Tetsuo Mohri

Thermal conductivity of B2-type aluminides and titanides

Thermal conductivity measurement by the laser-flash method adopted here is very suitable for intermetallic compounds, some of which are too brittle for preparing samples with good geometry. Thermal conductivity data at room temperature are presented in the series of aluminides, FeAl, CoAl and NiAl, titanides, FeTi, CoTi and NiTi, and gallides CoGa and NiGa, in order to reveal the role of 8A group elements of the first transition metal series. Thermal conductivity reaches a maximum at stoichiometry and decreases with deviations from stoichiometry on either side in most of these compounds. The largest thermal conductivities observed are 92 W m−1 K−1 of NiAl among the aluminides, 23 of NiGa among the gallides and 73 of FeTi among the titanides. It is discussed that the thermal conductivity is further reduced as the position of the partner components becomes distant from that of a host component.

First principles calculation of thermodynamic properties of noble-metal alloys

Abstract The electronic structure calculation is incorporated in the cluster-variation method, and thermodynamic properties of disorder phase of noble-metal alloys are studied. For Cu-Au and Ag-Au systems, the heats of formation are estimated as functions of concentration, and the influence of short range order on the lattice constant is quantitatively discussed at Cu3Au stoichiometry. For Ag-Cu system, phase diagram is obtained based on the same model.

L12-long-range order in Cu3Au: kinetics and equilibrium as studied by residual resistivity

Abstract The evolution of L12-long-range order in Cu3Au was studied as a function of temperature by means of residual resistivity measurement. Kinetics as well as temperature dependence of equilibrium values of the LRO-parameter were studied during isothermal annealing and the LRO-equilibrium values obtained were compared with model calculations and X-ray diffraction measurements. Comparing the analyses of order-order relaxations with those of disorder–order relaxations, information is gained on the growth kinetics of ordered antiphase domains.

Mechanical properties of Rh-based L12 intermetallic compounds Rh3Ti, Rh3Nb and Rh3Ta

Abstract A preliminary study of the mechanical properties of Rh-based L1 2 intermetallic compounds Rh 3 Ti, Rh 3 Nb and Rh 3 Ta is carried out by micro-vickers hardness tests at room temperature and compression tests at various temperatures ranging from room temperature to 1673 K. A cold-rolling test is also conducted on a Rh 3 Ti plate. Although high temperature ductility-loss is observed in all compounds, Rh 3 Ti shows good ductility at all temperatures investigated. Both Rh 3 Nb and Rh 3 Ta show a weak positive temperature dependence of strength (stress anomaly) at around 1273 K which is about half the melting point for both intermetallic compounds. The stress anomaly is discussed in terms of a phase stability concept based on the Kear–Wilsdorf (K–W) mechanism. High work hardening rates of Rh 3 Nb and Rh 3 Ta, which cause high vickers hardness at room temperature, are also attributed to the K–W mechanism.

Thermal conductivity in Pt-based alloys

Abstract Thermal conductivity of Pt alloys with an fcc single phase was comprehensively surveyed by employing the laser flash method. Thermal conductivity is dominantly determined by alloy composition and temperature and is scarcely influenced by fabrication condition. An addition of solute definitely decreases the thermal conductivity of Pt, and the conductivity-composition relationship is characterized by a sharp maximum at pure Pt. The Wiedemann-Franz relation held for Pt alloys suggests the dominant carrier of thermal conduction is an electron. An empirical rule is proposed that the thermal conductivity decreases significantly as the position of the solute element in the periodic table becomes horizontally more distant from that of Pt. Thermal conductivity of Pt alloys increases with increasing temperature in a range between 300–1100 K, and the temperature coefficient is found to be inversely correlated with the thermal conductivity.

First-principles study for ordering and phase separation in the Fe-Pd system

The main features of the phase diagram of the Fe-Pd system are characterized by phase separation in the Fe-rich region and appearance of two ordered phases in the Pd-rich region. The total-energy FLAPW electronic structure calculations are attempted on a set of selected ordered compounds as well as Fe and Pd to clarify the origin of these features. It is revealed that (i) magnetism plays a dominant role in the phase stability of the system and (ii) the system has an intrinsically large driving force for ordering, while the elastic energy originating from size mismatch causes the phase separation in the Fe-rich portion.

