Substitution Effect of the Third Element X on the Morphology of A1/L1<sub>2</sub>/D0<sub>22</sub> Three-Phase Microstructure in Ni-V-X (X=Si, Al) Alloy

Morphology of phase-separated microstructure consisting of cubic disordered A1 and ordered L12 phases and tetragonal ordered D022 phase in Ni-V-X (X=Al, Si) alloys has been investigated by transmission electron microscopy. Ternary Ni-V-Al alloy showed the lamellar structure of D022 phase with spherical L12 particles, while Ni-V-Si ternary alloy indicated the anisotropic microstructure consisting of plate- or diamond-shaped D022 and plate-shaped L12 phases. When a part of element Al is substituted by Si, the morphology of three-phase microstructure (A1/L12/D022) varied depending on the amount of element substitution. When Ni-16.1V-4.0Al-2.8Si (at.%) alloy was aged at 1173K for long time, D022 plate-like particle aligned along <110> direction and L12 particles formed between them in the shape of sphere. In the case of Ni-16.6V-2.8Al-4.0Si (at%) alloy more substituted for Al by Si, cuboidal L12 phases were first formed aligning along <100> direction and then plate-shaped D022 phases precipitated along the <100> direction in the channel of cuboidal L12 particles. Such morphological changes with the substitution of the third elements X by Al or Si were able to be explained based on lattice mismatch.