Beyond Ni-base superalloys: Influence of Cr addition on Co-Re base alloys strengthened by nano-sized TaC precipitates

The high-temperature Co-Re alloy [1], a promising candidate for gas turbine applications, has a complex microstructure containing various kinds of precipitates in different size scales. The nano-sized fine spherical TaC (< 100 nm) precipitates are the most important for strengthening of the alloy. In this contribution the influence of Cr addition (5 at.% and 15 at.%) on the strengthening TaC precipitates in Co-Re-Ta-C alloy with a C/Ta ratio of 0.9 is studied. The Cr content is important for the oxidation resistance of the alloy, but it also influences the size and volume fraction of TaC precipitates and consequently the possible formation of the Cr-rich Cr2Re3 (sigma phase) and Cr23C6 precipitates. In addition, Cr influences the temperature of the allotropic phase transformation of the Co matrix phase. It was found that additions of 5 at.% or 15 at.% Cr in the alloy does not result in the formation of Cr2Re3 and Cr23C6 phases. Further, the addition of such smaller amounts of Cr does not retain the Co gamma (fcc) phase (stable or metastable) at room temperature after cooling from high temperatures. Only the e (hcp) phase is present at low temperatures. The TaC precipitates are slightly influenced in size and volume fraction with 5 at.% and 15 at.% Cr.