Microstructure, Lattice Misfit, and High-Temperature Strength of γ′-Strengthened Co-Al-W-Ge Model Superalloys

The quaternary alloy system Co-Al-W-Ge was investigated and it was found that a continuous \(\gamma /\gamma ^\prime \) two-phase field extends between the systems Co-Al-W and Co-Ge-W. All alloys examined comprised cuboidal L1\(_2\) precipitates coherently embedded in an A1 matrix. Differential scanning calorimetry measurements revealed that the liquidus, solidus, and \(\gamma ^\prime \)-solvus temperatures decrease when the Ge content is increased. The lower liquidus temperature and the capability of \(\gamma ^\prime \)-strengthening in the Ge-rich alloys make them interesting as potential candidates for brazing applications of Co-base superalloys. The \(\gamma /\gamma ^\prime \) lattice misfit was determined by high-resolution X-ray diffraction and found to be positive for all alloys investigated, decreasing with increasing Ge content. The mechanical properties of the Al-rich alloys surpass those rich in Ge.