Surface energies of the solid solutions between Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W

The surface energy for different surface orientations of the solid solutions as a function of concentration formed by Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W is computed and analyzed using the BFS method for alloys. Similarities and differences among the different binary alloys are examined in terms of strain and chemical effects.     

Modeling of the deposition of Ni and Pd on Mo(1 1 0)

Recent experimental work on the deposition of fcc metals on a bcc substrate motivates this atomistic modeling analysis of Ni and Pd deposition on Mo(1 1 0). A detailed atom-by-atom analysis of the early stages of growth, focusing on the formation of surface alloys and 3D islands is presented, identifying the interactions leading to each type of behavior. Further analysis describes the growth pattern as a function of coverage. Temperature effects are studied via Monte Carlo simulations using the Bozzolo-Ferrante-Smith (BFS) method for alloys for the energetics.     

Atomistic modeling of Zn deposition on the low index faces of Cu

The formation process of Zn/Cu surface alloys is investigated using the Bozzolo-Ferrante-Smith (BFS) method for alloys. The effects of the crystallographic orientation on the deposition process, formation of surface alloys as a function of temperature and coverage, Zn surface migration, and interdiffusion in the Cu substrate, are modeled and discussed with atom-by-atom energy analyses and large scale simulations.     

Phase equilibrium of Cd_(1-x)Zn_xS alloys studied by first-principles calculations and Monte Carlo simulations

The first-principles calculations based on density functional theory combined with cluster expansion techniques and Monte Carlo(MC) simulations were used to study the phase diagrams of both wurtzite(WZ) and zinc-blende(ZB)Cd_(1-x)Zn_xS alloys.All formation energies are positive for WZ and ZB Cd_(1-x)Zn_xS alloys,which means that the Cd_(1-x)Zn_xS alloys are unstable and have a tendency to phase separation.For WZ and ZB Cd_(1-x)Zn_xS alloys,the consolute temperatures are 655 K and 604 K,respectively,and they both have an asymmetric miscibility gap.We obtained the spatial distributions of Cd and Zn atoms in WZ and ZB Cd_(0.5)Zn_(0.5)S alloys at different temperatures by MC simulations.We found that both WZ and ZB phases of Cd_(0.5)Zn_(0.5)S alloy exhibit phase segregation of Cd and Zn atoms at low temperature,which is consistent with the phase diagrams.     

Comparison of formation and evolution of radiation-induced defects in pure Ni and Ni-Co-Fe medium-entropy alloy

High-entropy alloys (HEAs) and medium-entropy alloys (MEAs) have attracted a great deal of attention for developing nuclear materials because of their excellent irradiation tolerance. Herein, formation and evolution of radiation-induced defects in NiCoFe MEA and pure Ni are investigated and compared using molecular dynamics simulation. It is observed that the defect recombination rate of ternary NiCoFe MEA is higher than that of pure Ni, which is mainly because, in the process of cascade collision, the energy dissipated through atom displacement decreases with increasing the chemical disorder. Consequently, the heat peak phase lasts longer, and the recombination time of the radiation defects (interstitial atoms and vacancies) is likewise longer, with fewer deleterious defects. Moreover, by studying the formation and evolution of dislocation loops in Ni-Co-Fe alloys and Ni, it is found that the stacking fault energy in Ni-Co-Fe decreases as the elemental composition increases, facilitating the formation of ideal stacking fault tetrahedron structures. Hence, these findings shed new light on studying the formation and evolution of radiation-induced defects in MEAs.     

