Inter-relationships between composition, γ′ morphology, hardness, and γ-γ′ mismatch in advanced polycrystalline nickel-base superalloys during aging at 800 °C

In this article, two established nickel-based superalloys, Udimet 720 and RR1000, and a further experimental alloy, UC01, were studied in terms of their response to aging at 800 °C. The alloys were subject to a solution treatment above their γ′ solvus temperature to dissolve all γ′ before cooling at either a slow "billet" or a more complex and faster two-step "intermediate" cooling rate. The effect of alloy composition and cooling rate on γ′ morphology, alloy hardness, and γ-γ′ mismatch was studied. When cooling at a slow billet rate, all alloys followed the LSW theory for precipitate coarsening. This resulted in increasing precipitate size with increasing time, a peak hardness associated with a specific precipitate size, and increasing asis γ-γ′ mismatch with time. When subjected to a two-step cooling rate, complex morphological changes were observed for intragranular γ′ in RR1000 and alloy UC01. This was manifested by variations in hardness, γ-γ′ mismatch, and the elastic and unconstrained misfit strain within the system. These variations are ascribed to the presence of tantalum within both RR1000 and alloy UC01, which is slow to diffuse to the growing γ′ precipitates during cooling and drives the morphological instability during subsequent aging at 800 °C.