By extrapolating the mean grain size of nanocrystal to an infinitesimal value, an amorphous phase has been obtained from the Voronoi construction. The molecular dynamics simulations indicated that for nanocrystal, the grain size variation of melting temperature exhibits two characteristic regions. As mean grain size above about 4 nm for Ag, the melting temperatures decrease with decreasing grain size. However, with grain size further shrinking, the melting temperatures almost keep a constant. This is because the dominant factor on the melting temperature of nanocrystal shifts from grain phase to grain boundary. As a result of fundamental difference in structure, the amorphous phase has a much lower solid-to-liquid transformation temperature than that of nanocrystal.

The thermodynamic data, such as the formation enthalpies of disordered solid solutions and intermetallic compounds, and the lattice parameters in the RE–Au (RE = Sc, Y, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) systems are calculated with the modified analytical embedded atom method (EAM). This work has been undertaken to investigate systemically in RE–Au alloying energies, and to augment available calorimetric data for enthalpies of formation in support of the development of accurate multicomponent thermodynamics databases for these technologically interesting systems. The accuracy of our calculations is assessed through comparisons with the experimental measurements and the theoretical results from the first-principles VASP as well as Miedema’s theory. The composition dependence of the heats of formation for these 10 binary systems is similar, and there is a minimum at 50 at.% Au for REAu (CsCl-type) ordered phase. In all 10 binary systems, the calculated zero-temperature intermetallic formation energies generally agree well with the calorimetric data obtained by the direct reaction synthesis and/or VASP results except for REAu2 (MoSi2type) phase. For the intermetallic phase, the calculated EAM and VASP zero-temperature lattice parameters agree well with the experimental data at ambient temperature. The largest discrepancy between the EAM calculated lattice parameters and experimental data [K. Fitzner,W.G. Jung, O.J. Kleppa, Metall. Trans. A 22 (1991) 1103 [16]; K. Fitzner, O.J. Kleppa, Metall. Trans. A 24 (1993) 1827 [17]; K. Fitzner, O.J. Kleppa, Metall. Trans. A 25 (1994) 1495 [18]; R. Ferro, G. Borzone, N. Parodi, J. Alloys. Compd. 321 (2001) 248 [19]] is approximately 10-18% for some intermediate phases, such as REAu (CrB-type) and RE3Au4 (Pu3Pd4-type).

Analytic modified embedded atom method (AMEAM) type many-body potentials have been constructed for ten hcp metals: Be, Co, Hf, Mg, Re, Ru, Sc, Ti, Y and Zr. The potentials are parametrized using analytic functions and fitted to the cohesive energy, unrelaxed vacancy formation energy, five independent second-order elastic constants and two equilibrium conditions. Hence, each of the constructed potentials represents a stable hexagonal close-packed lattice with a particular non-ideal c/a ratio. In order to treat the metals with negative Cauchy pressure, a modified term has been added to the total energy. For all the metals considered, the hcp lattice is shown to be energetically most stable when compared with the fcc and bcc structure and the hcp lattice with ideal c/a. The activation energy for vacancy diffusion in these metals has been calculated. They agree well with experimental data available and those calculated by other authors for both monovacancy and divacancy mechanisms and the most possible diffusion paths are predicted. Stacking fault and surface energy have also been calculated and their values are lower than typical experimental data. Finally, the self-interstitial atom (SIA) formation energy and volume have been evaluated for eight possible sites. This calculation suggests that the basal split or crowdion is the most stable configuration for metals with a rather large deviation from the ideal c/a value and the non-basal dumbbell (C or S) is the most stable configuration for metals with c/a near ideal. The relationship between SIA formation energy and melting temperature roughly obeys a linear relation for most metals except Ru and Re.

In the atomic scale, the melting behaviors of nanocrystalline Ag with mean grain size ranging from 3.03 to 12.12 nm have been investigated with molecular dynamics simulations, and a method to determine the melting temperatures of the infinite polycrystalline nanostructured materials is presented. It is found that the melting in nanostructured polycrystals starts from their grain boundaries, and the relative numbers of the three typical bonded pairs, (1551), (1431), and (1541), existing in the liquid phase, increase rapidly with the evolvement of melting. The melting temperatures of nanocrystalline Ag decrease with decreasing mean grain size, and it can be estimated from the size-dependent melting temperature of the corresponding nanoparticles.

