A TEM study of microstructure inhomogeneity in a superalloy powder: Implications for microstructure development

For nickel-base superalloys fabricated from gas-atomised powders, the final microstructure must be related to the characteristics of the powder itself. This work provides detailed characterisation, primarily using transmission electron microscopy, to link microstructure, composition and crystallography of the nickel-base superalloy, RR1000. In the virgin state, the powder has significant chemical inhomogeneity between and within powder particles, including evidence of ${\gamma }^{\prime }$ structural ordering. From solute enrichment remnant from solidification, these regions contain multiple phases including MC, M₂₃C${}_6$ and sigma ($\sigma$). Where the local composition is close to the stoichiometric chemistry of the respective phase, they evidently formed without the need for long range diffusional processes that are typically required for their formation. When the powder is heated, these phases, including ${\gamma }^{\prime }$, are retained. This study highlights how non-equilibrium characteristics of a gas atomised powder must infleuence microstructural development and, consequently, the selection of subsequent processing steps.