Variant selection of intragranular Ni2(Mo,Cr) precipitates (γ′) in the Ni-Mo-Cr-W alloy

Abstract

Variant selection of Ni2(Mo,Cr) precipitates (γ′) in a low coefficient of thermal expansion Ni-Mo-Cr-W alloy has been investigated as a function of precipitate size and applied stress. The lenticular-shaped, coherent γ′ precipitates form with a {110}M/(010)p habit plane. During conventional aging, all 6 variants of the Ni2(Mo,Cr) precipitates form and coarsen with aging time. With the application of external loading below the yield stress during aging, variants with {110}M habit planes that experience the largest dilations resulting from the externally applied stress are favored. Phase field modeling, coupled with elasticity theory, has been employed to simulate the development of variant selection of γ′ precipitates with and without applied external loading. Similar to the experimental results, the γ′ precipitates tend to form parallel to each other during coarsening and display variant selection when external stress is applied during aging. The mechanism is believed to be the reduction of elastic strain energy caused by the precipitate/matrix lattice mismatch. Thus, the nature of the selection indicates that the sign of the misfit is positive along [010]p. Because the misfit strain energy increases with increasing precipitate size, variant selection occurs during coarsening.