Bimetallic core-shell nanoparticles such as NiCu are of great interest not only due to their excellent stability, selectivity, and magnetic and catalytic properties, but also because they are tunable by changing the morphology, surface element distribution, and particle size of the nanoparticles. The surface segregation and structural features of NiCu bimetallic nanoparticles, the deposition growth and the surface diffusion of Cu adsorbed atoms on the Ni substrate surface are studied by using molecular dynamics and the Montero method combined with embedded atomic potential. The results show that the Cu atom has a strong tendency of surface segregation. With the increase of concentration of Cu atoms, Cu atoms preferentially occupy the vertex, edge, (100), and (111) facet of nanoparticles due to the difference in configuration energy between Cu atoms and surface Ni atoms with different coordination numbers after the exchange, and finally form perfect Ni-core/Cu-shell nanoparticles. When growth temperature <i>T</i> = 400 K, the Ni-core/Cu-shell structure formed is the most stable. By observing the NiCu core-shell structure’s growth sequence, it is found that a few Ni atoms are replaced by Cu atoms on the step edge of the Ni substrate. The diffusion energy barrier of Cu atoms adsorbed on a Ni substrate surface is calculated by using the nudged elastic band method. The results show that Cu atoms adsorbed need to overcome a large ES barrier for both exchange and diffusion, making it difficult to diffuse between the facets of Ni substrate surface in a temperature range of 200–800 K. The lowest energy barrier for the diffusion of Cu atoms between facets of Ni substrate surface is 0.43 eV, and the diffusion path is from (111) facet to (100) facet. In contrast to Ni substrate, Ni atoms deposited on Cu substrate can easily migrate from the (111) facet to the (100) facet with a diffusion energy barrier of only about 0.12 eV, and at the present simulated temperature, Ni adsorbed atoms are unable to migrate on the (100) facet, resulting in a growth configuration toward an octahedral shape with its eight apex angles almost occupied by Ni atoms. In this paper, a new idea and method are provided for the preliminary design of NiCu nano-catalysts from atoms.

The rapid solidification process of liquid Cu64Zr36 alloy is simulated using a molecular dynamics method. The evolution in micro-structures are analyzed by means of pair distribution functions (PDF), Honeycutt-Andersen (H-A) bond-type index method and cluster-type index method (CTIM). It is found that both of liquid and rapidly solidified solid mostly consist of (12 0 12 0) icosahedra and their distorted (12 8/1551 2/1541 2/1431) configurations at a cooling rate of 50 K/ns, most of which are Cu-centered Cu8Zr5 clusters, followed by Cu7Zr6 and then Cu9Zr4 clusters. Size distribution of icosahedral medium-range order (IMRO) clusters linked by intercross-sharing (IS) atoms in the liquid and the glassy solid presents the magic number sequences of 13, 19, 25,···and 13, 19, 23, 25, 29, 37···, respectively. The track of atoms reveals no icosahedral clusters in rapidly solidified solid that can be detected in the liquid alloy. Onset temperature of configuration heredity emerges in the supercooled liquid region of Tm–Tg. A direct and perfect heredity of icosahedra is found to be dominant and a distinct ascent in heredity fraction takes place at Tg. Compared with (12 8/1551 2/1541 2/1431) distorted icosahedra, (12 0 12 0) standard icosahedra are of high structural stability and configurational genetic ability below Tg, whereas only a few can keep their chemical composition unchanged. By partial heredity, even some IMRO clusters in super-cooled liquid can be transmitted to glassy alloy.

The morphology and physical properties of crystal as well as the glass-forming ability (GFA) of metals are closely related to the evolution pathway of atomic structures in the early stage of nucleation in supercooled liquids. Therefore, the study of the evolution of atomic structures in the isothermal crystallization process of supercooled liquids, is of great significance not only for predicting and accurately controlling the crystal nucleation and growth, but also for understanding the local atomic structural origin of the GFA. In the present work, the atomic-level mechanism for isothermal crystallization in the supercooled liquid tantalum is studied by molecular dynamics (MD) simulation. The microstructural evolution of metal Ta system is characterized and analyzed by using the potential energy per atom (<i>PE</i>), the pair distribution function (PDF) g(r), and the largest standard cluster (LSC). Two crystallization paths of Ta supercooled liquid can be observed during isothermal relaxations. For each pathway the incubation time of the formation critical nucleus increases with annealing temperature (<i>T</i>) rising. At 1800 K ≤ <i>T</i> ≤ 1850 K, the crystallization of supercooled liquid Ta conforms to the Ostwald's step rule: first, Z12 (i.e. icosahedron) and Z14 (Kasper cluster with 14 coordination number) clusters in supercooled liquids are hinged into medium-range order (i.e., Z-MRO); then the Z-MRO are merged and ordered into A15 crystal phase; finally, BCC crystal nucleus inside of the A15 phase grows rapidly into BCC single crystal at the cost of the atoms in A15 phase. While at 1900 K ≤ <i>T</i> ≤ 1950 K, Ta supercooled liquid is directly transformed into A15 phase. The A15 crystal phase is mainly formed by the continuous merging of the largest Z-MRO with the small Z-MRO, which is similar to the picture of the classical nucleation theory (CNT). However, whether the phase transition from A15 to BCC will occur above 1900 K remains to be further confirmed by a longer-time MD simulation. Relative to the supercooled liquids of monoatomic metals with lower melting point, the good GFA of Ta may originate from the slowly growing A15 crystal nucleus in its supercooled liquid.

