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[ES] El titanio comercialmente puro (CP-Ti) proporciona una excelente resistencia a la corrosión y tiene un amplio abanico de aplicaciones siendo el Ti6Al4V la aleación más utilizada en el ámbito de la biomedicina. Sin embargo, a pesar de sus características y comportamiento a corrosión, el titanio y sus aleaciones tienen un elevado precio debido a su complicada extracción y procesamiento. De esta situación surgen las técnicas de manufactura por pulvimetalurgia que han ganado una aceptación considerable en la fabricación en grandes cantidades de otros metales ligeros como el aluminio. Estas técnicas pueden proveer una forma económica de procesar aleaciones de titanio. El desarrollo de aleaciones de titanio mediante pulvimetalurgia implicaría una gran oportunidad para expandir las aplicaciones de estas aleaciones en un gran número de industrias como automoción, aeroespacial y particularmente la biomédica. Aunque los estudios de la adición de estaño en titanio para obtener aleaciones mediante pulvimetalurgia han sido limitados es interesante estudiar el comportamiento de estas debido a su buena biocompatibilidad y a sus propiedades mecánicas en contraposición al Ti6Al4V ya que al utilizarlo para implantes en óseos se ha comprobado que se producía un desgaste considerable en el hueso. Para estudiar el comportamiento de estas aleaciones se han generado muestras de Ti-1Sn, Ti-2Sn y Ti-4Sn mediante técnicas de pulvimetalurgia convencionales y se han estudiado sus propiedades mecánicas mediante ensayos de flexión a cuatro puntos, microdureza y excitación por impulso. También se ha estudiado su microestructura mediante microscopía óptica, electrónica de barrido y EBSD además de sus propiedades químicas mediante ensayos de liberación de iones y corrosión., [CA] El titani comercialment pur (CP-Ti) proporciona una excel·lent resistència a la corrosió i té un ampli ventall d'aplicacions sent el Ti6Al4V l'aliatge més utilitzat en l'àmbit de la biomedicina. No obstant això, malgrat les seues característiques i comportament a corrosió, el titani i els seus aliatges tenen un elevat preu degut a la seua complicada extracció i processament. D'aquesta situació sorgeixen les tècniques de manufactura per pulvimetalurgia que han guanyat una acceptació considerable en la fabricació en grans quantitats d'altres metalls lleugers com l'alumini. Aquestes tècniques poden proveir una forma econòmica de processar aliatges de titani. El desenvolupament d'aliatges de titani mitjançant pulvimetalurgia implicaria una gran oportunitat per a expandir les aplicacions d'aquests aliatges en un gran nombre d'indústries com a automoció, aeroespacial i particularment la biomèdica. Encara que els estudis de l'addició d'estany en titani per a obtindre aliatges mitjançant pulvimetalurgia han sigut limitats és interessant estudiar el comportament d'aquestes a causa de la seua bona biocompatibilitat i a les seues propietats mecàniques en contraposició al Ti6Al4V ja que en utilitzar-lo per a implants en ossis s'ha comprovat que es produïa un desgast considerable en l'os. Per a estudiar el comportament d'aquests aliatges s'han generat mostres de Ti-1Sn, Ti-2Sn i Ti-4Sn mitjançant tècniques de pulvimetalurgia convencionals i s'han estudiat les seues propietats mecàniques mitjançant assajos de flexió a quatre punts, microduresa i excitació per impuls. També s'ha estudiat la seua microestructura mitjançant microscòpia òptica, electrònica d'escombratge i EBSD a més de les seues propietats químiques mitjançant assajos d'alliberament d'ions i corrosió., [EN] Commercially pure titanium (CP-Ti) provides excellent corrosion resistance and has a wide range of applications, with Ti6Al4V being the most widely used alloy in the field of biomedicine. However, despite their characteristics and corrosion behavior, titanium and its alloys have a high price due to their complicated extraction and processing. Because of this situation, powder metallurgy manufacturing techniques arose and have gained considerable acceptance in the large quantity manufacturing of other light metals such as aluminum. These techniques can provide an inexpensive way to process titanium alloys. The development of titanium alloys by powder metallurgy would imply a great opportunity to expand the applications of these alloys in many industries such as automotive, aerospace and particularly biomedical. Although the studies of the addition of tin in titanium to obtain alloys by powder metallurgy have been limited, it is interesting to study their behavior due to their good biocompatibility and their mechanical properties in contrast to Ti6Al4V since, when used for implants in bone, it has been proven that there was considerable wear on the bone. To study the behavior of these alloys, samples of Ti-1Sn, Ti-2Sn and Ti-4Sn have been generated using conventional powder metallurgy techniques and their mechanical properties have been studied using four-point bending, microhardness and impulse excitation tests. Its microstructure has also been studied using optical, scanning electron and EBSD microscopy, as well as its chemical properties through ion release and corrosion tests.

