Dendritic segregation and arm spacing in directionally solidified CMSX-4 superalloy

Abstract

Dendritic segregation of directionally solidified columnar and single-crystal CMSX-4 was investigated. The partition coefficients derived from EDS measurements on as-cast samples in both transversal and longitudinal sections have shown strong segregation tendency of elements either to dendrite cores (Re, W) or to the interdendritic regions (Al, Ti, Ta). The dendritic segregation as a function of various secondary dendrite arm spacings (SDAS) has highlighted that Ti segregation is more sensitive to the SDAS than that of the other elements. It was also found that Cr has a tendency to accumulate at the coarse gamma prime–matrix interface. The solidification partition coefficients calculated with Pandat™ and the PanNi thermodynamic database showed a good agreement with the experimental data. Pandat™ calculations using the local chemical composition of segregated areas were also used for the optimisation of the solutioning heat treatment window. Primary (λ1) and secondary (λ2) arm spacings as important parameters that influence the dendritic segregation were measured for single-crystal rods at different distances from the cooling plate. Fitting equations were established for the experimental λ1 and λ2 as a function of cooling rate, the V−0.25G−0.5 product and the local solidification time. Analytical calculations of the dendrite arm spacing compared with the experiments and ProCAST calculation showed good agreement.