‒1110℃/1‒10 s^(-1) and 1146‒1180℃/1‒10 s^(-1),and the optimal hot working conditions for the holding time of 10 min are 1080‒1090℃/1‒10 s-1 and 1153‒1160℃/1‒10 s^(-1),indicating that the safe processing window can be obviously enlarged by shortening the holding time reasonably.In the absence of cracking,the dynamic recrystallization(DRX)grain size increases gradually with increasing the deformation temperature and holding time,but it first decreases and then increases with the increase in strain rate.When the deformation temperature is below 1100℃,the DRX mechanism is mainly the particle-induced continuous DRX.As the temperature is raised to above 1130℃,the main DRX mechanism changes to discontinuous DRX.

为了改善难变形高温合金U720Li的热加工性能,研究了变形前保温时间(5和10 min)对该合金的热变形行为的影响。结果表明,流动应力随应变速率的提高而增大,但随变形温度和保温时间的增加而减小。依据所建立的Arrhenius本构模型计算的峰值应力与实验值吻合良好,说明该模型可准确预测U720Li合金热变形行为。保温时间为5和10 min的热变形激活能分别为992.006和850.996 kJ·mol^(-1)。建立了U720Li合金在这2种保温时间下的热加工图。通过分析热加工图各区域对应的变形组织,明确了保温5 min的最佳热加工条件为1090~1110℃/1~10 s^(-1)和1146~1180℃/1~10 s^(-1),而保温10 min的最佳热加工条件为1080~1090℃/1~10 s^(-1)和1153~1160℃/1~10 s^(-1)。可见适当缩短变形前保温时间可扩大该合金的安全加工窗口。在未发生开裂的情况下,动态再结晶(DRX)晶粒随变形温度和保温时间的增加而逐渐增大,但随应变速率的增加先减小后增大。当变形温度低于1100℃,主要DRX机制为颗粒诱导连续DRX。当变形温度高于1130℃,主要DRX机制转变为不连续DRX。