Optimal precipitate shapes in nickel-base γ–γ′ alloys

Abstract

Precipitate shapes in nickel-base superalloys vary substantially with alloy composition, partitioning of elements to the disordered gamma matrix and the ordered gamma prime precipitates and the degree of coarsening during elevated temperature exposures. These shapes, which vary from spherical to cuboidal to rod-like, have typically not been quantified in Ni-base alloys containing relatively high γ′ volume fractions, in spite of their importance to mechanical properties. Precipitate shapes in a series of new platinum group metal (PGM)-containing Ni-base alloys have been quantified by their two-dimensional moment invariants. Precipitate morphologies were characterized in a total of 17 PGM-containing alloys in the solution treated and aged condition. The average γ′ volume fraction was measured as 0.60, typical of highly creep resistant Ni-base alloys. PGM additions resulted in an unusually large range of precipitate shapes. Precipitate morphologies were quantitatively analyzed using a shape parameter derived from the absolute moment invariant. For the compositions examined, the shape parameter reaches a maximum value at a precipitate–matrix misfit magnitude of 0.4%. A large set of commercial single crystals exhibits the same range in shape parameter values around this misfit magnitude, suggesting that favorable high temperature properties are correlated with an optimum precipitate shape.