Investigation of the γ′ Precipitates Dissolution in a Ni-Based Superalloy During Stress-Free Short-Term Annealing at High Homologous Temperatures

The equiaxed Ni-based superalloy Rene 108 was subjected to short-term annealing at five temperatures between 900 degrees C and 1100 degrees C. The phase composition, phase lattice parameters, microstructure, stereological parameters, and chemical composition of gamma ' precipitates were investigated by thermodynamic simulations, X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis of the gamma and gamma ' lattice parameters using the Nelson-Riley extrapolation function showed that the misfit parameter for temperatures 900 degrees C to 1050 degrees C is positive (decreasing from 0.32 to 0.11 pct). At 1100 degrees C, the parameter becomes negative, delta = - 0.18 pct. During the short-term annealing, gamma ' precipitates dissolution occurred progressing more rapidly with increasing temperatures. The surface fraction of gamma ' precipitates decreased with increasing temperature from 0.52 to 0.34. The dissolution of gamma ' precipitates did not only proceed through uninterrupted thinning of each individual precipitate, but also included more complex mechanisms, including splitting. Based on transmission electron microscopy, it was shown that after gamma ' precipitates dissolution, the matrix close to the gamma/gamma ' interface is strongly enriched in Co and Cr and depleted in Al.