[en] Undercooling experiments have been realized on the Re-W system by using a 48 meters hugh drop-tube facility. A double recalescence phenomenon is systematically obtained within the (W) solid solution range between 65 and 85 at%W. Is is shown that the first recalescence event involves the transitory appearance of a metastable A15 phase. Confidence in the nucleation path is gained with the help of calculations from first principles. The metastable phase is also detected in some droplets by X-ray diffraction. (authors)

[en] We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory.

[en] Li⁺ ion incorporation into the V₂O₅ lattice after reducing V₂O₅ single crystals with butyllithium has been studied by ESR. X band ESR spectra are presented and discussed

[en] We present microscopic calculations of the chemical short-range order in amorphous and liquid alloys of two transition metals. Our approach is based on a model one-band tight-binding Hamiltonian and a thermodynamic variational technique founded on the Gibbs-Bogolyubov inequality and a hard-sphere Yukawa fluid as a reference system. We calculate the Bhatia-Thornton partial structure factors of Ni/sub x/Ti/sub 1-//sub x/ and Ni/sub x/Zr/sub 1-//sub x/ alloys and find good agreement with experiment

[en] The enthalpies of formation are measured in the intermetallic compounds Zr₂Cu, ZrCu and the amorphous alloy Zrsub(0.55)Cusub(0.45) by means of a solution calorimeter based on liquid aluminium. The crystallization enthalpies of the amorphous alloys Zrsub(1-x)Cusub(x) with x = 0.38, 0.45, and 0.55 are determined by means of a differential scanning calorimeter. All these values are found to be relatively small compared with the activation energies for crystallization derived from a study of the crystallization temperature as a function of the heating rate. The enthalpy difference between amorphous and crystalline materials is analysed in terms of Miedema's model. The results indicate that the lower enthalpy content in the former materials is mainly due to a less effective surrounding of the atoms by dissimilar atoms. (author)

No abstract available

[en] A tight-binding recursion method is used to study the electronic and cohesive properties of stoichiometric and substoichiometric transition metal-carbides crystallizing with the NaCl structure. The electronic structure is analyzed in terms of a p-d interaction between the p orbitals of C and the d orbitals of the transition metal. Concerning the cohesive properties of these compounds, we have calculated the energies of formation of stoichiometric (Ti, Zr, Hf, V, Nb, Ta)-C compounds and compared with the experimental values. We have also studied the stability of vacancy-ordered compounds with a vacancy concentration of 50% on the carbon sublattice, i.e. M₂C compounds. (orig.)

[en] The formation enthalpies of CeNi₅ and YNi₅ derived from dissolution calorimetry measurements were found to be -199 kJ mol^-1 and -205 kJ mol^-1, respectively. On grounds of this small difference, it is suggested that the 4f electrons of Ce do not participate in the bonding in CeNi₅

[en] As it approaches the glass transition, particle motion in liquids becomes highly heterogeneous and regions with virtually no mobility coexist with liquid-like domains. This complex dynamic is believed to be responsible for different phenomena including non-exponential relaxation and the breakdown of the Stokes–Einstein relation. Understanding the relationships between dynamical heterogeneities and local structure in metallic liquids and glasses is a major scientific challenge. Here we use classical molecular dynamics simulations to study the atomic dynamics and microscopic structure of ${\mathrm{C}\mathrm{u}}_{50}{\mathrm{Z}\mathrm{r}}_{50}$ alloy in the supercooling regime. Dynamical heterogeneities are identified via an isoconfigurational analysis. We demonstrate the transition from isolated to clustering low mobility with decreasing temperature. These slow clusters, whose sizes grow upon cooling, are also associated with concentration fluctuations, characterized by a Zr-enriched phase, with a composition ${\mathrm{C}\mathrm{u}\mathrm{Z}\mathrm{r}}_2$. In addition, a structural analysis of slow clusters based on Voronoi tessellation evidences an increase with respect of the bulk system of the fraction of Cu atoms having a local icosahedral order. These results are in agreement with the consolidated scenario of the relevant role played by icosahedral order in the dynamic slowing-down in supercooled metal alloys. (paper)

No abstract available