Effect of 800 °C/10,000 h aging-induced μ phase on plasticity in Ni-Co-Cr based superalloys

Abstract

Since the concentration of refractory elements, such as W and Mo, increases in superalloys, the brittle μ phases, specifically (Co, Cr)₇(W, Mo)₆, tend to precipitate readily. These μ phases frequently serve as crack initiation sites, leading to a significant reduction in the lifetime of alloys. The present study focuses on the effect of μ phases on room-temperature tensile plasticity of a Ni-Co-Cr-based superalloy containing 4.0 wt% W and 4.0 wt% Mo during long-term aging. The objective is to analyze the precipitation behavior of μ phases and their influence on the elongation. With the precipitation of μ phases, W and Mo diffuse from the γ matrix into the μ phase, and meanwhile, the size and area fraction of μ phases initially increase before stabilizing. Notably, at an aging temperature of 800 °C, Mo exhibits a stronger diffusion ability compared to W. The depletion of solute atoms in the γ matrix, results the weakening of solid solution strength. Therefore, during tensile tests, more intergranular cracks propagate along the γ matrix interface near μ phases and γ' precipitates. Regarding the fracture characteristics, as the size and area fraction of μ phase increase, a greater number of dislocations pile-up near grain boundaries, leading to the formation of numerous creaks through shearing and bypassing. Moreover, the elongation significantly decreases from 17 to 19 % to 8–12 % after aging for more than 1000 h. This work enhances our understanding of μ precipitation and its effect on the material's plasticity.