单晶高温合金共晶溶解行为的差热分析

Investigation on Eutectic Dissolution Behavior of Single Crystal Superalloy by Differential Scanning Calorimetry

选用一种第二代单晶高温合金,基于差示扫描量热技术(DSC),采用对比法测量了铸态和完全热处理态样品的升温DSC曲线,研究了保温过程中单晶合金中γ′相、γ/γ′共晶相的相变温度变化规律。结果表明,1290和1300℃保温过程中,随着保温时间的延长,γ′相溶解温度和γ/γ′共晶相熔化温度先显著提高,然后缓慢增加。1300℃保温过程中,γ/γ′共晶体积分数随保温时间延长而逐渐降低。而1290℃保温过程中,随保温时间延长,共晶体积分数出现了先降低后增加的反常现象,这与金相实验方法相吻合。分析表明,枝晶间粗大γ′相未完全溶解,造成枝晶轴Ta元素向枝晶间扩散,促使共晶长大,从而使共晶体积分数增加。

Ni-based single crystal(SX) superalloys are used for the production of blades in gas turbines and aircraft engines because of their superior mechanical performance at high temperatures.To improve the temperature capabilities of modern SX superalloys,specific refractory elements are added to the alloys.This leads to micro-segregation in alloys,requiring a complex heat treatment process to eliminate γ/γ′ eutectic.Therefore,the dissolution process of γ/γ′ eutectic must be understood.In this study,a second-generation Ni-based SX superalloy was used to investigate the effect of extended holding time at1290 ℃ and 1300 ℃ on the γ′ phase dissolving temperature(T_(γ)′) and γ/γ′ eutectic phase-melting temperature(T_(γ/γ′)),respectively.The method involves measuring the differential heating curves of as-cast and as heat-treated samples using DSC.The results showed that T_(γ)′and T_(γ/γ′)increased at a holding time of 2 h.However,with an increase in the holding time,the temperature increase was not obvious.The volume fraction of γ/γ′ eutectic decreased with the extended holding at 1300 ℃,while the volume fraction of γ/γ′ eutectic increased after holding at 1290 ℃ for 8 h.This abnormal phenomenon was confirmed by the metallographic experiments.The analyses showed that the increase in the eutectic volume fraction was due to the incomplete dissolution of coarse γ′ phase at the inter-dendritic region,which resulted in the diffusion of Ta element from dendrite core to the inter-dendritic region,promoting eutectic growth.