Analysis of atomic-scale phenomena and the rhenium effect in nickel superalloys

Since the 1980s, small additions of rhenium have been found to markedly improve the creep properties of nickel based superalloys. However, the underlying causes for this effect are not yet fully understood. In this work, a variety of experimental and modelling techniques have been used to evaluate possible mechanisms improving creep properties at high temperature. Ab initio modelling (via the Density Functional Theory) has shown that rhenium clusters are unstable in the nickel lattice. This observation was confirmed by Extended X-ray Absorption Fine Structure, which showed that rhenium atoms are dispersed in the nickel lattice. Atom Probe Tomography also found no clusters in Ni-Re alloys and in CMSX-4, a commercial single-crystal superalloy. Atom Probe Tomography, in conjunction with Phase Field Modelling, was also used to show that the rhenium enrichment close to the [gamma]/[GAMMA] interfaces is formed upon cooling from service temperature. Thus means that the rhenium enrichment observed with the atom probe does not play a role in the strengthening effect played by rhenium. Finally, ab initio modelling (via the Density Functional Theory) was used to investigate the effect of rhenium on the stacking fault energy of nickel to ascertain whether rhenium content could increase the spacing between Shockley partial dislocations. The present work shows that rhenium clustering, the rhenium enrichment close to the [gamma]/[GAMMA] interfaces and the rhenium effect on stacking fault energy cannot be the mechanisms underlying the rhenium-effect in nickel based superalloys. It therefore appears that single atoms of rhenium may slow down dislocation motion in the [gamma] phase of nickel based superalloys. This is supported by the observation that rhenium is the slowest diffusing element in nickel and it is therefore expected to slow down all diffusion activated processes, such as the well-established climb-assisted glide of dislocations in the [gamma] phase of nickel based superalloys.EThOS - Electronic Theses Online ServiceEPSRCU.S. Dept. of Energy. Scientific User Facilities DivisionGBUnited Kingdo