Precipitation in a nickel-beryllium alloy

Abstract

Precipitation was studied in a nickel 1.89 wt% beryllium alloy over the temperature range 400 to 750° C. Below 650° C, the precipitation sequence is analogous to that in the widely studied Cu-Be system: $$\alpha (supersaturated solid solution) \to \alpha + GP zones \to \alpha + \gamma '_{NiBe} \to \alpha + \gamma _{NiBe} $$ where the equilibrium γ NiBe phase is formed by a discontinuous reaction consuming the intermediate γ∼' NiBe. The morphology of the discontinuous γ NiBe was too irregular to assign a habit plane. Numerous orientation relations were observed between the α Ni and the discontinuous γ NiBe. The kinetics of the reaction were found to follow a linear growth with time. The activation energy of 58±3 kcal mol−1 is less than bulk diffusion of nickel but greater than grain boundary self diffusion. Above 650° C, the discontinuous reaction front is arrested before completion because the equilibrium γ NiBe forms by continuous coarsening of the γ′ NiBe phase. The continuous γ NiBe has a habit plane of {113}α. Three closely related orientations were observed at constant ageing parameters for the equilibrium continuous phase. Each has been separately proposed in the literature as a unique relation for the copper-beryllium system. For nickel-beryllium either all three are uniquely present or the accuracy of selected area diffraction technique is such that the small differences in orientation cannot be distinguished.