Despite the existence of previous studies on the heat treatment of the MAR-M247 superalloy, there is a lack of microstructural characterization data that support the heat-treatment conditions that are proposed in this study. Thus, the aim of this study is to investigate the changes in microstructure that occur in this alloy when subjected to different solutioning and aging heat treatments. Thermodynamic calculations and differential thermal analysis guided the experimental design and the analysis of experimental results. The MAR-M247 superalloy was produced via vacuum induction melting and investment casting. The samples were solutioned between 1185 and 1270 °C and aged between 770 and 980 °C. The as-cast and heat-treated samples were characterized using scanning electron microscopy in backscattered electron and secondary electron modes. Thermodynamic calculations have shown that the minimum solutioning temperature is approximately 1220 °C, occurring in a γ + MC + MB2 three-phase field (M = metal). The samples were solutioned at 1250 °C for 310 min before aging heat treatment. During solutioning the carbide composition is the MC phase shifts to higher hafnium (Hf) and lower tantalum (Ta) content, which is in agreement with the thermodynamics calculations. After solutioning, residual aluminum (Al) segregation leads to the formation of large γ′ particles in certain regions of the material following one-step aging heat treatment at 770 and 870 °C. However, a nearly uniform size distribution of γ′ particles was observed after aging at 980 °C as well as after double aging heat treatment at 980 °C for 300 min + 870 °C for 1200 min.

Solutioning and Aging of MAR-M247 Nickel-Based Superalloy

A 2nd stage turbine blade from an aircraft engine, which was made of wrought Ni-based superalloy GH4033, was investigated after service exposure of 1600 engine operating hours with regard to multiple microstructural degradations and the corresponding stress rupture lives. Thermal exposure experiments were also carried out to simulate the microstructural evolution of GH4033 alloy. The gradual change of GB carbide morphology and coarsening of γ′ precipitates were observed after thermal exposures at 650 °C and 700 °C for 100–2000 h, and the corresponding TTP diagram of GB carbides as well as the coarsening kinetic equation of γʹ precipitates were determined. In this study, apparent microstructural degradation in terms of grain growth, GB carbide degradation, coarsening of γʹ precipitates and dislocation substructure as well as obvious reduction of creep life were not identified in the serviced blade after service exposure. The service temperature from the middle to the tip of the airfoil was estimated to be 650–700 °C based on microstructural features of γʹ precipitates and GB carbides.

H. Yoshizawa

Papers

Microstructural changes in a Ni-base superalloy during service