Interfaces in Iron-Rich Ordered Fe-Al Alloys: An Atomic-Scale Simulation Study

The aim of this paper is to investigate the low-temperature structure of interfaces (the (3 1 0) [0 0 1] symmetrical tilt grain boundary (GB) and the (3 1 0) surface) in stoichiometric ordered Fe-Al alloys with B2 and DO₃ structures. In both alloys, (i) the GBs cannot be realistically described by geometrical models, (ii) GBs and surfaces show strong segregation effects. A simple independent-defect model cannot be applied: the interactions between point defects sometimes lead to results opposite to those predicted from the formation energies of isolated point defects. The excess energies and configurations of the most stable interface variants are determined. All interfaces show a tendency to Al segregation except the B2 GB for which the most stable structure is an Fe-rich one. The interface structures are more complex in the DO₃ than in the B2 alloy, with a high multiplicity of DO₃ configurations with close energies. Finally, values of the GB and surface energies are introduced into a Griffith model of brittle fracture, in order to assess the trends of both alloys to intergranular fracture. Comparisons are also drawn with the similar Ni-Al ordered alloys.     

Radiation-induced glide motion of interstitial clusters in concentrated alloys

We propose a mechanism for glide motion, i.e. one-dimensional (1D) migration, of interstitial clusters in concentrated alloys driven by high-energy particle irradiation. Interstitial clusters are fundamentally mobile on their respective 1D migration tracks, but in concentrated random alloys they are stationary at the position where the fluctuating formation energy achieves a local minimum. Irradiation changes the microscopic distribution of solute atoms through atomic displacement and recovery of the produced Frenkel pairs, which causes cluster 1D migration into a new stable position. In molecular dynamics simulations of interstitial clusters up to 217i in Fe–Cu alloys, stepwise 1D migration was observed under interatomic mixing or shrinkage of the cluster: a single 1D migration was induced by two exchanges per atom or cluster radius change by two interatomic distances. The 1D migration distance ranged up to several nanometers. We compared the frequency and distance of 1D migration with those for in situ observation using high-voltage electron microscopy, allowing for the extremely large rate of interatomic mixing and cluster shrinkage in the present simulation.     

Atomistic simulation of Fe deposition and alloy formation on Pt substrates

Fe-Pt alloys are of significant importance toward future applications of high-density magnetic recording media. In this work, we apply the BFS method for alloys to study the energetic pathway for subsurface Fe-Pt alloy formation upon deposition of Fe atoms on Pt(1 0 0), Pt(1 1 1), and vicinal Pt(9 9 7) substrates. The simulation results indicate preference for Fe atoms to occupy sites in the Pt subsurface layers and form an ordered alloy phase upon deposition on a low-index Pt surface. This behavior results in Pt surface segregation leading to nucleation of 3D Pt islands. However, the energetics behind deposition of Fe on Pt(9 9 7) indicate that Fe atoms prefer decoration of Pt step edges prior to formation of the ordered Fe-Pt surface alloy, where the ordered alloy is observed to form at the edges of the monoatomic surface steps. In each case presented here, the results are in agreement with experiment, and the formation of a Fe-Pt subsurface alloy is explained by a simple analysis emerging from the competition between BFS strain and chemical energy contributions.     

Atomic-Scale Simulation Study of Equilibrium Solute Adsorption at Alloy Solid-Liquid Interfaces

Equilibrium structural properties of solid-liquid interfaces in Cu-Ni alloys are studied by Monte-Carlo simulations employing interatomic potentials based on the embedded-atom method. We describe a thermodynamic-integration approach used to derive bulk concentrations and densities for solid and liquid phases in two-phase thermodynamic equilibrium. These results are used as a basis for constructing three-dimensional supercell geometries employed in Monte-Carlo-simulation studies of solid-liquid interface properties for {100} and {111} crystallographic orientations. At a temperature of 1750 K (four percent below the calculated melting point of pure Ni) equilibrium density and concentration profiles have been derived, allowing a calculation of the relative Gibbsian adsorption, , of Cu (solute) relative to Ni (solvent) at solid-liquid interfaces in Ni-rich alloys. We derive absorption values of and –0.23 ± 0.50 atoms/nm² for {100} and {111} interfaces, respectively. These results are discussed in the context of available experimental measurements and continuum-theory results for adsorption at heterophase interfaces.     

