Hiroshi Numakura

Hydride precipitation in titanium

The crystal structure and morphology of hydride (deuteride) precipitates are investigated on ..cap alpha..-titanium specimens containing 1-3 at.% H or D by transmission electron microscopy. The hydride is found to have a face-centered tetragonal structure (c/a = 1.09) with an ordered arrangement of hydrogen, being isomorphous to ..gamma..-zirconium hydride. Two types of precipitation mode are observed with the habit planes (0110) and near (0225).

Self-diffusion mechanism in Ni-based L12 type intermetallic compounds

Abstract We propose a model for the self-diffusion in L12 type A3B ordered compounds. In this structure, the atoms of the major element A can diffuse on their own sublattice (α sublattice) via the ordinary vacancy mechanism without disturbing the ordered arrangement. For the atoms of the minor element B, the same mechanism is possible for those distributed over the α sublattice as antisite atoms. If the number of the antisite atoms is not negligibly small, the long-range diffusion of B can be due largely to their migration over the α sublattice as impurity diffusion. The self-diffusion data in Ni3Al, Ni3Ga and Ni3Ge are discussed in the light of this model.

Diffusion in Ni3Al, Ni3Ga and Ni3Ge

Abstract An overview is given on recent progress in the study of atomic diffusion in three nickel-based L1 2 -ordered intermetallic compounds, Ni 3 Al, Ni 3 Ga and Ni 3 Ge. Data of the diffusion of constituent species and those of chemical diffusion have been accumulated by radioactive tracer experiments and single-phase interdiffusion experiments. The tracer diffusion coefficients of the majority component, Ni, in the three compounds are of the same order of magnitude when normalized to the melting temperature, while in contrast, those of the minority component are widely different. Nevertheless, it appears possible to understand these common and distinctive features in terms of a simple model of diffusion, where both of the two species of atoms diffuse primarily over the sublattice of the majority component via the ordinary vacancy mechanism.

〈1 123〉{1011} slip in zirconium

Abstract Electron microscopy observations were made on Zr polycrystals deformed at room temperature. Deformation substructure consists of ⅓〈1120〉 dislocations mostly in screw orientations, ⅓〈1123〉 dislocations on {1011} planes, and deformation twins with a small volume fraction. The observed configurations of the ⅓〈1123〉 dislocations indicate that their mobility is low when they lie along a close-packed direction, supported by a computer simulation study.

Calculation of the strength of Snoek relaxation in dilute ternary B.C.C. alloys

Abstract The distortion around a pair of an interstitial solute atom and a substitutional solute atom in the b.c.c. lattice has been calculated by the method of molecular statics. The model crystal consists of a finite number of atoms forming a b.c.c. crystallite and an elastic continuum surrounding the crystallite. The atoms are assumed to interact through pairwise interatomic potentials. The atomic displacements in the vicinity of an interstitial—substitutional solute pair and the elastic displacements at large distances have been calculated for the first to the fifth neighbour configurations. To describe the elastic distortion of the outer region, two elastic displacement fields of isotropic and cylindrical symmetry are employed and are utilized to estimate the volume expansion and the uniaxial distortion due to the pair. Internal friction profiles expected for dilute ternary alloys are calculated by using the results of the simulation. The observed effect of manganese on the Snoek relaxation of carbon in α iron has been discussed in terms of the trapping of carbon to manganese and an associated decrease of the uniaxial distortion.

Site occupation preference of Fe in Ni3Al: an atom-probe study

Abstract The substitution behaviour of Fe in Ni 3 Al has been studied by atom-probe microanalysis for Fe concentrations from 0.4 to 1.9 at%. Effects of the matrix composition and annealing temperature have also been examined. Fe atoms have been found to occupy mostly Al sites, regardless of the Fe concentration and the matrix composition. The present observation and the experimental results for higher Fe levels reported in the literature are discussed in the light of the site preference predicted by various empirical rules and theories.

A Bragg-Williams model for the thermodynamic activity and the thermodynamic factor in diffusion for ordered alloys with substitutional defects

Abstract A pair interaction model for describing the thermodynamic activities in binary ordered alloys with substitutional defects (antisite atoms) has been formulated in the framework of the Bragg–Williams approximation, taking the ordering energy as the basic parameter. Analytical expressions have been derived for the thermodynamic factor in chemical diffusion. The thermodynamic factor can readily be calculated using the ordering energy obtained by analysing activity data. As examples, experimental data of the thermodynamic activity in AgMg (B2), NiZn (L10) and Ni3Al (L12) have been analysed and the thermodynamic factor vs composition curves are presented.

Atomistic study of ⅓〈1 123〉{1011} dislocations in h.c.p. crystals. I. Structure of the dislocation cores

Abstract The core structure of ⅓〈1 123〉 dislocations on {1011} planes in h.c.p. crystals has been studied by computer simulation using a truncated Lennard-Jones potential. An edge dislocation and a mixed dislocation parallel to the [2113] direction (30°dislocation) extend over the (1011) plane. A mixed dislocation parallel to the [1210] direction (105° dislocation) assumes either of the following three configurations, depending on initial elastic dislocations introduced: (i) a planar configuration spread over the (1011) plane, (ii) a nonplanar configuration spread over the (1011), (0001) and (1010) planes, (iii) another nonplanar configuration in which the disregistry is not confined to any low-index planes. The result is interpreted in terms of stacking faults in h.c.p. crystals.

The homogeneity ranges of the L12-type intermetallic compounds Ni3Ga and Ni3Ge

The homogeneity ranges of the L12-ordered compounds Ni3Ga and Ni3Ge and the boundaries of the neighboring phases have been examined by electron probe microanalysis using heterophase alloys in the temperature ranges 1073 to 1373 K (Ni-Ga) and 1073 to 1346 K (Ni-Ge). The composition ranges of the L12-ordered phases have been found to be narrower than those reported in the literature, on the Ga-rich side for Ni3Ga and on the Ge-deficient side for Ni3Ge.

Anelastic relaxation due to interstitial solute atoms in face-centred cubic metals

Abstract The behaviour of light interstitial elements dissolved in transition metals of the face-centred cubic structure has been studied by measurements of mechanical relaxation. Those interstitial solute atoms such as C and H give rise to anelastic relaxation effects as they form complexes, such as close pairs. Detailed characterization of such complexes is possible by examining the effects of crystal orientation, solute concentration and temperature on the relaxation effect. Recent experimental results on Ni–C, Ni–C–H and Pd–C systems are presented.