Microstructural Characterization of XH 67 Nickel-Based Superalloy Under Different Heat Treatment Conditions

XH 67 is a nickel-based superalloy, which is used in critical parts of modern aerospace engines. In the present study, XH 67 specimens in five different heat treatment conditions selected based on the processing and application have been investigated for their microstructures, morphology, and composition of the matrix and second-phase particles. Optical microscopic (OM) analysis revealed the presence of bi-model neck-lace type grains and equiaxed grains as a result of different heat treatment conditions. Scanning electron microscopy (SEM) and elemental composition analysis revealed precipitates and second-phase particles. Carbides such as Ti (N, C) and (W, Mo, Ti) C were observed having different sizes and shapes. Cr-rich carbide phases were found to be preferentially segregated along the grain boundaries. X-ray diffraction analysis was carried out for assessing the phases present in the alloy. X-ray diffraction analysis revealed the gamma matrix and the precipitate phase are crystallographically identical (coherent). Transmission electron microscopic (TEM) analysis of the specimens confirmed the presence of uniformly distributed spherical Ni3 (Al, Ti) precipitates which impart thermal stability to the superalloy. Presence of different types of carbides is also revealed by TEM analysis.