Papers by Author: Takao Kozakai

Abstract: 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.

Abstract: When Fe-10.3mol%Ni-14.3mol%Al alloy is heated at 1173 K for 8.64104 s, a number of B2 precipitates are dispersed in the A2 matrix. When the two-phase microstructure of A2+B2 is aged at 973 K, the phase-separation of B2 precipitate particles takes place to form a new A2 phase in each B2 particle. In the course of further ageing at 973 K, the new A2 phase grows but decreases in number, and finally only one A2 particle is left in the individual B2 particles. The appearance of new A2 phase in each B2 precipitate is due to the difference in the volume fraction of A2 phase that should exist in A2+B2 two-phase system depending on the heating temperature: i.e., the phase-separation of B2 precipitates starts with the aid of chemical free energy.

Abstract: Phase separations of A1 (γ) supersaturated solid solution into γ, cubic-L12 (γ') and tetragonal-D022 (γ") phases were investigated in Ni-V-Ge alloys by means of transmission electron microscopy (TEM). When Ni-15.8at%V-9.0at%Ge alloy is aged at 1073 K, at the early stage of ageing γ" phases are observed in the γ matrix as triangle- or diamond-shaped precipitates. With further ageing, colonies of lamellar structure consisting of two variants of γ" phase are dispersedly formed in the γ matrix and then γ" plates grow along the <110> direction. When Ni-14.5at%V-8.8.at%Ge alloy is isothermally annealed at 1023 K, first spherical γ' particles precipitate homogeneously in the γ matrix, followed by the formation of lamellar structure of γ" phases. In the course of further ageing, the lamellar structure develops, and only γ' particles around the lamellar structure grow and others gradually fade out.

Abstract: The phase-separation behaviour of γ’ precipitates in Ni-7.1Al-6.7Si alloy was investigated by means of transmission electron microscopy (TEM). When the alloy is aged at 1173K, coherent spherical γ’ particles having ordered L12 structure appear in γ matrix having disordered A1 structure. When the two-phase microstructure of γ + γ’ is aged at 973K, spherical γ particles precipitate in the individual γ’ precipitates. In the course of ageing at 973K, the new γ particles grow keeping the spherical shape, their number gradually decreases and finally γ particles aging at 1173K gradually change their shape from sphere to cuboid, but do not practically change their size, i.e. such phase-separation behaviour brings the decelerated growth of γ’ precipitates.

Abstract: Phase-separation of D03 precipitates in A2 matrix of Fe-Si-V alloys was investigated with TEM. When Fe-14.5at%Si-12.9at%V alloy is aged at 873 K, the phase-separation of cuboidal D03 precipitates occurs and A2 particles newly appear in each D03 cuboid. The A2 particles grow to become plates, then the A2 plates elongate along {100} to reach the A2 matrix, and finally the split of D03 cuboid is realized to form smaller cuboids. When Fe-15.5at%Si-5.0at%V alloy is aged at 873 K, the phase-separation of rod-shaped D03 precipitates occurs and A2 particles newly appear in each D03 rod. The A2 particles elongate along the long axis of D03 rod to reach the A2 matrix, and the split of D03 rod is realized to form thinner rods. The split in each alloy brings the refinement of two-phase microstructure, which is a result of not only the elastic energies but also the chemical free energy.

Abstract: When the Al/Ge/SiO2 bilayer films are annealed in-situ in a scanning electron microscope (SEM) at the temperatures lower than the crystallization temperature of amorphous Ge itself, the so-called metal-mediated-crystallization (MMC) takes place. In the course of MMC, crystalline Ge aggregates (Ge clusters) form in the bilayer films, which results in the formation and the evolution of impressive fractal patterns with branching on the free surface. In-situ SEM observations of annealed Al/Ge/SiO2 bilayer films indicate that the grain size of polycrystalline Al-layer influences the nucleation of Ge clusters and hence of fractal patterns. For the bilayer films containing larger Al grains, the nucleation rate of fractal patterns (Ge clusters) is faster and the number of patterns is larger.

Abstract: Phase separation of γ (A1) supersaturated solid solution into A1, γ’ (L12) and γ” (D022) phases was investigated in two Ni-rich Ni-V-Si ternary alloys by means of transmission electron microscopy. When the alloys are annealed at 1073K, two different sequences of the phase separation are observed, depending on the chemical composition of the alloy: In Ni-17.0at%V-6.9at%Si alloy (A) at the D022 corner of three-phase field, first many D022 particles precipitate aligning along the <110> direction of the matrix and the so-called chessboard pattern is observed, followed by the formation of L12 phase at the interface between D022 and A1 phases. In Ni-12.1at%V-11.3at%Si alloy (B) at the L12 corner of the Gibbs triangle, cuboidal L12 particles precipitate arranging along the <100> direction, and then D022 phase is formed. As the phase separation proceeds, a selective growth/formation of the third phase (L12 in the alloy A, D022 in the alloy B) occurs: In the alloy A, L12 phase grows into D022 particle inside along the diagonal direction of D022 cube which is parallel to the a-axis of D022 tetragonal phase. In the alloy B, D022 forms on the {100} cube face of cuboidal L12 particle, arranging the c-axis of D022 perpendicular to the {100} cube face of L12 phase. As a result of such a selective growth/formation, the first phase D022/L12 is split off into two particles, which results in the formation of laminated structure consisting of D022 and L12 phases. The selective growth/formation is considered to occur so as to maintain the less elastic strain state.

Abstract: In the elastically constrained Ni-Al-Ti alloy system, three kinds of phase-separations, i.e. microstructure changes, take place to bring the two-phase state of γ+γ’ depending on the alloy compositions and heat treatments: 1) in Ni-8at%Al-6at%Ti, the phase-separation of γ phase takes place and γ’ particles appear in the γ matrix, 2) in Ni-13at%Al-9at%Ti, the phase-separation of γ’ intermetallic phase takes place and γ particles appear in the γ’ matrix, 3) in Ni-8.5at%Al-5.4at%Ti, the phase-separation of γ’ precipitate phase takes place and γ particles appear in the γ’ precipitate.

Abstract: The critical minimum size of stable precipitate in the vicinity of edge of miscibility gap is experimentally determined for the Ni3Si precipitate particle in Ni-Si, Ni3Al in Ni-Al, Cu4Ti in Cu-Ti and Co in Cu-Co binary alloy systems by utilizing the macroscopic composition gradient method recently proposed. The results obtained are as follows: The critical nucleus size shows a rapid increase to several tens of nm in a very narrow composition region less than 0.3at% from the phase boundary. Such a big critical size of nucleus is statistically rationalized by the conventional nucleation theories.

Abstract: The present study examined the effects of heat treatment and the addition of Cu-Ni alloy on the corrosion resistance of the matrix of spheroidal graphite cast iron in aqueous environments. Test materials of white cast iron and carbon steel were used for comparison with spheroidal graphite cast iron. The alloy spheroidal graphite cast iron that added Cu and Ni was prepared. The spheroidal graphite cast iron was subjected to three kinds of heat treatment to adjust the matrix: annealing, oil quenching, and austemper heat treatment. In electrochemical tests, measurements of corrosion electrode potential and cathode and anode polarization were used. The following was clarified from the relationship between the electrode potential and current density of each of the materials in each of the solution. The alloy spheroidal graphite cast iron had a high corrosion electrode potential owing to the addition of Cu-Ni, and tended to have a low corrosion current density. This demonstrates that in any of the materials having a matrix adjusted by heat treatment, the addition of Cu-Ni increased the corrosion resistance. The corrosion current density was highest in a sulfuric acid environment.