Marcel Porta

Development of a tight-binding potential for bcc Zr: Application to the study of vibrational properties

We present a tight-binding potential based on the moment expansion of the density of states, which includes up to the fifth moment. The potential is fitted to bcc and hcp Zr and it is applied to the computation of vibrational properties of bcc Zr. In particular, we compute the isothermal elastic constants in the temperature range 1200 K,T,2000 K by means of standard Monte Carlo simulation techniques. The agreement with experimental results is satisfactory, especially in the case of the stability of the lattice with respect to the shear associated with C8. However, the temperature decrease of the Cauchy pressure is not reproduced. The T5 0K phonon frequencies of bcc Zr are also computed. The potential predicts several instabilities of the bcc structure, and a crossing of the longitudinal and transverse modes in the ~001! direction. This is in agreement with recent ab initio calculations in Sc, Ti, Hf, and La.

Vacancy-assisted domain growth in asymmetric binary alloys: A Monte Carlo study

A Monte Carlo simulation study of the vacancy-assisted domain growth in asymmetric binary alloys is presented. The system is modeled using a three-state ABV Hamiltonian which includes an asymmetry term. Our simulated system is a stoichiometric two-dimensional binary alloy with a single vacancy which evolves according to the vacancy-atom exchange mechanism. We obtain that, compared to the symmetric case, the ordering process slows down dramatically. Concerning the asymptotic behavior it is algebraic and characterized by the Allen-Cahn growth exponent x51/2. The late stages of the evolution are preceded by a transient regime strongly affected by both the temperature and the degree of asymmetry of the alloy. The results are discussed and compared to those obtained for the symmetric case. @S0163-1829~99!09329-7#

Effect of the vacancy interaction on antiphase domain growth in a two-dimensional binary alloy

Departament d’Estructura i Constituents de la Mate`ria, Facultat de Fi´sica, Universitat de Barcelona, Diagonal 647,E-08028 Barcelona, Catalonia, Spain~Received 26 March 1997; revised manuscript received 16 May 1997!We have performed a Monte Carlo simulation of the influence of diffusing vacancies on the antiphasedomain growth process in a binary alloy after a quench through an order-disorder transition. The problem hasbeen modeled by means of a Blume-Emery-Griffiths Hamiltonian whose biquadratic coupling parameterKcontrols the microscopic interactions between vacancies. The asymmetric term L has been taken as L50 andthe ordering dynamics has been studied at very low temperature as a function of K inside the range20.5<K/J<1.40 ~with J.0 being the ordering energy!. The system evolves according to the Kawasakidynamics so that the alloy concentration is conserved while the order parameter is not. The simulations havebeen performed on a two-dimensional square lattice and the concentration has been taken so that the systemcorresponds to a stoichiometric alloy with a small concentration of vacancies. We find that, independently ofK, the vacancies exhibit a tendency to concentrate at the antiphase boundaries. This effect gives rise, via thevacancy-vacancy interaction~described by K), to an effective interaction between bulk diffusing vacancies andmoving interfaces that turns out to strongly influence the domain growth process. One distinguishes threedifferent behaviors: ~i! For K/J,1 the growth process of ordered domains is anisotropic and can be describedby algebraic laws with effective exponents lower than 1/2; ~ii! K/J.1 corresponds to the standard Allen-Cahngrowth; ~iii! for K/J.1 we found that, although the motion of the interface is curvature driven, the repulsiveeffective interaction between both the vacancies in the bulk and those at the interfaces slows down the growth.@S0163-1829~97!06933-6#I. INTRODUCTION

Statistico‐thermodynamic properties of a binary alloy

We use the antiferromagnetic Ising model to obtain the equilibrium properties of a bcc binary alloy AxB1−x which undergoes an order–disorder transition. We use Monte Carlo simulations to obtain the μ–T phase diagram, for different values of x, and discuss how to interpret the results in relation to experiments.

Intermittent dynamics in externally driven ferroelastics and strain glasses

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Precursor Nanoscale Textures: from Tweed to Glassy Behaviour

We study spatially inhomogeneous states that occur as precursors of marten-sitic/ferroelastic transitions. We will show that cross-hatched modulations (tweed patterns)arise at temperatures above the phase transition in the limit of high elastic anisotropy or lowdisorder while a nano-cluster phase-separated state occurs at low anisotropy or high disorder.In the latter case, nanoscale inhomogeneities give rise to glassy behaviour while the structuraltransition is inhibited. Interestingly, in this case the ferroelastic system also displays a largethermo-mechanical response so that the low symmetry structure can be easily induced by theapplication of relatively small stresses within a broad temperature range.