Domain growth in a system with nonconserved order parameter is studied. We simulate the usual Ising model for binary alloys with concentration 0.5 on a two-dimensional square lattice by Monte Carlo techniques. Measurements of the energy, jump-acceptance ratio, and order parameters are performed. Dynamics based on the diffusion of a single vacancy in the system gives a growth law faster than the usual Allen-Cahn law. Allowing vacancy jumps to next-nearest-neighbor sites is essential to prevent vacancy trapping in the ordered regions. By measuring local order parameters we show that the vacancy prefers to be in the disordered regions (domain boundaries). This naturally concentrates the atomic jumps in the domain boundaries, accelerating the growth compared with the usual exchange mechanism that causes jumps to be homogeneously distributed on the lattice.

Geometric parameters of binary ~1:1! PdZn and PtZn alloys with CuAu-L1 0 structure were calculated with a density functional method. Based on the total energies, the alloys are predicted to feature equal formation energies. Calculated surface energies of PdZn and PtZn alloys show that ~111! and ~100! surfaces exposing stoichiometric layers are more stable than ~001! and ~110! surfaces comprising alternating Pd ~Pt! and Zn layers. The surface energy values of alloys lie between the surface energies of the individual components, but they differ from their composition weighted averages. Compared with the pure metals, the valence d-band widths and the Pd or Pt partial densities of states at the Fermi level are dramatically reduced in PdZn and PtZn alloys. The local valence d-band density of states of Pd and Pt in the alloys resemble that of metallic Cu, suggesting that a similar catalytic performance of these systems can be related to this similarity in the local electronic structures.

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, not considered in previous works. 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 x=1/2. 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., 21 pages, 9 figures, accepted for publication in Phys. Rev. B

In the study of out of equilibrium relaxation phenomena two processes are considered as prototypes. Both correspond to the evolution of a solid AB binary alloy quenched from a high-temperature disordered phase to a temperature below a phase transition. The first case is the growth of ordered domains @nonconserved order parameter ~NCOP!# and the second is the phase separation problem @conserved order parameter ~COP!#. There is general agreement, 1 in the fact, that in both cases the mean size of the domains R grows, during the late stages of the evolution, following a power law. Such laws @R(t);t x # are characterized by the growth exponent x that takes the value x NCOP51/2 ~Allen-Cahn law 2 ! for the nonconserved case and x COP51/3 ~Lifshitz-Slyozov law 3 ! for the conserved case. These values are considered to be universal. They have been corroborated by theoretical arguments, numerical analysis of Langevin equations and Monte Carlo ~MC! simulations of lattice systems. 1 In this report we

