加热工艺对难变形高温合金U720Li中γ′相溶解及晶粒长大行为的影响

Effect of heating process on γ′ dissolution and grain growth behaviors of hard-deformed superalloy U720Li

摘要:

变形前的加热工艺对高温合金的热加工性能具有重要影响。本文通过开展等温加热实验研究了难变形高温合金U720Li中γ′相特征及晶粒组织随加热温度及保温时间的演变行为，进而为改善其热加工性能提供理论指导。为了更准确地描述γ′相和晶粒的特征，本文采用电解腐蚀与化学腐蚀试样相结合的方法开展研究。结果表明，一次γ′的面积分数及尺寸随加热温度的升高及保温时间的延长逐渐减小。细小二次γ′比粗大一次γ′更易溶解，前者在1100 ℃保温30 min就可完全溶解，而后者在1180 ℃保温30 min才能完全溶解；一次γ′的不断溶解减弱了其对晶界的钉扎作用，导致合金的平均晶粒尺寸随加热温度升高及保温时间的延长逐渐增大，且在1180 ℃（高于γ′溶解温度）短时保温就发生了晶粒异常长大现象，形成混晶组织。最后，根据不同加热条件的晶粒长大行为，建立了U720Li合金在1050~1160 ℃的晶粒长大动力学模型。

关键词:U720Li合金;加热工艺;γ′相;晶粒长大

Abstract:

The heating process before deformation has significant influence on the hot workability of superalloys. In this paper, the evolution behaviors of γ′ characteristic and grain structure with the heating temperature and holding time in hard-deformed superalloy U720Li were studied by isothermal heating experiments, which can provide theoretical guidance for improving its hot working performance. The results show that the area fraction and size of primary γ′ decrease with the increase of heating temperature and holding time. The dissolution of fine secondary γ′ is easier than the coarse primary γ′. The former can be completely dissolved after holding at 1100 ℃ for 30 min, while the latter can be completely dissolved after holding at 1180 ℃ for 30 min. The continuous dissolution of primary γ′ weakens its pinning effect on grain boundaries, causing the average grain size to gradually increase with the increase of heating temperature and holding time. Moreover, as the temperature increases to 1180 ℃ (higher than γ′ dissolution temperature), the grains undergo abnormal growth in a short space of time, forming a mixed grain structure. Finally, according to the grain growth behaviors under different heating conditions, a grain growth kinetic model for U720Li alloy has been established.

Key words:U720Li alloy; heating process; gamma prime precipitate; grain growth