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

Abstract

The microstructure of a René 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 °C-2h) and aged (900 °C-8h). The γ matrix is mainly strengthened by the ordered L12 γ′ phase, with the mean γ/γ′ misfit, δ, +0.6%. The typical dendritic microstructure with considerable microsegregation of the alloying elements is revealed. Dendritic regions consist of secondary and tertiary γ′ precipitates. At the interface of the matrix with secondary γ′ precipitates, nano M5B3 borides are present. In the interdendritic spaces additionally primary γ′ precipitates, MC and nano M23C6 carbides were detected. The γ′ precipitates are enriched in Al, Ta, Ti, and Hf, while channels of the matrix in Cr and Co. The highest summary concentration of γ′-formers occurs in coarse γ′ 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 + γ → M23C6 + γ′ during the ageing treatment.

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

“…The STEM-EDX measurements showed that borides in the Rene´108 superalloy have significant amounts of W and Mo, which could increase their solvus temperature and consequently solidus temperature. [27] B. Influence of the Short-Term Annealing Temperature on the c/c¢ Misfit Coefficient…”

“…Dendrite cores and arms consist of secondary and tertiary c¢ precipitates and (Cr, W) 5 B 3 nanoborides with a size around 50 nm at the c/c¢ interfaces. [27] In interdendritic regions, primary c¢ precipitates, (Ta, Hf)C carbides with various morphologies and M 23 C 6 nanocarbides, have additionally been detected. The M 23 C 6 nanocarbides form during aging, according to the phase transformation reaction MC + cfiM 23 C 6 + c¢.…”

“…As the microstructure within the interdendritic regions is complex and far from the equilibrium state, special attention was given to the dendrite cores. [27] II. METHODOLOGY…”

Effect of γ′ Phase Elements on Oxidation Behavior of Nanocrystalline Coatings at 1050 °C

“…The γ′ phase is the precipitation strengthened phase with a stoichiometric ration of Ni 3 Al. Al, Ti, Ta, Nb, and so on can be dissolved, which are named as γ′ phase elements [ 8 , 9 , 10 , 11 , 12 ]. With the rapid consumption of a large number of alloying element on the high-temperature environment, the original stable γ/γ′ structure will be destroyed [ 10 , 11 ].…”

“…Al, Ti, Ta, Nb, and so on can be dissolved, which are named as γ′ phase elements [ 8 , 9 , 10 , 11 , 12 ]. With the rapid consumption of a large number of alloying element on the high-temperature environment, the original stable γ/γ′ structure will be destroyed [ 10 , 11 ]. Therefore, the long-term service performance of the superalloy in harsh temperatures is often related to its composition components and constitute elements [ 12 ].…”