K424高温合金凝固特征及冷却速度对其影响规律

Solidification characteristics and microstructures evolution of Ni-based superalloy K424 with different solidification cooling rates

采用等温凝固实验、差示扫描量热仪（DSC）研究了K424合金的凝固行为以及冷却速度对其影响.利用光学显微镜、扫描电镜以及能谱分析仪分析了合金在不同温度等温凝固、不同冷却速度下的微观组织以及凝固后期的元素的偏析行为,确定K424合金的固相线、液相线和主要相的析出温度等凝固特性以及冷却速度对γ＇相、MC碳化物以及共晶组织的影响规律.研究结果表明：K424合金的凝固顺序为：1345℃,γ相从液相析出,随后在1308℃析出MC型碳化物,在非平衡凝固条件下,共晶组织在1260℃析出,1237℃,凝固结束;共晶组织的形成与凝固末期Al、Ti元素的偏析行为以及冷却速度密切相关;随着冷却速度的增加,MC和共晶组织尺寸及数量均呈现先增大后减小的趋势;γ＇相形貌从花瓣形状向规则立方及球形转变,尺寸也从2μm减小至60 nm.

The solidification behavior and microstructures evolution of cast Ni-based superalloy K424 under different cooling rates were investigated by isothermal solidification quenching experiments and differential scanning calorimetry（ DSC）. The microstructures and segregation characteristics of K424 were analyzed at different isothermal temperatures and cooling rates using optical microscopy,scanning electron microscopy,and energy-dispersive spectrometry（ EDS）. The isothermal solidification,microstructure,and distribution characteristics of elements at the final solidification stage were also investigated,and the liquidus,solidus,and the formation temperatures of the main phases were evaluated. Furthermore,the influence of cooling rate on the morphology and size of MC carbides,（ γ＋ γ＇） eutectic,and precipitated γ＇ phase were discussed. The results show that the solidification of the K424 alloy follows the sequence：（ 1） L → L ＋ γ at 1345 ℃ ： the solidification begins with the formation of primary γ,and the liquidus temperature is 1345℃;（ 2） L → L ＋ γ ＋ MC at 1308 ℃ ： as the solidification continues,Ti and Nb are enriched in the liquid interdendrite,which results in the formation of MC carbides at 1308 ℃;（ 3） L → γ ＋（ γ ＋ γ＇） at 1260 ℃ ： the precipitation of the（ γ ＋ γ＇） eutectic occurs at about 1260 ℃ because of non-equilibrium solidification. Finally,the solidification ends with a solidus temperature of 1237 ℃.Furthermore,it is found that the precipitation of（ γ ＋ γ＇） eutectic at the interdendtitic regions is closely related to the cooling rate and the severe segregation behavior of Al and Ti into the residual liquid at the final solidification stage. With the increase of cooling rate,the quantity and size of MC carbides and eutectic first increase and then decrease. Moreover,with increasing cooling rate,γ＇shapes transform from irregular petal-like structure to near cuboidal and spherical patterns,and the size scale of γ＇ precipitates decreases from 2 μm to 60 nm.