Investigation of high-temperature homogenization of a single crystal nickel-based superalloy of the 3rd generation

Abstract

High-alloyed nickel-based superalloys used for casting of single crystal blades of aircraft gas-turbine engines have to be subjected to a long-term high-temperature homogenization annealing for dissolution of nonequilibrium γ′-eutectics and smoothing of dendritic segregation of alloying elements. Single crystal nickel-based superalloy of the 3-rd generation CMSX-10 developed by Cannon-Muskegon corporation for casting highly loaded uncooled single crystal blades of the first stage of a high pressure turbine of an aviation gas turbine engine Trent 800 belongs to such alloys. In the present work, the processes occurring during high-temperature homogenization annealing of this alloy have been investigated. It is shown that the kinetics of dissolution of nonequilibrium γ′-eutectics strongly depends on temperature: in the range of 1340 – 1360 °C the dissolution rate increases by about an order of magnitude with increasing temperature by 10 °C. Two types of homogenization annealing with a stepwise increase of temperature proposed for CMSX-10 by Cannon-Muskegon have been investigated: a long 45-hour annealing with 10 steps of temperature increase and a shortened 20-hour annealing with 6 temperature steps. The change in the solidus temperature of the alloy during the homogenization was studied by the method of measurement of the specific electrical resistance. It is shown, that during homogenization annealing of the both types the nonequilibrium eutectics is completely dissolved, and a longer 45-hour annealing provides a deeper chemical homogenization of the alloy. This is especially important for such alloying element as rhenium, which has a high segregation coefficient and very slow diffusion mobility in nickel. Deeper homogenization provides better thermal stability of the alloy microstructure and consequently higher long-term creep strength. A negative effect of high-temperature homogenization is increased porosity resulting from the dissolution of γ′-eutectics. Homogenization porosity can be reduced by optimizing the alloy composition and reducing the fraction of nonequilibrium γ′-eutectics.