Homogenization heat treatment and segregation analysis of equiaxed CMSX-4 superalloy for gas turbine components

Abstract

Homogenization heat treatment and segregation analysis of equiaxed CMSX-4 nickel-based superalloy were investigated. The researches were focused on studying the influence of a multi-cycle heat treatment on the evolution of CMSX-4 superalloy in terms of microstructure and porosity. The heat treatment temperature was selected based on the chemical composition of superalloy and thermodynamic calculations of solidification path and phase formation using a thermodynamic software. Elemental partition coefficients were calculated using the local chemical compositions in different areas of the samples, before and after each heat treatment cycle. It was observed that Al, Ti and Ta segregate in the interdendritic regions and eutectic γ–γ′ pools, while W, Re and Co segregate mainly in the dendrite cores and the dendrite arms. The compositional analysis shows an increase in the structure homogeneity after the heat treatment, but a certain compositional gradient between the dendrite cores and the interdendritic regions is still observed. The qualitative analysis based on the backscattered electron images sustains the results obtained from the compositional analysis regarding the homogenization of the alloy. A descending evolution of the cuboidal γ′ phase was observed until 6 h of holding, but after 9 h of holding at 1573 K the size of cuboidal γ′ phase starts to grow especially in the interdendritic regions. After long temperature holding time, a notable increase in porosity level was observed due to the partition coefficients of the alloying elements and dissolution of the residual phases (eutectic γ–γ′ pools, coarse γ′).