Transitions In Binary Alloys Research Articles

Order-disorder transitions in binary alloys with the fcc (100) surface

Order-disorder transitions in binary alloys A x B 1-x with the fcc(100) surface have investigated by use of the cluster variation method within a framework of the tetrahedron approximation. According to the theory developed by Sanchez et al, phase diagrams in surfaces composed of the first several layers have been calculated above the bulk transition temperature T B . Taking account of only the nearest neighbor pairwise interactions, we have found successive transitions and a surface re-entrant transition. At T n , only the nth layer orders and other layers remain unchanged. The transition temperature from the disordered phase to the ordered phase is given by the largest value among T n 's.

Soft-sphere model for the glass transition in binary alloys. II. Relaxation of the incoherent density-density correlation functions.

Using molecular-dynamics (MD) data on a binary-alloy model, we have computed the self (incoherent) -part of the density autocorrelation functions of both species in the supercooled liquid and near the glass transition, over an extensive range of wave numbers. Standard theoretical models of liquid-state theory fail to reproduce the data, while the Chudley-Elliott jump diffusion model yields reasonable results in the glass range. With a suitable scaling of the time axis, the data for different temperatures can be brought onto a single master curve, which is well fitted by a Kohlrausch (``stretched-exponential'') function with a wave-number-dependent exponent.

Soft-sphere model for the glass transition in binary alloys: Pair structure and self-diffusion.

Molecular-dynamics simulations have been carried out on a ``soft-sphere'' model for binary alloys quenched into supercooled and amorphous states. The main emphasis of the work is on the static and dynamic characterization of the glass transition. A comparison between molecular-dynamics data and the results of a self-consistent integral equation shows that the equation of state bifurcates into ``glass'' and ``fluid'' branches below a glass transition temperature ${T}_{g}$. The static pair structures differ significantly along the two branches. The structurally relaxed ``fluid'' branch leads to a phase separation at very low temperatures. Close to the glass transition, the atomic mean-square displacements of the two species go over more and more slowly to the asymptotic diffusive regime, due to the emergence of an intermediate time scale linked to the slowing down of structural relaxation. The diffusion constants of the two species follow closely a scaling law, as predicted by mode-coupling theory, except in the immediate vicinity of the glass transition where activated processes lead to residual diffusion.

ChemInform Abstract: Liquid Ionic Alloys

Abstract(advances that have been made in recent years in understanding the electrical transport and the continuous metal‐nonmetal transition in binary alloys of nearly‐free‐electron‐like liquid metals with special emphasis on the most ionic alloy Cs‐Au; 72 refs.

Landau theory of surface segregation and phase transitions in binary alloys with fcc crystal structure.

The order-disorder transition at the surface of binary alloys with fcc structure is studied within the Landau theory. The interplay of spatial order and segregation leads to results that provide a different mechanism for the bulk first-order phase transition characteristic of these systems. The surface transition may be of first or second order. A new instability temperature towards phase separation, regulated by the surface, is obtained.

Studies of the dislocation loops produced in III–V semiconducting compounds of B3 structure by irradiation in a high voltage electron microscope

Abstract Dr. F. Reynaud studied metallurgy at the School of Mines in Saint-Etienne (1962–1965). He joined the Laboratoire d'Optique Electronique du C.N.R.S. in Toulouse (France) first as a graduate engineer (1965), then as a scientist (1973). He received the Jules Garnier prize (1974) awarded by the French Society of Metallurgy and spent a sabbatical year (1975) in the Max-Planck-Institut für Metallforschung (Institut für Werkstoffwissenschaften) in Stuttgart (F.R.G.). His research work includes order-disorder transitions in binary alloys [he organized with N. Clement and J. J. Couderc a winter school in Aussois (I984)] and electron microscopic studies of the damage created by irradiation in a high voltage electron microscope of different materials: Nia Mn, semimetals (Sb, Bi), 111-V semiconducting compounds and V3Si. Dr. B. Legros-de Mauduit graduated in 1963 from the Université Paul Sabatier in Toulouse (France). She has been working there since 1964 as a research scientist of the C.N.R.S., in the Laboratoire de Physique Structurale which became in 1980 the Laboratoire de Miaoscopie et Structure des Matériaux. Her research work includes macroscopic and microscopic studies on plastic deformation of V B semimetallic (Bi, Sb) single crystals and electron irradiation of semimetallic (Bi, Sb) and recently 111-V semiconducting (InP, InSb, GaSb, etc.) thin films. A series of experiments was performed by high voltage electron microscopy (HVEM) to study the behaviour of radiation-produced defects above room temperature in semiconducting compounds InP, InSb, GaAs and GaSb with the B3 (zincblende) structure. During the observation by HVEM operating at 2 MV, i.e. much above the threshold voltages of both constituents of the compounds, the formation of interstitial dislocation loops was observed in InP, InSb, GaAs and GaSb above room temperature. The Burgers vectors of the majority of the loops were determined to be b=½<110>, so that there are six families of such perfect loops. This result is quite different from the case of II–VI semiconducting compounds with the B3 structure (CdTe, ZnSe and ZnS) where the Burgers vectors of the loops were determined to be mostly b=½<111>, sometimes b=½<110> and exceptionally b=<100>.

