Analysis of γ′ Precipitates, Carbides and Nano-Borides in Heat-Treated Ni-Based Superalloy Using SEM, STEM-EDX, and HRSTEM

The microstructure of a Ren&eacute
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 &deg
C-2h) and aged (900 &deg
C-8h). The &gamma
matrix is mainly strengthened by the ordered L12 &gamma
&prime
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&rsquo
s morphology. The nano M23C6 carbides enriched in Cr have been formed as a consequence of phase transformation MC + &gamma
&rarr
M23C6 + &gamma
during the ageing treatment.