INFLUENCE OF PHASE-COMPOSITION AND MICROSTRUCTURE ON THE HIGH-TEMPERATURE CREEP-PROPERTIES OF A MODEL SINGLE-CRYSTAL NICKEL-BASE SUPERALLOY - AN ATOM-PROBE AEM STUDY

Atom probe field ion microscopy (APFIM) and analytical electron microscopy (AEM) have been used to characterize the microstructure of as-quenched and creep-tested model single crystal nickel-base superalloys having the nominal bulk composition Ni-x at. % Mo-2.0% Ta-13.7% Al (x = 7.8-9.2%). The results showed that Ta partitioned preferentially to the gamma' phase whereas Mo partitioned to the gamma matrix. Atom probe composition profiles across gamma-gamma' interfaces did not reveal any evidence of solute segregation and statistical analyses of the data did not indicate ultrafine solute clusters in the gamma matrix. Acicular delta phase precipitates were detected in number densities approximately one order of magnitude higher in the 9.2% Mo alloy as compared to the 8.6 and 8.8% Mo alloy. There was some indication that the DO22-ordered gamma('') phase is stable at 982 degrees C, the temperature at which the creep tests were performed. The absence of interfacial solute segregation and clustering in the gamma matrix suggested that the high creep life of this model alloy system is influenced by the nature of the dislocation network at the gamma-gamma' interface with a possible strengthening contribution from the DO22-ordered phase.