M. Koiwa

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).

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Abstract Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

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 mechanisms in ordered alloys—a detailed analysis of six-jump vacancy cycle in the B2 type lattice

Abstract The diffusion coefficients of A and B atoms in the B2 type AB alloy are derived for the case that highly correlated vacancy jump cycle is operative. The effective frequencies of 100- and 110-type jumps of A and B atoms are expressed in terms of frequencies for individual vacancy jumps; the calculation is made by applying the concept of the mean first passage time known in the theory of stochastic processes. The correlation factors for diffusion of A and B atoms are calculated as a function of the wrong bond energy. The present analysis provides a sound and quantitative basis for the discussion of the ratio of diffusion coefficients for two types of atoms, D A D B , in ordered alloys.

Single-crystal elastic constants of intermetallic compounds

The elastic constants of single crystals of several intermetallic compounds have been measured in our laboratory by the rectangular parallelepiped resonance method. This paper first describes the experimental procedure and the method of data analysis of the resonance spectrum to derive values of the elastic constants, and then gives the numerical results for materials measured so far: Ni-base L12 compounds Ni3X (X = Mn, Fe, Al, Ga, Ge and Si), TiAl, Ti3Al, CoTi, Co3Ti and CoSi2. The elastic constants of various intermetallic compounds reported by other investigators are also given in tabulated form.

Elastic constants of Al-based icosahedral quasicrystals

The elastic constants of several Al-based icosahedral alloys were measured over a temperature range from 4K to 1073 K by the rectangular parallelepiped resonance method. The values of the bulk moduli of these quasicrystals are similar to those of conventional cubic crystals having similar melting temperatures, while the values of Youngs and shear moduli are larger in comparison with conventional aluminium alloys. The latter feature is considered to be due to the strong directional bonding in the quasicrystals.

Elastic constants and their temperature dependence for the intermetallic compound Ti3Al

Abstract Single-crystal elastic contents of Ti3Al have been measured from 3.3 to 290K by a rectangular parallelepiped resonance method. The elastic anisotropy of Ti3Al is similar to, but slightly larger than, that of pure Ti. The values of the measured elastic constants are compared with those calculated for pure Ti stressed to such an extent that it has the same lattice constants as Ti3Al. The difference is interpreted as indicating the existence of directional bonding in the compound.

Interdiffusion and structural relaxation in Mo/Si multilayer films

Interdiffusion and structural change on annealing of sputter‐deposited Mo/Si and Mo(N)/Si(N) multilayer thin films have been investigated over the temperature range from 674 to 1027 K. X‐ray diffractometry shows that in the as‐deposited Mo/Si multilayers the Mo is bcc with the stacking of (110) plane parallel to the substrate, and the Si is amorphous, while in the as‐deposited Mo(N)/Si(N) multilayers, both Mo and Si nitrides are amorphous. The interdiffusivities have been determined from the decay rate of satellite peak intensity around (000). The activation energies for the interdiffusion in Mo/Si and Mo(N)/Si(N) multilayers are 105±5 and 351±88 kJ mol−1, respectively. A drastic decrease in the satellite peak intensity on annealing is observed in the Mo/Si multilayer films, which is interpreted to be due to interdiffusion and structural relaxation. On the other hand, a remarkable increase in the satellite intensity is found for the nitride multilayer films, which is explained by crystallization into β‐Mo...

Elasticity of Ni-based L12-type intermetallic compounds

Abstract The three elastic stiffness constants of various Ni-based intermetallic Ni 3 X (X = Mn, Fe, Al, Ga, Ge and Si) with L1 2 structure have been determined at room temperature by the rectangular parallelepiped resonance method. The elastic anisotrophy factor, A , of the compounds is generally large, ranging from 2.5 to 3.3 except for Ni 3 Ge ( A = 1.71). The large anisotropy is believed to be responsible at least partly for the positive temperature dependence of the flow stress observed for these compounds. The elastic constants exhibit some correlation with the lattice constants. The minor element, X, can be regarded as exerting “chemical pressure” which causes the expansion of the Ni lattice, thus resulting in the decrease of the elastic constant.

The core structure of 1/3⟨1123⟩ {1122} edge dislocations in h.c.p. metals

Abstract A computer simulation has been carried out for the core structure of a 1/3⟨1123⟩ {1122) edge dislocation using the truncated Lennard-Jones (12–6) potential. The core is composed of two distinct structural units which can be interpreted in terms of {1121) and {1122) twins. This core structure differs markedly from that proposed by Rosenbaum (1964).