First-principles study of short range order and instabilities in AuCu, AuAg and AuPd alloys

Abstract Interaction energies derived from first-principles total energy calculation are used together with the Cluster Variation Method in order to study phase diagrams, ordering instabilities and the Fourier spectrum of the Warren-Cowley short range order parameters for Au Cu, Au Ag and Au Pd alloys. The present study is carried out using both the tetrahedron and the tetrahedron-octahedron approximations of the CVM. It is shown that the tetrahedron approximation predicts the wrong Fourier spectrum for the SRO parameters although it gives qualitatively correct phase diagrams. In particular, the tetrahedron approximation fails to reproduce the expected maxima in the fluctuation spectrum at the 〈1,0,0〉-type reciprocal space vectors, characteristics of L1 0 and L1 2 ordering systems. The correct features of the k -space fluctuation spectrum is obtained in the tetrahedron-octahedron approximation.

Computer simulation of local lattice distortion in Cu-Au solid solution

The local lattice-distortion effects in Cu-25 at Pct Au solid solution were investigated using a computer simulation in which the nearest-neighbor Lennard-Jones interactions with thermal vibration effects were assumed. The investigation revealed that the average displacements of Cu-Cu and Au-Au pairs in the nearest neighbor were negative and positive directions, respectively, with respect to the rigid lattice before the relaxation, and that the diffuse scattering intensity due to local lattice distortion and local atomic configuration can be calculated by taking the short-range configuration up to the 12th nearest-neighbor pairs into account. The results reproduced the overall tendency of the experimental observations.

Ternary eutectic dendrites: Pattern formation and scaling properties.

Extending previous work [Pusztai et al., Phys. Rev. E 87, 032401 (2013)], we have studied the formation of eutectic dendrites in a model ternary system within the framework of the phase-field theory. We have mapped out the domain in which two-phase dendritic structures grow. With increasing pulling velocity, the following sequence of growth morphologies is observed: flat front lamellae → eutectic colonies → eutectic dendrites → dendrites with target pattern → partitionless dendrites → partitionless flat front. We confirm that the two-phase and one-phase dendrites have similar forms and display a similar scaling of the dendrite tip radius with the interface free energy. It is also found that the possible eutectic patterns include the target pattern, and single- and multiarm spirals, of which the thermal fluctuations choose. The most probable number of spiral arms increases with increasing tip radius and with decreasing kinetic anisotropy. Our numerical simulations confirm that in agreement with the assumptions of a recent analysis of two-phase dendrites [Akamatsu et al., Phys. Rev. Lett. 112, 105502 (2014)], the Jackson-Hunt scaling of the eutectic wavelength with pulling velocity is obeyed in the parameter domain explored, and that the natural eutectic wavelength is proportional to the tip radius of the two-phase dendrites. Finally, we find that it is very difficult/virtually impossible to form spiraling two-phase dendrites without anisotropy, an observation that seems to contradict the expectations of Akamatsu et al. Yet, it cannot be excluded that in isotropic systems, two-phase dendrites are rare events difficult to observe in simulations.

Reinvestigation of Ni-Solid Solution/Liquid Equilibria in Ni-Al Binary and Ni-Al-Zr Ternary Systems

The present work reports the results of a reinvestigation of the γ liquidus and solidus temperatures of the Ni-Al system and the γ/Ni5Zr eutectic reaction temperatures in the Ni-Al-Zr system. In the Ni-Al binary system, it is found that the stability of the Ni-rich γ solid solution phase is enhanced by small additions of Al with the melting temperature of the γ phase and that the melting temperature reaches a congruent maximum at a few at.% Al. The temperature of the eutectic reaction L→γ+Ni5Zr in the Ni-Zr binary edge is confirmed to be 1196 °C by differential thermal analysis (DTA), rather higher than the value reported previously. The reaction temperature increases with Al addition to reach or exceed 1206 °C, forming a “saddle point,” then decreasing to reach 1187 °C or below by flowing into a ternary invariant reaction. These findings can be explained by γ/liquid equilibrium in the Ni-Al binary system.