On the Rate Dependence of Precipitate Formation and Dissolution in a Nickel-Base Superalloy

The temporal dependence of γ′ dissolution in the polycrystalline Ni-base superalloy RR1000 has been studied with implications to thermo-mechanical processing. A resistivity-based method using an electro-thermal mechanical testing (ETMT), which overcomes the drawbacks associated with other approaches, such as calorimetry, dilatometry, and diffraction, has been used to explore the effect of transient and isothermal thermal cycles. This is supplemented by DICTRA numerical models that simulate the diffusion within the γ phase up to the γ/γ′ interface. It is demonstrated that dissolution is affected by heating rate as well as the precipitate size. Below a threshold heating rate of ∼0.1 ∘C s−1, the dissolution kinetics are marginally affected, however, is sensitive to microstructure. The role of precipitate size during dissolution is governed by diffusion flux in the γ phase at the γ/γ′ interface, which is inversely proportional to size. It is argued that numerical simulations that predict constitutional liquation during rapid heating by altering the width of the computation domain to match the average precipitate size of the γ′ population will yield inaccurate predictions. The influence of the heating rate on the nucleation undercooling, during subsequent cooling, has also been addressed. With increasing heating rates, the local γ′ solvus temperature is shifted to progressively higher temperatures. Unless complete dissolution of γ′ occurs prior to subsequent cooling, erroneous interpretations of nucleation undercooling can arise.