Influence of composition on monomodal versus multimodal γ′ precipitation in Ni-Al-Cr alloys

This study investigates the influence of alloy composition on γ′ precipitation in Ni-8Al-8Cr and Ni-10Al-10Cr at.% during continuous cooling from a supersolvus temperature. When subjected to the same cooling rate, Ni-8Al-8Cr develops a monomodal population, whereas Ni-10Al-10Cr develops a multimodal (primarily bimodal) population of γ′ precipitates. The bimodal γ′ precipitate size distribution in Ni-10Al-10Cr alloy can be attributed to two successive nucleation bursts during continuous cooling while the monomodal γ′ size distribution in Ni-8Al-8Cr results from a single nucleation burst followed by a longer time - wider temperature window for nucleation resulting in a larger number density of precipitates. Three-dimensional atom probe investigations reveal that while local equilibrium is achieved across the γ/γ′ interface, far-field γ compositions still retain an excess of Al and a depletion of Cr with respect to equilibrium, thus giving direct experimental evidence of the driving force for subsequent nucleation bursts during continuous cooling in case of Ni-10Al-10Cr. Contrastingly, in case of the Ni-8Al-8Cr alloy, while the γ phase retains an excess of Al and a depletion of Cr with respect to equilibrium after continuous cooling to room temperature, the additional driving force is consumed during subsequent isothermal annealing by growth of the large number of γ′ precipitates without any new nucleation.