Using absorption-corrected energy-dispersive X-ray (EDX) spectroscopy within the scanning transmission electron microscope (STEM) [1, 2], compositional variations for secondary γ′ precipitates as a function of coarsening behaviour under ageing at 1073K have been investigated for the low misfit commercial powder metallurgy (PM) Ni-based superalloy RR1000. We have observed clear cyclic variations in the elemental concentrations of Co, Ti and Al within the secondary γ′ as a function of ageing time. STEM/EDX spectrum imaging and electron tomography on individual secondary γ′ have revealed local enrichment of Co within the core of secondary γ′. STEM-EDX analysis of the γ-γ′ interface revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix region near the γ-γ′ 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, we show that elemental inhomogeneities, produced within both matrix and precipitates, are induced by complex interactions between thermodynamics and diffusion kinetics. These elemental inhomogeneities will likely affect the kinetics of coarsening and therefore must be taken into account when predicting the microstructure likely to be produced when the material is exposed to different heat treatment regimes. More generally, our findings suggest the importance of considering diffusion kinetics when attempting to understand the microstructural evolution of advanced superalloys.
Reference:
[1] Chen YQ, Francis E, Robson J, Preuss M, Haigh SJ. Acta Mater 2015;85:199.