Creep properties beyond 1100 °C and microstructure of Co–Re–Cr alloys

The melting point of a novel Co–17Re–23Cr alloy (numbers given in at.%) could be increased by 250 °C as compared to established Ni-based superalloys, by optimising the content of Re. Samples were produced by vacuum arc-melting in order to evaluate the creep behaviour at temperatures beyond 1100 °C and for microstructural analysis. Three alloys (the Co–17Re–23Cr-based material, and the carbide strengthened alloys Co–17Re–23Cr–2.6C and Co–17Re–23Cr–2.6C–1.2Ta) were investigated. Creep properties, especially the minimum creep rate and the Larson–Miller plots, were compared. The Co–17Re–23Cr–2.6C–1.2Ta alloy has a higher minimum creep rate than Co–17Re–23Cr at 1200 °C but it has a lower minimum creep rate than Co–17Re–23Cr at 1100 °C. TaC coarsening, detected via transmission electron microscope (TEM) measurements may explain this effect. The overall creep behaviour of Co–17Re–23Cr–2.6C at 1200 °C is better than that of Co–17Re–23Cr–2.6C–1.2Ta, but worse than that of Co–17Re–23Cr. tructural investigations by scanning electron microscopy and TEM reveal a hexagonal closed-packed (hcp) matrix and σ-phases. The microhardness of the σ-phase was about 1570 HV (load: 1 g) and around 800 HV for the matrix. Pores and cracks occur along the brittle σ-phases and grain boundaries in the Co–Re–Cr alloys. A Norton exponent n in between 1.4 and 3.0 points to grain boundary dominated creep mechanisms.