Brittle-ductile behavior of a nanocrack in nanocrystalline Ni: A quasicontinuum study

The effects of stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy on the fracture behavior of nanocrystalline Ni are studied via quasicontinuum simulations. Two semi-empirical potentials for Ni are used to vary the values of these generalized planar fault energies. When the above three energies are reduced, a brittle-to-ductile transition of the fracture behavior is observed. In the model with higher generalized planar fault energies, a nanocrack proceeds along a grain boundary, while in the model with lower energies, the tip of the nanocrack becomes blunt. A greater twinning tendency is also observed in the more ductile model. These results indicate that the fracture toughness of nanocrystalline face-centered-cubic metals and alloys might be efficiently improved by controlling the generalized planar fault energies.     

Solute-atom segregation/structure relations at high-angle (002) twist boundaries in dilute Ni−Pt alloys

Monte Carlo simulations, utilizing embedded atom method (EAM) potentials, are employed to investigate in detail solute-atom segregation behavior at high-angle symmetrical (002) twist boundaries, at T=850 K, in Pt-3 at.% Ni and Ni-3 at.% Pt alloys. Solute enhancement in those alloys occurs on both sides of the phase diagram, although it is considerably higher on the Ni-rich side. The distributions of solute concentrations within the first and the second planes are very inhomogeneous, with the sites highly enhanced in solute being in the minority. The remaining sites exhibit little or no enhancement. The highest level of solute concentrations at individual sites continues to increase with the value of the rotations angle, , until saturation occurs at about the =5 misorientation. The large differences in concentrations between different types of sites suggest the possibility of an ordered grain-boundary phase. The correlation between the structure and solute species concentrations in most cases follows the trends observed for low-angle boundaries: Pt as a solute prefers the structural units of the perfect crystal type, while Ni as a solute tends to segregate at the filler units associated with the cores of the primary grain boundary dislocations. A strong correlation is observed between the position of a site in the first or second (002) plane and the plane of the interface. Rigid-body translations are detected for two boundaries on the Pt-rich side of the phase diagram. Roughening and possible structural multiplicity occur in the =5 boundary on the Ni-rich side. The same boundary on the Pt-rich side of the phase diagram exhibits a considerable amount of structural and chemical disorder.     

Self-consistent Monte Carlo simulations of the electron and ion distributions of inhomogeneous liquid alkali metals. II. Longitudinal and transverse density distributions in the liquid-vapor interface of binary metallic alloys

We present the results of Monte Carlo simulations of the liquid-vapor interface of sodium-cesium alloys. The longitudinal density profile of each alloy shows that the liquid-vapor interface consists of a well-defined monolayer of cesium sitting on top of a slab of the bulk alloy. Underneath the monolayer there is a slight excess of sodium. A comparison with a van der Waals analog of one of the alloys shows that the presence of the well-defined monolayer of cesium on the outside of the liquid-vapor interface is a feature peculiar to metallic mixtures. The transverse pair correlation functions of the cesium monolayer are insensitive to the composition of the bulk of the slab.     

The characterisation of atomic structure and glass-forming ability of the Zr–Cu–Co metallic glasses studied by molecular dynamics simulations

In the present work, the glass formation process and structural properties of Zr₅₀Cu_50-xCo_x (0 ≤ x ≤ 50) bulk metallic glasses were investigated by a molecular dynamics simulation with the many body tight-binding potentials. The evolution of structure and glass formation process with temperature were discussed using the coordination number, the radial distribution functions, the volume–temperature curve, icosahedral short-range order, glass transition temperature, Voronoi analysis, Honeycutt–Andersen pair analysis technique and the distribution of bond–angles. Results indicate that adding Co causes similar responses on the nature of the Zr₅₀Cu_50-xCo_x (0 ≤ x ≤ 50) alloys except for higher glass transition temperature and ideal icosahedral type ordered local atomic environment. Also, the differences of the atomic radii play the key role in influencing the atomic structure of these alloys. Both Cu and Co atoms play a significant role in deciding the chemical and topological short-range orders of the Zr₅₀Cu_50-xCo_x ternary liquids and amorphous alloys. The glass-forming ability of these alloys is supported by the experimental observations reported in the literature up to now.     

