Effect of Cr and Ni on the microstructural evolution in Co-Re-Cr-Ni alloys

The present study focuses on the microstructural evolution in Co-Re-Cr-Ni alloys at high temperatures. Four alloys with different Cr and Ni contents were studied. The microstructural examinations were performed using complementary methods of scanning electron microscopy, transmission electron microscopy, differential scanning calorimetry and neutron diffraction. Based on the experimental results and assisted by thermodynamic calculations, the effect of Cr and Ni on the microstructure could be identified. While both Ni and Cr, but Cr only if Ni is also present in the alloy, stabilize the fcc phase, the impacts of these elements on the sigma phase formation are different. It was found that the temperature stability of the sigma phase enhances with increasing Cr content, whereas Ni reduces the required Cr concentration for the sigma phase formation. The evaluation of the Cr and Ni effects on the microstructural evolution contributes to a fundamental understanding of the alloy behaviour and, thus, is a key aspect for successful alloy development.