Lattice parameter instabilities during multi-phase precipitation in Alloy 693

Ni-base superalloy Alloy 693 exhibits mono-modal, bi-modal and tri-modal distribution of γ′-phase precipitates depending upon heat treatments and subsequent cooling rates employed. This multimodal distribution of γ′-phase particles is a consequence of their formation during isothermal and continuous cooling of the alloy. Further, γ' particles that form under isothermal and continuous cooling are known to exhibit varying phase chemistry. This difference in their chemistry is expected to alter their lattice parameters. Aim of the present work is to study the variation in lattice parameters of γ′- phase particles that form during isothermal and continuous cooling. These studies have been carried out on the basis of Rietveld refinement of neutron diffraction data in conjunction with scanning electron microscopy and energy dispersive spectroscopy. This work also brings out the effect of precipitation of a chromium rich phase on the lattice parameters of γ- and γ′- phases during prolonged annealing of the alloy.