An investigation of diffusion-mediated cyclic coarsening and reversal coarsening in an advanced Ni-based superalloy

The anomalous cyclic coarsening behaviour of gamma' precipitates after ageing at 1073 K has been investigated for the low misfit commercial powder metallurgy (PM) Ni-based superalloy RR1000. Using scanning transmission electron microscope (STEM) imaging combined with absorption-corrected energy-dispersive X-ray (EDX) spectroscopy, the elemental segregation as a function of coarsening behaviour has been experimentally observed for secondary gamma' precipitates. Elemental EDX spectrum imaging has revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the gamma matrix close to the gamma-gamma' interface. Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, elemental inhomogeneities, as a result of complex diffusive interactions within both matrix and precipitates, play a crucial role in determining the rate of coarsening. Our findings provide important new evidence for understanding the microstructural evolution observed for advanced superalloys when they are exposed to different heat treatment regimes. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd.