Papers by Keyword: Order-Disorder Transformation

Abstract: Segregation of solute atoms and vacancies to migrating D03–type antiphase boundaries (APBs) in Fe3Al of the stoichirometric and an Al-rich compositions has been investigated in detail by a phase-field method developed in the previous study [Koizumi et al. Acta Mater 2009;57:3039] focusing on the quantitative relationship among the segregation, APB energy, driving-force, drag-force and APB mobility. It has been revealed that the drag-force by solute segregation depends considerably on the alloy composition and is responsible for the significant composition dependence of the migration kinetics of D03-APB whereas the composition dependences of driving-force and mobility are negligibly small.

Abstract: Owing to the technological importance of M-type hexaferrites of general formula MFe12O19 (M = Ba, Sr and Pb) and their substituted analogues, a staggering number of research activities on their synthesis, magnetic properties and thermal properties are being pursued in recent years. However, many critical issues on the effect of aliovalent ion substitution either on the M-site or Fe-sites are still unexplored. This paper deals with a survey of crystallochemical and thermodynamic properties of this important class of ferrimagnetic oxides.

Abstract: Within the framework of the lattice-statics and static fluctuation-waves’ methods, the available energiesof strain-induced interaction of interstitial–interstitial, interstitial–substitutional and substitutional–substitutional impurity atomic pairs are collected and analysed for f.c.c.-(Ni,Fe)–C solutionsallowing for discrete atomic structure of the host-crystal lattice. The lattice spacings, elasticity moduliand/or quasi-elastic force parameters of the host-crystal lattice, and concentration coefficients of thedilatation of solid-solution lattice due to the respective solutes are selected as the input numerical experimentaldata used. The above-mentioned interaction energies prove to have non-monotonically decreasing(‘quasi-oscillating’) and anisotropic dependences on discrete interatomic radius-vector, andthemselves are strong and long-range. In all f.c.c.-(Ni,Fe)-base solutions, there is strain-induced attractionin many co-ordination shells. In general, the strain-induced interaction between impurity atomsin γ-Fe is weaker than in α-Ni (but in some solid solutions, it may prove to be of the same order).The verification of applicability of the approximation of strain-induced interaction of impurities forf.c.c.-(Ni,Fe)–C alloys (by means of analysis of thermodynamic C activity and ‘short-range order’ parametersof C-atoms’ distribution revealed by Mössbauer spectroscopy) showed that it must be supplementedwith additional short-range (‘electrochemical’) repulsion in the first co-ordination shell.Nevertheless, in any case, the strain-induced interaction of impurity atoms must be taken into accountfor analysis of structure and properties of f.c.c.-(Ni,Fe)-base solutions.

Abstract: Using both the statistical-thermodynamics methods within the scope of the selfconsistent field approximation and the diffraction data on coherent (or diffuse) scattering of X-rays (or thermal neutrons) from (dis)ordered f.c.c.-Ni–Fe alloys of various compositions, the estimation of interatomic interactions (including their magnetic contribution) and their temperature– concentration dependences were obtained. Based on the static concentration-wave representation, the expressions for configuration free energies of L12-Ni3Fe-type permalloy, L10-NiFe-type elinvar and L12-Fe3Ni-type invar were analyzed, considering explicit expressions for configuration entropies of atomic and magnetic subsystems with their configuration internal energies. Phase diagram of a system at issue was plotted within the field of the presence of f.c.c.-Ni–Fe alloys; their phase boundaries, equilibrium (static) properties near critical points (order parameter, etc.), and possible microstructures developed by composition-controlled magnetic transitions and/or order–disorder constant-composition solid–solid phase transformations were discussed. The obtained results were compared with available experimental data.

Abstract: Using the self-consistent field approximation, the static concentration waves approach and the Onsager-type kinetics equations, the descriptions of both the statistical thermodynamics and the kinetics of an atomic ordering of D019 phase are developed and applied for h.c.p.-Ti–Al alloy. The model of order–disorder phase transformation describes the phase transformation of h.c.p. solid solution into the D019 phase. Interatomic-interaction parameters are estimated for both approximations: one supposes temperature-independent interatomic-interaction parameters, while the other one includes the temperature dependence of interchange energies for Ti–Al alloy. The partial Ti–Al phase diagrams (equilibrium compositions of the coexistent ordered α2-phase and disordered α-phase) are evaluated for both cases. The equation for the time dependence of D019- type long-range order (LRO) parameter is analyzed. The curves (showing the LRO parameter evolution) are obtained numerically for both temperature-independent interaction energies and temperature-dependent ones. Temperature dependence of the interatomic-interaction energies accelerates the LRO relaxation and diminishes a spread of the values of instantaneous and equilibrium LRO parameters versus the temperature. Both statistical-thermodynamics and kinetics results show that equilibrium LRO parameter for a non-stoichiometry (where an atomic fraction of alloying component is more than 0.25) can be higher than for a stoichiometry at high temperatures. The experimental phase diagram confirms the predicted (ordered or disordered) states for h.c.p.-Ti– Al.

Abstract: The temperature dependence of single-crystal elastic constants of L10-ordered single-crystals of FePd . A complete set of elastic constants has been determined with the resonance ultrasound spectroscopy technique. The compounds clearly show a tetragonal elastic anisotropy, c11 < c33 and c44 < c66. The temperature dependencies of the anisotropies are not simply explained by the variation of axial ratio (c/a) of the crystal.