铸态GH742合金中存在复杂的第二相组织和严重的枝晶偏析,Nb、Ti大量富集于枝晶间和MC碳化物、 Laves相、δ相、(γ+γ´)等析出相中.本文确定了γ基体的熔化温度为1160℃,并通过不同的均匀化处理确定了γ´相、(γ+γ´)共晶、MC碳化物、Laves相、δ相、Ni5Ce相及含氧硫稀土相等析出相的溶解温度.由于稀土相在1120℃发生熔化,提出一种GH742合金低温预处理加高温扩散的二次均匀化制度来扩大其均匀化温度区间,提高扩散退火温度, 从而获得均一的组织. 元素扩散计算结果表明,提高均匀化温度可以提高元素的扩散系数; Nb的偏析系数比Ti的小得多,达到完全均匀化需要很长的时间.
The precipitation of as-cast GH742 alloy is complicated and the dendritic segregation is severe. Niobium and titanium are segregated to the interdendritic region and enriched in the precipitates such as MC carbide, Laves phase, δ phase, (γ+γ´) eutectic. The melting temperature of as-cast GH742 alloy is 1160 determined by differential thermal analysis and metallography, while the soluble temperatures of γ´ phase,(γ+γ´)eutectic, MC carbide, Laves phase, δ phase, Ni5Ce phase and RE-O-S phase were determined by different homogenization treatments. In order to obtain uniform austenitic microstructure, a two-step homogenization treatment via low temperature pretreatment followed by high temperature diffusion is established due to the melting of RE-O-S phase at 1120 , which broadens the window of the homogenization temperature and increases the annealing temperature. The diffusion coefficient of niobium is much smaller than that of titanium, which requires more time to be absolutely homogenized.
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