Tetsuo Mohri

The phase separation behaviors from a single B2 ordered phase into two separate B2 and L2 ₁ ordered phases in X–Al–Ti (X: Fe, Co, and Ni) alloys are analyzed utilizing the cluster variation method (CVM), based on interaction energies derived from electronic band structure calculations. A cubic approximation of the CVM is adopted for X ₂ Al _{2− x} Ti _x ( ) alloys limiting the interchange between Al and Ti atoms on the - and -sublattices of an L2 ₁ ordered structure with X atoms fixed on the -sublattice. The phase stabilities of the B2 and L2 ₁ structures are examined, and phase diagrams at the pseudo-binary section, XAl–XTi, are determined. The two-phase regions of the B2 and L2 ₁ phases (i.e. phase separation behavior) are successfully produced in Co– and Ni–Al–Ti alloy systems, and no phase separation is observed in the Fe–Al–Ti alloy. The origins of phase separation in the Co– and Ni–Al–Ti alloys are mechanical instability and a combination of mechanical instability and chemical repulsions of unlike pairs, respectively.

We present the ab-initio calculations for the solvus temperatures (T_solvus = 820∼860 K) of Pd_1−cRh_c (0.09 ≤ c ≤ 0.12) in which the Rh atoms are treated as impurities in Pd. The interaction energies (IEs) among the Rh impurities in Pd, being used in the real-space cluster expansion for the internal energies in the free energies, are determined by the ab-initio calculations based on the full-potential Korringa-Kohn-Rostoker Green’s function method, combined with the generalized gradient approximation in the density functional theory. The configurational entropy calculations are based on the cluster variation method within the tetrahedron approximation in which the 2∼4 body IEs are treated exactly within a tetrahedron of the 1st-nearest neighbor (nn) pairs. In order to take into account the 2-body IEs at the long-distance neighbors, we renormalize the 1st-nn 2-body IE by including the 2-body IEs up to the 10th-nn, because the 9th-nn 2-body IE is comparatively large. To realize the precise calculations for the T_solvus of Pd_1−cRh_c, we also investigate the following three effects on the IEs among the Rh impurities: (1) the electron excitation due to the Fermi-Dirac distribution, (2) the thermal lattice vibration by the Debye-Grüneisen model, and (3) the local lattice distortion for the 1st-nn 2-body IE. The calculated results for the T_solvus of Pd_1−cRh_c agree fairy well (within the error of ∼50 K) with the observed T_solvus.

Investigation on the crystallographic orientation relationships among D022-Al3Ti, A15-Mo3Al and high temperature bcc phase consisting of an Al-Mo-Ti ternary alloy with an equi-axed two-phase structure was conducted by FESEM/EBSD analysis. The grains of intermetallic phases have certain crystallographic orientations each other, while the crystallographic orientation distributions become random after a high temperature deformation. This strongly suggests the grain boundary sliding and grain rotating govern the high temperature deformation, which is consistent with the fact that the strain-rate sensitivity m is 0.3 or higher during a steady-state compressive deformation.