NiAl nanoparticles possess high-energy density and good mechanical properties at elevated temperatures, and are considered as an important material. However, the differences in the diffusion behavior of Al adsorbed atoms on different Ni substrate surfaces and the effects of different diffusion mechanisms on the deposition growth of Al atoms on the Ni substrate surface are highly desired to be clarified. Therefore, in the present work, the diffusion behavior of single Al adsorbed atoms and nanoparticle cluster growth on the Ni substrate surface of decahedral (DEC), cuboctahedral(CUB) and icosahedral(ICO) structures are systematically studied by molecular dynamics (MD) throuh analyzing the embedded atom potentialand using the nudged elastic band method. The diffusion barriers of Al adsorbed atoms on three different Ni substrates are calculated by nudged elastic band methodand analyzed, showing that the diffusion barrier is greatly affected by the smoothness of the step edge and the atomic coordination number of substrate as well. The diffusions of Al adsorption atoms on the surfaces of three Ni substrates are realized by two mechanisms, namely exchanging or hoping, and the lowest Ehrlich-Schwoebel (ES) barrier is 0.38 eV for exchange CUB{111} → {100}, 0.52 eV for exchange DEC{111} → {100}, and 0.52 eV for hoping ICO {111} → {111}. The exchanging mechanismsupports Al adatoms diffusing from {111} to {100} facet on the three Ni substrates, while the diffusion between two adjacent {111} facets is mainly driven by the hoping mechanism. On this basis, atom-by-atom growth MD simulation is used to study the structure of the Ni-Al cluster. The deposited Al atoms first tend to diffuse near the edges of the steps and the vertices. The deposited Al atoms begin to aggregate into islands with the increase of their number. For Al atoms on the Ni cluster, a good Ni-core/Al-shell structure can be obtained by depositing Al atoms on the surface of Ni substrate at lower temperatures. In this core-shell structure, Al atoms have a larger surface energy and atom radius compared with Ni atoms. For the ICO substrate, the corresponding defect number of core-shell clusters is smaller than for the CUB and the DEC substrate, which is in good agreement with the diffusion behavior of Al adsorbed atoms on the Ni substrate cluster surface. The surface of Ni-Al bimetal is gradually alloyed with the increase of growth temperature. This study provides a good insight into the diffusion and growth of Al adsorbed atoms on Ni substrates surface on an atomic scale.

To explore the origin of glassy transition and glass-forming abilities (GFAs) of transition metal-transition metal (TM-TM) alloys from the microstructural point of view, a series of molecular dynamics (MD) simulation for the rapid solidification processes of liquid Cu56Zr44alloys at various cooling rates and pressures P are performed by using a LAMPS program. On the basis of Honeycutt-Andersen (H-A) bond-type index (ijkl), we propose an extended cluster-type index (Z, n/(ijkl)) method to characterize and analyze the microstructures of the alloy melts as well as their evolution in the rapid solidification. It is found that the majority of local atomic configurations in the rapidly solidified alloy are (12 12/1551) icosahedra, as well as (12 8/1551 2/1541 2/1431) and (12 2/1441 8/1551 2/1661) defective icosahedra, but no relationship can be seen between their number N(300 m K) and the glassy transition temperature Tg of rapidly solidified Cu56Zr44alloys. By an inverse tracking of atom trajectories from low temperatures to high temperatures the configuration heredity of icosahedral clusters in liquid is discovered to be an intrinsic feature of rapidly solidified alloys; the onset of heredity merely emerges in the super-cooled liquid rather than the initial alloy melt. Among these the (12 12/1551) standard icosahedra inherited from the super-cooled liquids at Tm-Tg is demonstrated to play a key role in the formation of Cu56Zr44 glassy alloys. Not only is their number N300 KTgP inherited from Tg to 300 K closely related to the GFA of rapidly solidified Cu56Zr44alloys, but a good correspondence of the onset temperatures of heredity (Tonset) with the reduced glass transition temperature (Trg= Tg/Tm) can be also observed. As for the influence of and P on the glassy transition, a continuous tracking of descendible icosahedra reveals that the high GFA of rapidly solidified Cu56Zr44 alloys caused by big and P can be attributed to their elevated inheritable fraction (fp and ftotal) above Tg.