Morphological changes of gamma prime precipitates in nickel-base superalloy single crystals

Changes in the morphology of the γ' precipitate were examined during tensile creep at temperatures between 927 and 10380C in [001]-oriented single crystals of a Ni-Al-Mo-Ta superalloy. In this alloy, which has a large negative misfit of -0.80 pct., the γ' particles link together during creep to form platelets, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ' phases were measured as directional coarsening developed and were related to creep time and creep strain in an attempt to trace the changing morphology under various stress levels. In addition, the effects of initial microstructure as well as slight compositional variations, were related to raft development and creep properties. The results showed that directional coarsening of γ' began during primary creep, and under certain conditions, continued to develop after the onset of steady-state creep. The length of the rafts increased linearly with time up to a plateau region. The thickness of the rafts, however, remained constant up through the onset of tertiary creep; this is a clear indication of the stability of the finely-spaced γ-γ' lamellar structure