The influence of high temperature annealing and creep on the microstructure and chemical element distribution in the γ, γ′ and TCP phases in single crystal Ni-base superalloy

The development of the microstructure in the CMSX-4 superalloy caused by high temperature annealing and creep and its relation with the chemical composition evolution of the γ, γ′ and topologically close-packed (TCP) phases was investigated at the micro- and nanoscales. The samples were creep deformed at a temperature of 900 °C and 1000 °C without previous treatment, and after pre-annealing at a temperature of 1100 °C. The changes in microstructure and elemental composition of phases were investigated using analytical electron microscopy, electron tomography and X-ray diffractometry methods. The combination of the different imaging techniques with the high precision of chemical composition microanalysis in nanoareas allowed to examine the compositional changes which accompany coalescence of the γ′ particles as well as precipitation and growth of the TCP phases in single-crystal nickel base superalloys. It was determined that in the temperature range of 900-1100 °C the P and μ phases with TCP structure are present in the CMSX-4 superalloy. The μ phase exhibits almost perfect crystal structure, while the P phase contains numerous planar defects. Precipitation of the TCP phases at the expense of the γ phase dissolution in their vicinity leads only to the localized depletion in refractory elements in the surrounding γ′ phase. The concentration of these elements in the γ phase remains on the similar level close to the TCP phases and away from them.