Nickel-Based Single Crystal Superalloys with Different Rhenium Contents

Abstract

Nickel-based single crystal superalloys were widely used in modern aero-engine turbines and large industrial gas turbins due to their excellent mechanical properties and outstanding oxidation and hot corrosion resistance. The continuous demand of the gas turbine engine manufacturers for an increasing turbine inlet temperature had pushed the alloy designers to develop superalloys with higher mechanical strength. Re was a crucial element in the history of single crystal superalloy development. In this article, the development history of nickel-based single crystal superalloys was briefly described, the role of Re in nickel-based single crystal superalloys, including its directly and indirectly effects on the microstructure, creep property, high temperature oxidation and hot corrosion behavior were reviewed. It was found that Re addition gathered mainly in the γ matrix and its solubility in the γ' phase was very low, in spite of that there was still controversy on Re distribution in γ: in the form of Re-clusters, or in the γ/γ' interface, or distributed uniformly in γ. The addition of Re had two main influences on the alloy microstructure. On the one hand, Re addition affected the growth rate and morphology evolution of γ' phase at high temperature. The addition of a certain amount of Re delayed the coarsening of γ' and maintained the cubic morphology of them. On the other hand, Re addition changed the distribution ratio of other elements in γ phase and γ' phase. Re promoted the distribution of Al into the γ' phase, and the distribution of Cr, Ta and W into the γ matrix. The altered distribution ratio also affected the morphology of γ'. Re addition significantly improved the creep performance of single crystal superalloys. The size, shape and morphology of γ' phase during long-term service were the main factors that influenced the mechanical property of nickel-based superalloys. The following mechanisms were put forward to explaining the improved creep performance by Re addition: (1) distribution of Re in γ phase caused solid solution strengthening of the substrate, (2) Re changed the γ/γ' lattice misfit to more negative value, (3) large atomic radius of Re lowered down the diffusion rate of other elements, retarded the dislocation motion and coarsening of γ', (4) Re clusters acted as obstacles of dislocation movement and put off the γ' rafting, (5) Re segregation strengthened the γ/γ' interface. The interfaces decorated with higher Re concentration had higher density of dislocation network and could effectively prevent the shearing of the dislocations. The mechanisms, however, still remained controversial. For coatings, the oxidation rate constant was reduced and the oxidation resistance was significantly improved with an appropriate amount of Re addition. The possible mechanisms were reduced depletion rate of β-NiAl, improved stability of α-Cr and subsequent increased adhesion between the protective α-Al2O3 scale and the substrate, and enhanced transformation rate from θ-Al2O3 to α-Al2O3. For bare superalloys, beneficial and deteriorated effects of Re addition were both reported. The deteriorated effect was supported by observation of discontinuous or porous α-Al2O3 scale caused by aggravated micro-segregation of Al elements or formation of volatile Re2O7 raised by Re addition. The opposite results, however, were reported by other researchers. It was probably because Re could slow down the diffusion rate of Cr in the alloy, promote the formation of continuous α-Al2O3 scale, and inhibit the formation of nitrides in the alloy. The Cr2O3 scale formed on alloys with high Re content was denser than that on Re-free alloys. Moreover, Re promoted the formation of TiO2 and NiTiO3. The reason was probably attributed to the increased activities of elements Cr and Ti caused by the Re addition, which allowed more Cr to diffuse outward to heal the cracks in the protective scale. However, the distribution of Re in the alloys and its mechanisms on the properties still requires in-depth research. The effect of Re addition, the optimization of Re alloying, the interaction of Re and other elements and the development of new Re-bearing alloys were still the direction of numerous researchers. Computer-assisted method and high-throughput method were prospected to provide more effective ways for single crystal superalloy research and development.