Surface Energy Anisotropy of Diffusion-Induced Ni 3 A1 Crystals
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SURFACE ENERGY ANISOTROPY OF DIFFUSION-INDUCED Ni3 Al CRYSTALS
T. C. CHOU* and Y. T. CHOU** *Research and Development, Engelhard Corp., Edison, NJ 08818 **Dept. of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015
ABSTRACT Faceted crystals with well-defined {100}, {110}, and {1111 surfaces were observed in the interdiffusion zone during chemical interdiffusion of 0 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 Ni Al crystals showed a 26-sided polyhedron (rhombicuboctahedroný with a m3m point group symmetry at low temperature range (8000 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, y{lll}/ y{lOOI}, and y{llO}/ y{1001, were obtained. The surface-energy anisotropy was then calculated from the crystal dimension. For the equilibrium crystals with tetrakaidecahedral form, the inequality y{llO} > y{[10} > y{llI} holds. On the other hand, for crystals of rhombicuboctahedral form, the inequality becomes y{ll0} > y{llll > y{lO0. The experimental values were consistent with theoretical calculations based on the broken-bond model.
I.
INTRODUCTION
Since the recent finding [1,2] that the addition of a small amount of boron (about 0.05 wt%) greatly improves the room temperature ductility of polycrystalline Ni Al, considerable research efforts on the physical and mechanical properties of Ni3Al-based alloys have been rekindled [3-12]. In this paper, we report an intriguing observation of Ni 3 Al crystals with well-defined habit planes during the chemical interdiffusion study. Based upon the crystal geometry, equations of surface energy anisotropy were formulated. The anisotropies were calculated according to the crystal dimensions and the experimental results were compared with theoretical calculations based on the broken-bond model.
II.
EXPERIMENTAL
The Ni 3Al alloys and Ni metal used in this study were supplied by the Oak Ridge National Laboratory. For the interdiffusion study, the diffusionbonding method was employed in making diffusion couples. Seven Ni3 Al alloys were used with the following compositions: Ni-24A1(#1), Ni-25A1(#2), Ni-26AI(#3),
Ni-24A1-O.2B(#4), Ni-25AI-O.2B(#5) Ni-26AI-0.2B(#6)
Ni-23.5A1-O.5Hf-0.2B(#7)
where the number preceding an element refers to its atomic percent.
Mat. Res. Soc. Symp. Proc. Vol. 133.
01989Materials Research Society
162
During the diffusion-bonding process, two surfaces of pure Ni and a Ni Al alloy were brought into close contact under an applied force normal to the bonding interface. To improve the flatness and perfection of the adjacent contact surfaces, the samples were polished to 0.5 Vi grit. For a close contact along the bonding plane, a specially designed bonding die was used. The bonding pressu
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