Influence of rhenium on the microstructures and mechanical properties of a mechanically alloyed oxide dispersion-strengthened nickel-base superalloy

Abstract

The influence of a 3 wt% Re addition on the creep strength and microstructure of a mechanically alloyed and oxide dispersion-strengthened nickel-base superalloy was investigated. Two alloys, Ni–8Cr–6.5Al–6W–3Ta–1.5Mo–6Co–1Ti–3Re–0.15Zr–0.05C–0.01B–0.9Y₂O₃ (3Re alloy) and a non-rhenium containing (0Re) alloy were prepared for this study.

The 3Re alloy showed two-fold improvement in creep life compared with that of 0Re alloy, presumably due to a change in the mode of the precipitate-dislocation interaction. For the 3Re alloy, finer, more cuboidal and aligned γ′ precipitates are formed, which force the mobile dislocations at the γ–γ′ interfaces to cut precipitates in order to proceed. Shearing of precipitates is evinced by the existence of stacking faults and results in an increase of creep strength. In constrast, lower creep strength was observed for 0Re alloy because a dislocation looping mode is dominant with coarser and more irregularly shaped γ′ precipitates present in this alloy. Another possible explanation for an improved creep strength of 3Re alloy is related to the tangled dislocation structure formed by the interaction between glide dislocation and interfacial dislocation, which also acts as an effective barrier for further glide dislocation motion. A 3 wt% Re addition significantly retards γ′ coarsening kinetics. Rhenium acts as a rate-controlling species upon the volume diffusion-controlled coarsening process because it is a heavy elemenet and also it almost solely partitions to the γ matrix. X-ray diffraction experiments showed that the magnitude of the lattice mismatch between γ and γ′ increased with the 3 wt% Re addition from 0% to −0.26% at room temperature. Increased lattice mismatch for 3Re alloy causes the formation of more aligned and cuboidal γ′ precipitates rather than random and odd-shaped γ′ precipitates for 0Re alloy, and it also accelerates the coalescence between cuboidal γ′ precipitates.