Using the modified analytic embedded atom method and molecular dynamics, the binding energies and their second order finite differences stability functionsd of icosahedral Ni clusters with shell and subshell periodicity are studied in detail via atomic evolution. The results exhibit shell and sub shell structures of the clusters with atoms from 147 to 250 000, and the atomic numbers corresponding to shell or subshell structures are in good agreement with the experimental magic numbers obtained in time-of-flight mass spectra of threshold photoionization, and Martin’s theoretical proposition of progressive formation of atomic umbrellas. Clusters with size from 147 to 561 atoms are energetically investigated via one-by-one atomic evolution and their magic numbers are theoretically proved. For medium-size Ni clusters with 561 to 2057 atoms, the prediction of magic numbers with atomic numbers is performed on the basis of umbrellalike subshell growth in near face-edge-vertex order. The similarity of the energy curves makes it possible to extend the prediction to even larger Ni nanoclusters in hierarchical Mackay icosahedral configurations.

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs 555 , 433 , and 544 , existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.

Using molecular dynamics simulations with an analytical embedded-atom method (MAEAM) model, the diffusion behavior of helium atoms at two Σ=5 symmetric grain boundaries of Ni was investigated in the present paper. The simulations demonstrate that the interstitial helium atoms are stable at the grain boundaries. Helium atoms in Ni lattices could diffuse easily to the grain boundary and precipitate into a cluster, which confirmed the fact that helium atoms could nucleate and grow up into bubbles at the grain boundaries. The diffusion activation energy of interstitial helium atom at grain boundaries was also determined.

In the present paper, the surface and size effects on the alloying ability and phase stability of immiscible alloy nanoparticles have been studied with calculating the heats of formation of Au-Pt alloy nanoparticles from the single element nanoparticles of their constituents (Au and Pt) with a simple thermodynamic model and an analytic embedded atom method. The results indicated that, besides the similar compositional dependence of heat of formation as in bulk alloys, the heat of formation of alloy nanoparticles exhibits notable size-dependence, and there exists a competition between size effect and compositional effect on the heat of formation of immiscible system. Contrary to the positive heat of formation for bulkimmiscible alloys, a negative heat of formation may be obtained for the alloy nanoparticles with a small size or dilute solute component, which implies a promotion of the alloying ability and phase stability of immiscible system on a nanoscale. The surface segregation results in an extension of the size range of particles with a negative heat of formation. The molecular dynamics simulations have indicated that the structurally and compositionally homogeneous Au Pt nanoparticles tend to form a core-shell structure with temperature increasing.

The surface premelting, melting behavior, melting competition, and structural transition of shell-closed icosahedral ICO and cuboctahedral CUB nickel clusters with atoms from 309 to 2057 were discussed extensively by using quantitative caloric curves based on the modified analytic embedded atom method and molecular dynamics, qualitative three-dimensional structural visualization of symmetric truncation, and the radial number distribution function. These studies reveal that smaller clusters melt at lower temperatures and a solid-to-solid structural transition occurs from CUB to ICO structure during melting process. The shell-closed ICO clusters could only be preferred until 923 atoms at temperatures no higher than 1380 K, which is in agreement with the experiments. The melting temperature of larger clusters would depend on their starting structures, which can be attributed to surface premelting.

Analytic embedded-atom method (EAM) interatomic potentials for body-centered cubic (bcc) transition metals have been constructed. The total energy is regarded as consisting of a pair potential part, an embedding interaction part and a modification term. All these parts are analytic functions of the atomic separations only and are fitted to various bulk properties, such as cohesive energy, vacancy formation energy, lattice constant and elastic constants. The present potentials are shown to predict a more realistic pressure–volume relationship so that interactions at separations smaller than that of the first-nearest neighbors can be treated. Interstitial formation energies are calculated for various configurations, using quenched molecular dynamics. Vacancy, surface and phonon spectra have been also studied andsatisfactory agreement with available experimental data has been found.