Ordering in binary FCC alloys

A new method of approximation for studying the order-disorder transition in binary alloys is presented. The formulation is in terms of face-centered cubic alloys, although the method can also be applied to other structures. The technique is essentially a generalization of the constant-coupling approximation. We have applied the principles of that method to derive the free energy of the alloy using a tetrahedral cluster of nearest neighbouring sites as the basic unit of the calculation. We only consider the case of nearest neighbour pair interactions, but show how the method can be generalized to include many body and second neighbour interactions. Numerical results are presented and comparison of these results is made with results of cluster variation calculations on the same system and with experimental results on the copper-gold alloy system.

Electronic Theory of Order-Disorder Transformations in Binary Alloys

ABSTRACTA microscopic theory of order-disorder phase transitions in binary alloys is oresented. The internal energy of the system is calculated within the tight-binding Hamiltonian and long- and short-range order effects are included by solving the equations of motion for the Green's functions in the Be the lattice approximation. Results are presented for the electronic local density of states and for various values of the long- and short-range order parameters.

Experimental Study on the Critical Dynamics of the Order-Disorder Transition in Binary Alloys

By the experiments of the relaxation process of the electrical resistivity in Ni 3 Mn alloy its relaxation time was found to become longer as temperature approaches the transition point. It was verified that below the transition temperature thus obtained relaxation time coincides with that previously obtained for the order parameter by Collins and Teh's neutron diffraction experiments. From the results in the wide temperature range the critical index of the linear relaxation time was analyzed to be 1.0. In CuZn alloy the tendency of critical slowing-down was revealed to take place by measuring the similar electrical resistivity relaxation.

Behavior of the electrical resistivity at phase transitions in binary alloys

The excess resistivityΔ ρ of binary alloys is calculated in the Born approximation for the scattering of quasifree electrons from the atomic disorder.Δ ρ can be represented as the convolution of the structure functionS(q) and the fourier transform of the effective potential. The critical anomaly ofΔ ρ is derived from scaling expressions forS(q), both for alloys which undergo unmixing and for order-disorder transitions. The variation ofΔ ρ with concentration and temperature in the vicinity of a critical point is obtained. The time-dependence of the resistivity of alloys after a sudden quench into the two-phase region is also calculated, using computer simulation data forS(q,t), and a maximum ofΔ ρ is found for zones with linear dimension of 8–12 lattice spacings. All these results are in fair agreement with the available experiments. As a further possible application, we obtain the critical exponent for the nonlinear relaxation of the resistivity of an alloy close to the order-disorder transition.

Coherent potential approximation to order-disorder transition in binary alloys

A band theory of order-disorder transition in an AB alloy is studied using the locator formalism in the coherent potential approximation. The influence of the difference in the band widths of A and B metals on the transition temperature is discussed.

Charge transfer and ordering energy in a model binary alloy

Intra- and inter-atomic Coulomb interactions have been accounted for in the band theory of the order-disorder transition in binary alloys. The variation of the energy levels of the two kinds of atoms on the two sublattices is determined self-consistently using an extension of the coherent potential approximation (CPA). As an application, the authors investigate the role of the charge transfer in the order-disorder transition of CsCl type structure transitional alloys such as VMn, TiFe and ScCo.

Order-disorder transitions in binary alloys

Abstract A quantum theory of order-disorder transitions in binary alloys is developed by studying the behaviour of the Gibbs free energy as a function of the distribution of positive ions on lattice sites and the accompanying distribution of valence electrons around them. An expression is derived determining the transition point in terms of the alloying potential, the Coulomb and exchange interactions between the valence electrons, the ionic and core repulsions between neighbouring positive ions, the relative concentration and the generalized susceptibility function of the electrons of the solid solution. The physical interpretation of the result is that the feasibility of a transition depends on the balance of Gibbs free energy which is restricted by the electron distribution in reciprocal space because of the geometry of the Fermi surface and of the Pauli exclusion principle. The compact analytic result provides a very convenient tool for testing various possible modes of transformation. The transfer of...

Order-disorder transitions in binary alloys

The result of the preceding paper has been applied to the Cu3Au system to determine the transition temperature. If the short-range repulsion due to overlap of closed d shells is ignored, using recent data on pseudo-potential form factors a transition temperature of Tc = 508 °K is obtained. If an estimate of the short-range repulsion from measured Born-Mayer constants is included, a higher value of Tc = 955°K is obtained.

Order-Disorder Phase Transition in Binary Alloys—Coherent Potential Approximation

The electronic density of states of a one-dimensional $\mathrm{AB}$ model alloy with long-range order is calculated in the coherent potential approximation. Impurity states are found inside the energy gap and the energy gap disappears long before the alloy becomes completely disordered. The temperature-dependent long-range order exhibits a first-order order-disorder phase transition which causes a discontinuity in the electronic density of states at the middle of the energy gap at the critical temperature ${T}_{c}$.