Single crystalline γ′-strengthened Co-base superalloys – Alloy development, heat treatment strategies and mechanical properties

The γ′-hardened Co-base superalloys are a rather novel class of materials (only discovered in 2006). However, its fundamental characteristics are similar to those of Ni-base superalloys, which are known for more than 60 years. Ni-base superalloys are especially used as turbine blades or disks in aircraft engines und stationary gas turbines. In the regions of highest gas temperatures, they are mainly used as single crystals. The absence of grain boundaries and the orientation of the turbine blades along specific crystallographic directions significantly decrease the velocity of diffusion processes and thermal stresses. One apparent aspect that Ni- and Co-based alloy systems have in common is their microstructure, consisting of a face-centered cubic (fcc) solid solution matrix phase and a high-volume fraction (about 70 %) of coherent intermetallic L1₂ precipitates usually exhibiting a cuboidal particle shape. Despite many similarities, the novel Co-base superalloys also exhibit significant differences compared to the Ni-base superalloys and can therefore be used to gain a more comprehensive understanding of the underlying mechanisms that are responsible for the excellent properties of superalloys. One of these differences, for example, is the sign of the lattice misfit. In contrast to most of the Ni-base superalloys, Co-base superalloys have a positive lattice misfit (lattice parameter of the precipitate phase is higher than the lattice parameter of the matrix phase).

This project is integrated in the Sonderforschungsbereich/Transregio 103 „ From atoms to turbine blades – a scientific basis for a new generation of single crystal superalloys“. The aim of this project is to investigate whether and to what extent the new γ′-strengthened Co-base superalloys can compete with or even replace Ni-base superalloys or conventional carbide- and solid solution-hardened Co-base alloys. In this project (B3), fundamental investigations are carried out on the designed alloys regarding

• Castability
• Segregation behavior, diffusivity, elemental partitioning behavior and solid solution hardening effects of various alloying elements
• Microstructure and phase stability
• Lattice misfit between the γ phase (matrix) and the γ′ phase (precipitates).
• High temperature strength and underlying deformation mechanisms.

Furthermore, the strong networking of project B3 within the SFB/TR 103 enables the characterization of the developed alloys using state-of-the-art methods such as 3D atom probe tomography (A4) or high-resolution electron microscopy (A7). Two further projects also deal with oxidation and corrosion properties (A5) and develop potential protective coatings (B6) for the alloys developed within this project B3.