Equilibrium ordering properties of Au–Pd alloys and nanoalloys

Equilibrium configurations of bulk and surface Au–Pd alloys as well as of nanoalloy clusters are studied using Metropolis Monte Carlo importance sampling and the embedded atom method. The clusters contain about 1000 atoms. Three ordered bulk phases are predicted at low temperature, centred on compositions around 25%, 50%, and 75% Pd. The predicted order–disorder transition temperatures partially disagree with the available experimental results, but they are in good agreement with ab initio calculations. Surface enrichment in Au is systematically predicted, accompanied by partial subsurface enrichment in Pd, best enhanced around the equiatomic overall composition. The subsurface enrichment in Pd is suggested to play a decoupling role between surface and bulk conditions and, subsequently, ordered surface structures not induced by the order in the bulk are predicted at low temperatures. Clusters display similar segregation and ordering properties as flat infinite surfaces. However, the stability of the ordering at cluster surfaces is not globally characterized. The order–disorder transitions in the clusters occur at temperatures between 50 and 100 K lower than in the bulk. The disorder appears at the surface and proceeds to the core as the temperature is increased.     

On the entropic nucleation barrier in a martensitic transformation

The nucleation of martensite in alloys is hindered by a free energy nucleation barrier, hence comprising contributions of the potential energy and the entropy. The leading effect is commonly attributed to the potential energy barrier due to strain fields. In this contribution, we investigate the nature of the entropic barrier by means of molecular dynamics (MD) simulations. We study a transformation process of an undercooled single crystal and examine two nucleation events observed under adiabatic conditions using vibrational mode analysis of the atomic trajectories. Our analysis shows that martensitic nucleations are indicated by transit from a state of uncorrelated into a state of correlated atomic motions. This correlation process is built up locally by a small group of atoms even before the product lattice can be recognized morphologically and it produces vibrational ‘soft’ modes along transformation paths. Phase space analyses unveil that the correlation process is characterized by narrow domains – ‘nucleation channels’ – the atomic trajectories have to pass, connecting the phase space domains of the parent and the product lattice. For a successful nucleation event, the nucleus atoms have to pass this channel collectively, which stochastically represents a rare event. Thermal fluctuations prevent finding the channel at elevated temperature and give rise for entropic stabilization of the parent phase. This ‘entropic nucleation barrier’ is reduced in the undercooled state but still effective, thus preventing the parent phase from collapsing into the product. The entropic barrier may be interpreted as the probability of a group of atoms to simultaneously pass the nucleation channel. Such group then represents a nucleus.     

Preparation and magnetic properties of ultrafine particles of Fe---Ni alloys

Ultrafine particles (UFP) of six kinds of Fe---Ni alloys were synthesized by the method of hydrogen plasma reaction. The prepared UFP samples were examined by X-ray diffraction, electron transmission microscopy and magnetic measurement. The spherical Fe---Ni UFP alloys with a mean particle size less than 35 nm can be prepared with a production rate much higher than by conventional methods. The phase constitution of UFP alloys is different from the equilibrium phase diagram owing to rapid condensation of evaporated metal gases. Although the magnetization for the UFP alloys has almost the same temperature dependence as that of the bulk alloys, the saturation magnetization remarkably decreases as the bulk alloys change into the UFP alloys.     

Preparation and magnetic properties of ultrafine particles of FeNi alloys

Ultrafine particles (UFP) of six kinds of FeNi alloys were synthesized by the method of hydrogen plasma reaction. The prepared UFP samples were examined by X-ray diffraction, electron transmission microscopy and magnetic measurement. The spherical FeNi UFP alloys with a mean particle size less than 35 nm can be prepared with a production rate much higher than by conventional methods. The phase constitution of UFP alloys is different from the equilibrium phase diagram owing to rapid condensation of evaporated metal gases. Although the magnetization for the UFP alloys has almost the same temperature dependence as that of the bulk alloys, the saturation magnetization remarkably decreases as the bulk alloys change into the UFP alloys.