铸造镍基高温合金IN792的凝固和偏析行为研究

Solidification and Segregation Behavior of Cast Ni-Base Superalloy IN792

采用等温凝固淬火试验(ISQ)与差热扫描量热(DSC)分析的方法对铸造镍基高温合金IN792的凝固过程和元素在固、液相中的分配进行研究,得到了合金的等温凝固组织、相析出顺序图、凝固特性曲线以及元素在不同温度下的偏析特性。ISQ结果表明:IN792合金的液相线温度为1328℃,开始析出MC碳化物的温度为1310℃,开始形成(γ+γ’)共晶的温度为1225℃,于1190℃开始析出二次γ’相。由DSC测得的合金宏观固相线温度(尚存体积分数5%残余液体)和由ISQ测得的微观固相线温度(终凝温度)分别为1250和1180℃,两者相差达70℃,这一温度范围往往是热裂缺陷形成的敏感区间。IN792合金在液相线下30℃范围内液体量锐减,析出了体积分数约85%的γ枝晶。1300～1270℃是合金枝晶间液体由连通转为断开的温度范围,它与铸件疏松形成密切相关。在1325～1190℃温度区间,IN792合金中元素W、Co分配系数大于1,倾向富集于枝晶干固相,为负偏析元素;Zr、Mo、Ti、Ta、Cr分配系数小于1,倾向富集于枝晶间液相,为正偏析元素;Al在凝固早期富集于液相而后来倾向于分布在枝晶干固相区,随着凝固温度降低逐步转变为负偏析元素。在1325～1210℃温度范围内,随着凝固温度降低,Al、Ni的分配系数升高,而Mo、Cr的分配系数降低。相反,在1210～1180℃温度区间,随着凝固温度降低,Ni、Al的分配系数降低,而Mo、Cr的则升高。

The solidification process of cast Ni-base superalloy IN792 was comparatively investigated by differential scanning calorimeter（DSC） and isothermal solidification quenching（ISQ） experiment.The distribution characteristics of elements in solidified solid and residual liquid at different temperatures were also concerned.The isothermal solidification microstructure,the phase precipitation diagram,the solidification characteristic curve and the segregation behavior of elements at different temperatures in liquidus and solidus range were obtained.The results of ISQ indicate that the IN792 alloy possesses a liquidus temperature of 1328 ℃.The onset of MC carbides and eutectic（γ＋γ＇） formation take place at 1310 and 1225 ℃,respectively.The secondary γ＇ is precipitated at 1190 ℃.The macro-solidus temperature determined by DSC was 1250 ℃.The volume fraction of residual liquid at this temperature can reach about 5%.The end of the solidification happened at 1180℃,designated as micro-solidus temperature.No liquid remained at this temperature.There is a 70 ℃ gap between macro and micro-solidus temperature.The hot tearing tends to occur in this temperature range.About 85 vol% solid phase was formed within a range of 30 ℃ below the liquidus temperature.In 1300～1270 ℃ range,the residual liquid in the interdendritic region changed from connected channels to the isolated micro-liquid pools,which is related to the formation of micro-porosity defect.The distribution coefficient of element W and Co are more than 1,indicating that they tend to distribute in solid phase in dendritic region,which are negative segregation elements.The distribution coefficients of element Zr,Mo,Ti,Ta and Cr are less than 1,indicating that they tend to distribute in liquid phase in interdendritic region,which are positive segregation elements.The element Al tends to distribute to liquid phase at the beginning of the solidification,and then be prone to concentrate in the solid phase in the dendritic region and transform to a negative segregation element followed by the solidification temperature decreased.The distribution coefficient of element Al and Ni increases but Mo and Cr decreases followed by the solidification temperature decreasing in 1325～1210 ℃ range.On the contrary,The distribution of element Al and Ni decreases but Mo and Cr increases followed by the solidification temperature decreasing in 1210～1180 ℃ range.