Analysis Of Gamma ' Precipitates, Carbides And Nano-Borides In Heat-Treated Ni-Based Superalloy Using Sem, Stem-Edx, And Hrstem

The microstructure of a Rene 108 Ni-based superalloy was systematically investigated by X-ray diffraction, light microscopy, energy-dispersive X-ray spectroscopy, and electron microscopy techniques. The material was investment cast in a vacuum and then solution treated (1200 degrees C-2h) and aged (900 degrees C-8h). The gamma matrix is mainly strengthened by the ordered L1(2) gamma ' phase, with the mean gamma/gamma ' misfit, delta, +0.6%. The typical dendritic microstructure with considerable microsegregation of the alloying elements is revealed. Dendritic regions consist of secondary and tertiary gamma ' precipitates. At the interface of the matrix with secondary gamma ' precipitates, nano M5B3 borides are present. In the interdendritic spaces additionally primary gamma ' precipitates, MC and nano M23C6 carbides were detected. The gamma ' precipitates are enriched in Al, Ta, Ti, and Hf, while channels of the matrix in Cr and Co. The highest summary concentration of gamma '-formers occurs in coarse gamma ' surrounding MC carbides. Borides M5B3 contain mostly W, Cr and Mo. All of MC carbides are enriched strongly in Hf and Ta, with the concentration relationship between these and other strong carbide formers depending on the precipitate's morphology. The nano M23C6 carbides enriched in Cr have been formed as a consequence of phase transformation MC + gamma -> M23C6 + gamma ' during the ageing treatment.