We have performed a Monte Carlo simulation of the influence of diffusing vacancies on the antiphase domain growth process in a binary alloy after a quench through an order-disorder transition. The problem has been modeled by means of a Blume-Emery-Griffiths Hamiltonian whose biquadratic coupling parameter $K$ controls the microscopic interactions between vacancies. The asymmetric term $L$ has been taken as $L=0$ and the ordering dynamics has been studied at very low temperature as a function of $K$ inside the range $\ensuremath{-}0.5l~K/Jl~1.40$ (with $Jg0$ being the ordering energy). The system evolves according to the Kawasaki dynamics so that the alloy concentration is conserved while the order parameter is not. The simulations have been performed on a two-dimensional square lattice and the concentration has been taken so that the system corresponds to a stoichiometric alloy with a small concentration of vacancies. We find that, independently of $K$, the vacancies exhibit a tendency to concentrate at the antiphase boundaries. This effect gives rise, via the vacancy-vacancy interaction (described by $K)$, to an effective interaction between bulk diffusing vacancies and moving interfaces that turns out to strongly influence the domain growth process. One distinguishes three different behaviors: (i) For $K/Jl1$ the growth process of ordered domains is anisotropic and can be described by algebraic laws with effective exponents lower than $1/2$; (ii) $K/J\ensuremath{\simeq}1$ corresponds to the standard Allen-Cahn growth; (iii) for $K/Jg1$ we found that, although the motion of the interface is curvature driven, the repulsive effective interaction between both the vacancies in the bulk and those at the interfaces slows down the growth.

A Monte Carlo study of the late time growth of L12-ordered domains in a fcc A3B binary alloy is presented. The energy of the alloy has been modeled by a nearest-neighbor interaction Ising Hamiltonian. The system exhibits a fourfold degenerated ground state and two kinds of interfaces separating ordered domains: flat and curved antiphase boundaries. Two different dynamics are used in the simulations: the standard atom-atom exchange mechanism and the more realistic vacancy-atom exchange mechanism. The results obtained by both methods are compared. In particular we study the time evolution of the excess energy, the structure factor and the mean distance between walls. In the case of atom-atom exchange mechanism anisotropic growth has been found: two characteristic lengths are needed in order to describe the evolution. Contrarily, with the vacancyatom exchange mechanism scaling with a single length holds. Results are contrasted with existing experiments in Cu3Au and theories for anisotropic growth.

We use a nearest-neighbor antiferromagnetic Ising model with spin-exchange dynamics to study by Monte Carlo simulations the dynamics of ordering in low-temperature quenched nonstoichiometric A xB 1 − x binary alloys. By implementing the conserved spin-exchange dynamics into the Monte Carlo method the system evolves so that the density is preserved while the order parameter is not. The simulations have been carried out on a two-dimensional square lattice and the stoichiometric value of the composition x is x 0 =0.50. By using different values of x ranging from 0.60≤x≤ x 0 =0.50, we study the influence of the off-stoichiometry on the dynamics of ordering. Regarding the behavior of the excess particles all along the ordering process, we obtain two different regimes. (i) At early to intermediate times the density of excess particles at the interfaces rapidly increases, reaching a saturated value. This density of saturation depends on both composition and temperature. As a consequence of this, since the disorder tends to be localized at the interfaces, the local order inside the growing domains is higher than the equilibrium value. (ii) Once saturation is reached, the system evolves so that the density of excess particles at the interfaces remains constant. During this second regime the excess particles are expelled back to the bulk as the total interface length decreases. We use two different measures for the growth: the total interface length and the structure factor. We obtain that during the second regime scaling holds and the domain-growth process can be characterized, independently on x, by a unique length which evolves according to l(t)∼ t n being n ∼ (0.50–0.40). Although the growth process tends to be slower as x increases, we find that the domain-wall motion follows the main assumptions underlying the Allen-Cahn theory. This is indicative that the coupling between diffusive excess particles and curvature-driven interface motion does not modify the essential time dependence but varies (slows down) the growth rate of the growth law, i.e., l(t)= k 1 / 2 xt , with k x decreasing with x. We suggest that the logarithmic growth experimentally observed in some nonstoichiometric binary materials has to do with the existence of specific interactions (not present in our case) between diffusive particles and domain walls. These interactions are of crucial importance in determining the essential time dependence of the growth law.

El fenomen de les transicions de fase ha estat, durant molts anys, un tema de gran interès dins de la comunitat científica. Actualment, aquest interès continua vigent i es reflecteix en la gran quantitat de treballs, tant teòrics com experimentals, publicats sobre el tema durant els darrers anys. La raó d'aquest interès és, en part, de tipus fonamental, però també es deu a les repercussions de tipus tecnològic que comporta.

