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

The influence of a 3wt% 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-0.9Y(2)O(3) (3Re alloy) and a non-rhenium containing (ORe) 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 gamma' precipitates a re formed, which force the mobile dislocations at the gamma-gamma' 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 contrast, lower creep strength was observed for ORe alloy because a dislocation looping mode is dominant with coarser and more irregularly shaped gamma' 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 3wt% Re addition significantly retards gamma' coarsening kinetics. Rhenium acts as a rate-controlling species upon the volume diffusion-controlled coarsening process because it is a heavy element and also it almost solely partitions to the gamma matrix. X-ray diffraction experiments showed that the magnitude of the lattice mismatch between gamma and gamma' increased with the 3wt% Re addition from 0% to -0.26% at room temperature. Increased lattice mismatch for 3Re alloy causes the formation of more aligned and cuboidal gamma' precipitates rather than random and odd-shaped gamma' precipitates for ORe alloy, and it also accelerates the coalescence between cuboidal gamma' precipitates. (C) 1998 Kluwer Academic Publishers.