Linking the Properties, Processing and Chemistry of Advanced Single Crystal Ni-Base Superalloys

Abstract

As nickel-base single crystals are being implemented in gas turbine engines operating at temperatures in excess of 1600K, maintaining structural properties at these elevated temperatures has become an increasingly important problem. Typically, elevated levels refractory alloying elements are added to these single crystal alloys to enhance the degree of solid solution strengthening within the microstructure. The presence of Re, W, Mo, Ta in Ni-base superalloys strongly influences parameters, such as the stacking fault energies, lattice misfit, and shear modulus of the crystalline lattices, that govern the creep response of the material. However, this is a challenge when dealing with high refractory alloys that are susceptible to the precipitation of deleterious topologically-close-packed (TCP) phases. Recent investigations have demonstrated Ru additions to be beneficial with respect to hindering the formation of these TCP phases and consequently improving the creep properties of these advanced single crystal alloys. Substantial effort has been dedicated towards understanding the fundamental effects and the mechanisms by which Ru additions alter the structure of the γ and γ' phases. Much of this knowledge has been effectively applied to the development of a new class of single crystal Ni-base superalloys with improved strength and a temperature capability significantly higher than those of existing "second" or "third" generation alloys. Nevertheless, optimization of the alloy chemistries remains notoriously difficult as various component design requirements related to the solidification characteristics, overall blade density, coating compatibility and cost must also be carefully considered when developing these materials. This investigation addresses some of the issues associated with the development of Ru-bearing, high refractory content Ni-base superalloys and describes how interrelationships between the properties, processing, microstructure and chemistry of these alloys can be balanced to yield a practical "fourth" generation single crystal superalloy.

Recrystallisation behaviour and mechanical properties of a novel Re-containing nickel-base superalloy

“…It should be noted that the superalloy composition is close to that of some single-crystal superalloys and casting superalloys [1,4–8]. The present work demonstrated that heavy alloying of nickel-base superalloys with refractory elements including rhenium is not contraindicative for wrought processing.…”

“…To improve the fuel efficiency of GTE, it is of great importance to develop new Ni-based superalloys to withstand higher operating temperatures and loadings [1–3]. Regarding polycrystalline nickel-base superalloys, which are the most competitive from the viewpoint of the production cost, novel nickel-base superalloys are often designed towards heavier alloying [1–11]. Particularly, the alloy designing approaches include an increase in the volume fraction of the γ′ phase precipitates by increased concentrations of precipitate forming elements [1,2], heavy alloying with elements providing effective substitutional solid solution strengthening and a higher γ′ solvus temperature [1,4–6] etc.…”

Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades

“…In addition, it is also known that there is a strong correlation between the level of refractory alloying additions, such as Mo, Hf, Ta, W and Re in CMSX-10 in this study, and the occurrence of the freckle chains 44) . Therefore, solute segregation probably induces the buoyancy driven convective uid ow and result in the formation of freckle defects during dendritic solidi cation 44) . However, despite much research on the nature of freckle chains, there is little literature showing the boundary of freckle chains and matrix.…”

Design of Laves phase-reinforced compositionally complex alloy

“…As already predicted by the calculations, the literature data supports that Nb and Mo additions to the alloy composition are expected to govern the formation of the TCP Laves phase 26 , 27 . Additionally, it is expected that Nb further increases the APB energy of the -phase 28 , 29 and Mo contributes to the solid solution strengthening (SSS) 17 , 30 of the multi-component matrix, further contributing to material strength at elevated temperatures. It is also reported that both elements increase the lattice parameter in the CCA’s matrix 31 , leading to a lower lattice misfit between the matrix and the -phase, as the latter commonly exhibits a larger lattice parameter in these types of alloys 3 , 32 .…”