Satoshi Takizawa

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.

Synthetic approach to alloy thermodynamics and kinetics by cluster variation method and path probability method

The cluster variation method of equilibrium thermodynamics is employed to calculate the Ll0-disorder phase diagram. The stability analysis based on the second-order derivative of the free energy functional with respect to the deviation of correlation functions from their equilibrium values yields the 〈100〉 instability locus in the phase diagram. The time evolution of cluster probabilities at the phase transition is investigated on the basis of the path probability method of non-equilibrium statistical mechanics. Thereby the synthetic study of thermodynamics and kinetics based on the cluster variation method and the path probability method is attempted. It is shown that the fluctuation is necessary to drive the ordering reaction above the instability temperature, whereas the spinodal-like ordering transition takes place below the instability temperature. The extension of the present study of a spin system to an alloy system is briefly discussed and a critical problem to be settled is pointed out.

Theoretical investigation of the phase stability of intermetallic compounds by CVM and PPM

Theoretical investigations on the phase stability of intermetallic compounds are categorized into three groups: studies for the ground-state stability, the relative stability at finite temperatures and time evolution and devolution kinetics in a non-equilibrium state. Electronic structure calculations, the cluster variation method and the path probability method are proposed as potential theoretical tools, respectively, for each category. A number of examples of our previous calculations are presented to demonstrate that these methods are fully compatible and provide a sound basis for a general framework of the phase stability theory of intermetallic compounds.

A Monte Carlo Simulation of Melting in Prototype Crystal

We investigate the melting transition of the solids interacting through a simple pairwise potential using conventional and Wang-Landau Monte Carlo simulation. In the simulations, the atomic displacement is discretized for describing the atomic vibration and each atom is confined within its Voronoi polyhedron. The melting point can be uniquely determined by Wang-Landau approach while the temperature hysteresis inevitably appears in the conventional method. The obtained results show typical feature of first-order transition which is the discontinuous change in the internal energy. We discuss the relation between the limit of superheated state and intrinsic instability of the system through the comparison with two results.

Investigation on Additional Hardening Mechanisms in Off-Stoichiometric Al-Rich Ni 3 Al Alloys in Terms of Anti-Site Defect Configuration

One of the unsettled issues of the mechanical properties of L1 2 -Ni 3 Al compound is the effect of off-stoichiometry on the 0.2% flow stress at low temperature region. 0.2% flow stress at 77 K increases as deviating from the stoichiometric composition to both Ni-rich and Al-rich sides, but in Al-rich side the increasing rate is higher. According to the XRD results for the binary Ni 3 Al alloys, however, the composition dependence of the lattice constant is the same in both sides of the stoichiometric composition, which implies that the magnitude of lattice strain induced at off-stoichiometric composition is symmetrical. And it is not rational to associate the strengthening mechanism to a sole effect of isotropic strain due to an atomic size difference. In this study, attempts are made to explain the additional strengthening in Al-rich alloys in terms of the tetragonal distortion hardening mechanism, which arises from anisotropic atomic configuration around an anti-site Al atom.

FIRST-PRINCIPLES STUDY ON THE PHASE STABILITY OF Au-Pd ALLOYS

The effective interaction energies including first and second nearest neighbor pair interactions are derived for Au-Pd system by operating the cluster expansion method on the total energies of nine kinds of ordered phases in addition to pure Au and Pd which are obtained by electronic structure calculations. By combining with the Cluster Variaton Method, the heats of formation of a disordered phase at 298K are calculated as a function of composition.