Influence of high-temperature annealing on morphological and compositional changes of phases in Ni-base single crystal superalloy

Microstructure evolution during high temperature annealing of the CM186LC single crystal nickel-base superalloy in the temperature range of 900-1100 °C for up to 2500 h was investigated. Microstructural analyses were aimed at observing the changes in morphology of precipitates, in both two dimensions (2D) and three dimensions (3D). The changes in the chemical composition of constituting phases, caused by annealing, were also determined. Although no external stress was applied, after annealing at 1000 °C and 1100 °C the directional coalescence of the γ' phase in directions parallel to secondary dendrite arms occurred. In the temperature range of 900-1100 °C, decomposition of the primary MC carbides took place. The primary carbides were fragmented into small plate-shaped particles, and secondary carbides were also formed. Additionally, in the investigated temperature range, the precipitates of topologically close packed (TCP) phases were found. Those precipitates, plate-like in shape, grew up to a length of several micrometres. Coalescence of the γ′ phase and formation of TCP precipitates during high temperature exposure of CM186LC was accompanied by decreases in the concentrations of Co, Cr, W and Re in the γ phase.