Driving force for discontinuous coarsening in a Ni-Al-Mo base superalloy

The driving force and mechanism for discontinuous coarsening (DC) in a Ni-Al-Mo base superalloy were studied. Samples were solutionized at 1300 °C, cooled, and aged at 1150 °C to produce DC. Both the intragranular and DC-produced microstructure were studied using optical and scanning electron microscopy of metallographically prepared surfaces and scanning transmission electron microscopy (STEM) of thin foils. Resulting microstructural and microchemical information were analyzed to elucidate the DC process. Grain boundary diffusion of Mo appeared to control the DC growth rate. The principal contributor to the DC driving force was determined to be coarsening of semicoherentγ′ with coarsening of the Mo-rich a phase makingα minor contribution. Using the estimated driving force, grain boundary diffusivities were calculated which were within a factor of 3 of published values for stationary boundaries of comparable misorientation.