Carbides in Co-Re-Cr-based high-temperature alloys

Co-Re-base alloys with very high melting point are being developed to supplement Ni-superalloys in future gas turbines in which much higher gas entry temperatures are expected. The microstructure of Co-17Re-23Cr-2.6C and Co-17Re-23Cr-1.2Ta-2.6C alloys has been investigated by scanning and transmission electron microscopy. These alloys are mainly strengthened by carbides of Cr and/or Ta, but it is not trivial to accurately quantify the composition of the carbides. The compositions of the various carbides in Co-Re-base alloys were quantified with near-atomic resolution using atom probe tomography. The Cr as well as Ta carbides exists in the alloys with different morphologies, ranging from extremely fine (nm scale) to large (mu m scale) sizes. The composition and the crystal structure of the investigated phases were measured and reported. It is shown that both types of carbides are stable up to 1000 degrees C but the Ta carbides are more stable than the Cr carbides at 1200 degrees C after long-time ageing. They are also effective as strengthening precipitates.