对U720LI镍基高温合金连续冷却过程中析出的高温γ' 相的分解现象进行了观察研究. 结果表明,自固溶温度快冷至室温时, 发现有大量细小的γ'$相析出,其尺寸大多位于40-60 nm之间, 均匀分布. 当试样自固 溶温度缓冷至650℃然后快冷至室温时, 发现有两类大小的γ'相析出, 一类在200 nm左右, 为高温析出γ'相; 另一类集中分布在50-65 nm之间,为低温析出γ'相. 重点研究了高温析出γ'相的分解现象, 在高温析出$\gamma'$/$\gamma$相界, 适合非经典的形核和长大的条件, 在高温γ'/γ相界面上的Al和Ti等元素的偏聚区, 可以作为过饱和γ基体析出γ'相的析出核心, γ'相中的Al和Ti的自由能梯度导致高温析出γ'相中Al和Ti等γ'形成元素向细小γ'相核心作短距离扩散,使细小γ'相长大和高温粗大γ'相分解. 此种析出的γ'相的尺寸只有10-20 nm.上述缓冷试样经时效后, 发现高温粗大γ'相的分解现象更加明显.
The decomposition of spherical γ' phase precipitated at high temperature during continuous cooling of U720LI Ni-based alloy has been studied. Fast cooling from supersaturated γ solid solution of the alloy could not inhibit fine γ'precipitates, and the size of the γ' precipitates is about 40-60 nm.When slow cooling to 650℃ and then fast cooling to the room temperature, two kinds of γ' precipitates were found. One was precipitated at high temperature with the size about 200 nm and another was precipitated at low temperature with the size of 50-65 nm in average. Moreover, the big γ' precipitates which precipitated at high temperature were found to be decomposed to form finer γ' precipitates. Because the high temperature γ'\γ interface offers necessary condition for non-classical nucleation theory, any segregation area of γ' formation elements at the high temperature γ'/γ interface can act as pre-existed γ'nuclei and thus promote the precipitation of finer γ' from the saturated γ matrix. Through the short-distance diffusion of the γ' formation elements from the high temperature γ' to pre-existed γ' nuclei, the pre-existed γ' nuclei can grow to be 10-20 nm in size. At the same time, the high temperature γ' precipitates became instable due to loss of γ' formation elements and began to be decomposed as a result. The subsequent ageing promotes the decomposition of the big high temperature γ'.
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