The relationship between the elastic shear modulus C'=1/2(${C}_{11}$-${C}_{12}$) and the atomic order state in a shape-memory binary alloy ${A}_{x}$${B}_{1\mathrm{\ensuremath{-}}x}$ above its martensitic transition temperature is analyzed. We first present a simple method to evaluate the elastic constants in binary alloys, assuming the atoms interact via a two-body Morse potential. For CuZn and AgZn alloys, the potential parameters corresponding to the different A-A, B-B, and A-B pairs are determined from experimental data of the elastic constant C' for different alloy compositions. We next calculate C' at 0 K as a function of the ordering state. To do this, we use atomic configurations obtained with a Monte Carlo simulation of the Ising model for a bcc binary alloy, at each temperature ${T}_{i}$. We obtain a linear relationship between C' and the short-range-order parameter \ensuremath{\eta}. We also show that the deviations from the linear behavior observed when C' is represented against the square of the long-rang-order parameter 〈S〉 come mainly from the critical behavior of the system near the order-disorder temperature ${T}_{c}$.

The dynamic collective properties of the liquid Li0.7-Mg0.3 alloy are studied by molecular-dynamics simulation. The dynamic structure factors and longitudinal current correlations at wave vectors between the hydrodynamic and kinetic regime (0.2,k, 4A 21 ) are analyzed. In order to discuss the influence of the mass difference between particles on the longitudinal modes, the same alloy, except that the mass of the heavy atoms was increased by a factor of 10, was simulated. The resulting properties are compared with those of the ordinary Li0.7-Mg0.3 alloy. It is shown that at wave vectors prior to the hydrodynamic region both fast and slow longitudinal modes of kinetic character propagate through light and heavy particles, respectively. In the hydrodynamic limit fast and slow modes merge into a single acoustic mode. It is corroborated that fast propagating modes in disparate mass liquid mixtures have a kinetic character. The number-number and concentration-concentration time correlation functions were also determined. The former shows a behavior analogous to that of the corresponding function in one-component liquids. The second reflects the existence of propagating concentration modes.

We use a nearest-neighbor antiferromagnetic Ising model with spin-exchange dynamics to study by Monte Carlo simulations the dynamics of ordering in low-temperature quenched nonstoichiometric A xB 1 − x binary alloys. By implementing the conserved spin-exchange dynamics into the Monte Carlo method the system evolves so that the density is preserved while the order parameter is not. The simulations have been carried out on a two-dimensional square lattice and the stoichiometric value of the composition x is x 0 =0.50. By using different values of x ranging from 0.60≤x≤ x 0 =0.50, we study the influence of the off-stoichiometry on the dynamics of ordering. Regarding the behavior of the excess particles all along the ordering process, we obtain two different regimes. (i) At early to intermediate times the density of excess particles at the interfaces rapidly increases, reaching a saturated value. This density of saturation depends on both composition and temperature. As a consequence of this, since the disorder tends to be localized at the interfaces, the local order inside the growing domains is higher than the equilibrium value. (ii) Once saturation is reached, the system evolves so that the density of excess particles at the interfaces remains constant. During this second regime the excess particles are expelled back to the bulk as the total interface length decreases. We use two different measures for the growth: the total interface length and the structure factor. We obtain that during the second regime scaling holds and the domain-growth process can be characterized, independently on x, by a unique length which evolves according to l(t)∼ t n being n ∼ (0.50–0.40). Although the growth process tends to be slower as x increases, we find that the domain-wall motion follows the main assumptions underlying the Allen-Cahn theory. This is indicative that the coupling between diffusive excess particles and curvature-driven interface motion does not modify the essential time dependence but varies (slows down) the growth rate of the growth law, i.e., l(t)= k 1 / 2 xt , with k x decreasing with x. We suggest that the logarithmic growth experimentally observed in some nonstoichiometric binary materials has to do with the existence of specific interactions (not present in our case) between diffusive particles and domain walls. These interactions are of crucial importance in determining the essential time dependence of the growth law.