Abstract The microstructure degradation and subsequent phase transformations in Waspaloy nickel-based superalloy during thermal exposure at 780 °C for 10,000 h were investigated. Two paths of η phase formation in the centre of extra-large γ’ (EL-γ’) following the formation of EL-γ’ were observed: (i) η phase directly precipitated within EL-γ’ when the coalescence of γ’ reached a critical stage; (ii) η phase precipitated at the interface of small size MC carbide and EL-γ’, with both MC and η embedded inside EL-γ’. The phase transformation process including the formation of EL-γ’ were experimentally observed and the formation sequences were schematically suggested. Two criteria of η formation and growth within EL-γ’ were established: (i) stacking faults formation in the nucleation site and (ii) sufficient atom diffusion during nuclei growth. The study of kinetics of η formation through two different paths revealed the critical role of small size carbides in promoting η nucleation and growth. It is concluded that η formation may be suppressed by controlling the size and density of MC carbides during materials processing.

中文翻译：

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镍基高温合金长期热暴露过程中η相的形核和生长

摘要 研究了 Waspaloy 镍基高温合金在 780 °C 下 10,000 小时的热暴露过程中的微观结构退化和随后的相变。观察到EL-γ'形成后在超大γ'（EL-γ'）中心形成η相的两条路径：（i）当γ'聚结时η相直接沉淀在EL-γ'内到了关键阶段；(ii) η 相在小尺寸 MC 碳化物和 EL-γ' 的界面处析出，MC 和 η 均嵌入 EL-γ' 内部。实验观察了包括 EL-γ' 形成在内的相变过程，并示意性地提出了形成顺序。建立了 EL-γ' 内 η 形成和生长的两个标准：(i) 成核位点中的堆垛层错形成和 (ii) 原子核生长过程中足够的原子扩散。通过两种不同途径对 η 形成的动力学研究揭示了小尺寸碳化物在促进 η 成核和生长中的关键作用。结论是，在材料加工过程中控制 MC 碳化物的尺寸和密度可以抑制 η 的形成。