Recoating and Refurbishment of Single crystal Ni- Based Superalloys for Advanced Gas Turbines

本工作旨在明确单晶镍基高温合金CMSX-2和CMSX-4由于局部塑性变形而导致的异常组织或胞状转变的形成，为建立先进燃气轮机部件的翻新技术奠定基础。在研究过程中，着重从晶体学和热力学的角度对其成核和生长行为进行了认识。探讨了变形与实际破坏损伤之间的关系。进一步研究了局部细胞转变对高温疲劳强度的影响。结果表明：(1)CMSX-2和CMSX-4在局部塑性变形损伤后进行再热处理，其细胞转变为片状或等轴阵列的γ/γ′组织粗化。这不是一种特殊的现象，但它实际上是在材料遭受疲劳和热机械疲劳损伤时再现的。(2)相变的发生和发展具有高度的各向异性，受以下因素的影响：局部塑性变形引起的应变场、晶体取向、再热处理温度和时间以及材料中的微偏析。再加热过程中的γ′-溶剂显著加速了局部再结晶。(3)应变过程中γ′析出物的剪切作用可能在细胞转化中起着内在的作用，当{111}<110>体系被激活时，形成显著。另一方面，当{100}<110>滑移系统被激活时，例如在较高温度下蠕变破坏时，它可以忽略不计。(4)用Johnson-Mehl型方程成功地表示了相变的生长过程。相变的生长必须由A1和Ni的扩散过程控制，其中活化能约为36 kcal/mol。(5)变形区域的小疲劳裂纹扩展速率显著提高。少

This work has been conducted to make clear the formation of the abnormal microstructure, or cellular transformation, resulting from the local plastic deformation in single crystal Ni-base superalloys, CMSX-2 and CMSX-4, in order to get fundamental understanding to establish the refurbishment technology for advanced gas turbine components. During the study special attention was paid to get understandings on the nucleation and growth behavior from viewpoints of crystallography and thermo dynamics. The relationship between the transformation and the actual failure damages was also explored. Furthermore, the effect of the local cellular transformation on high temperature fatigue strength was investigated.The conclusions obtained are summarized as follows :(1) When CMSX-2 and CMSX-4 were subjected to damages associated with local plastic deformation, followed by the re-heat treatments, the cellular transformation associated with γ/γ' microstructure coarsened in lamellar or equiaxed array wa … More s originated. This was not a special phenomenon, but it was actually reproduced in the material subjected to fatigue and thermo-mechanical fatigue damages.(2) The transformation was originated and developed with high anisotropy, being influenced by the following many factors : i.e., the strain field by the local plastic deformation, the crystallographic orientation, the re-heat treatment temperature and time, and the microsegregation in the material. The γ'-solvus in the reheating process drastically accelerated the local recrystallization.(3) The shearing of γ' precipitates in the straining process may play an intrinsic role in the cellular transformation : when the {111}<110> systems were activated, the formation was significant. When the {100}<110> slip systems were activated such as in creep failure at higher temperature, on the other hand, it was negligible.(4) The growth of the transformation was successfully represented by the Johnson-Mehl type equation. The growth of the transformation must be controlled by diffusion process of A1 and Ni, where activation energy was determined by about 36 kcal/mol.(5) The small fatigue crack propagation rate was remarkably enhanced at the transformed region. Less