Surface Energy Anisotropy of Diffusion-Induced Ni₃ A1 Crystals

Abstract

Faceted crystals with well-defined {100}, {110}, and {1111 surfaces were observed in the interdiffusion zone during chemical interdiffusion of Ni/Ni 3 Al diffusion couple in the temperature range of 800°−1200°C. Composition of the diffusion-induced crystals was determined to be 87.6Ni-12.4A1 (wt%) by electron microprobe.

The shape of Ni₃ Al crystals showed a 26-sided polyhedron (rhombicuboctahedron) with a m3m point group symmetry at low temperature range (800° to 1000°C). At high temperatures, for example at 1212°C, the crystal shape changes to a 14-sided polyhedral form (tetrakaidecahedron) with the same point group symmetry.

Based on the crystal geometry, equations describing the surface energy anisotropy, γ{111}/γ{100}, and γ{100}/γ{100}, were obtained. The surface-energy anisotropy was then calculated from the crystal dimension. For the equilibrium crystals with tetrakaidecahedral form, the inequality γ{110} > γ{110} > γ{111} holds. On the other hand, for crystals of rhombicuboctahedral form, the inequality becomes γ{110} > γ{111} > γ{100}. The experimental values were consistent with theoretical calculations based on the broken-bond model.