A DISLOCATION BASED CRITERION FOR THE RAFT FORMATION IN NICKEL-BASED SUPERALLOYS SINGLE-CRYSTALS

A dislocation based criterion is proposed for describing the raft form ation in single crystals of nickel-based superalloys subject to creep at high temperature. Those of the precipitate faces exhibiting disloca tions after the onset of creep are thought to expand by directional co arsening of gamma' precipitates. The proposed criterion predicts corre ctly the raft morphology for creep along crystallographic directions o f high and low symmetry for alloys exhibiting a positive or a negative misfit. A diffusional mechanism based on the effect of creep dislocat ions on the local gamma phase chemical composition is suggested to acc ount for the observed relationship between dislocation activity and di rectional coarsening of precipitates.