Kinetics of rafting in a single crystal superalloy: effects of residual microsegregation

Abstract

The kinetics of directional γ′ coarsening, known as rafting, are examined in the nickel based single crystal superalloy CMSX-4. Electron microscopy and image analysis are used to characterise heat treated specimens, prestrained in creep to a plastic strain beyond the threshold required to initiate rafting. The kinetics of rafting are found to be sensitive to the presence of microsegregation from casting, incompletely removed by the heat treatment process; thus the dendritic regions raft appreciably faster than the interdendritic ones – this despite rhenium enrichment at the dendrite cores consistent with its preferred partitioning during solidification. Estimates of the activation energy required for rafting in the two regions are established and Avrami-type equations developed. Since the important γ′ forming elements aluminium and tantalum are known to partition interdendritically causing an enhanced γ′ fraction, it is suggested that the correspondingly smaller γ channel width so caused is responsible for the greater resistance to rafting displayed by the interdendritic regions.