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

Abstract

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 (μm 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 °C but the Ta carbides are more stable than the Cr carbides at 1200 °C after long-time ageing. They are also effective as strengthening precipitates.