Atomistic simulation of the point defects in B2-type MoTa alloy

The formation and migration mechanisms of three different point defects (mono-vacancy, anti-site defect and interstitial atom) in B₂-type MoTa alloy have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM). From minimization of the formation energy, we find that the anti-site defects Mo_Ta and Ta_Mo are easier to form than Mo and Ta mono-vacancies, while Mo and Ta interstitial atoms are difficult to form in the alloy. In six migration mechanisms of Mo and Ta mono-vacancies, one nearest-neighbor jump (1NNJ) is the most favorable due to its lowest activation and migration energies, but it will cause a disorder in the alloy. One next-nearest-neighbor jump (1NNNJ) and one third-nearest-neighbor jump (1TNNJ) can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by straight [1 0 0] six nearest-neighbor cyclic jumps (S[1 0 0]6NNCJ) or bent [1 0 0] six nearest-neighbor cyclic jumps (B[1 0 0]6NNCJ) and [1 1 0] six nearest-neighbor cyclic jumps ([1 1 0]6NNCJ), respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.     

Atomistic simulation of point defects at low-index surfaces of noble metals

The formation energies and the migration energies of an isolated vacancy and adatom formed on low-index surfaces are calculated with MAEAM for three noble metals Cu, Ag and Au. The results indicate that the formation energies of an isolated vacancy or adatom increase with increasing atom density in the sequence (1 1 0) → (1 0 0) → (1 1 1), and it is more difficult to form an adatom than to form a vacancy at the same surface. For the mobility of an isolated vacancy, the migration energy grows in the sequence (1 0 0) → (1 1 0) → (1 1 1) for each noble metal. However, a much less migration energy is obtained for the migration of an adatom on (1 1 1) surface.     

模型二元有序合金固液界面结构的分子动力学研究

采用分子动力学方法,研究了模型二元有序合金体系的平衡界面结构和界面处原子的扩散行为.计算结果表明,该二元有序合金的固液界面属于光滑界面.由于固体中同时存在结构和化学有序,从而导致界面处的原子结构与单质以及异质固液界面的结构明显不同.在界面法向方向上,粒子数密度呈复杂的波动行为,并延伸到液体中约30 (A).对界面层的二维结构分析表明,固液转变层部分原子形成了二维固体团簇.从固体到液体,扩散系数从零逐渐增加到一个饱和值.在界面处附近,平行于界面方向的扩散系数明显比垂直于界面方向的大.     

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.     

Atomistic simulation of the interaction between mobile edge dislocations and radiation-induced defects in Fe-Ni-Cr austenitic alloys

The classical molecular dynamics method is employed to simulate the interaction of edge dislocations with interstitial Frank loops (2 and 5 nm in diameter) in the Fe-Ni₁₀-Cr₂₀ model alloy at the temperatures T = 300–900 K. The examined Frank loops are typical extended radiation-induced defects in austenitic steels adapted to nuclear reactors, while the chosen triple alloy (Fe-Ni₁₀-Cr₂₀) has the alloying element concentration maximally resembling these steels. The dislocation-defect interaction mechanisms are ascertained and classified, and their comparison with the previously published data concerning screw dislocations is carried out. The detachment stress needed for a dislocation to overcome the defect acting as an obstacle is calculated depending on the material temperature, defect size, and interaction geometry. It is revealed that edge dislocations more efficiently absorb small loops than screw ones. It is demonstrated that, in the case of small loops, the number of reactions accompanied by loop absorption increases with temperature upon interaction with both edge and screw dislocations. It is established that Frank loops are stronger obstacles to the movement of screw dislocations than to the movement of edge ones.     

Formation energy of planar superstructure defects in ordered FCC and BCC alloy lattices

A solid sphere model is used to develop a general procedure for calculating the energy of planar superstructure defects of any type in ordered alloys with arbitrary unit cells. As an example we obtain an analytic expression for the energy required to form c-domain boundaries for L1₀ superstructures in a face-centered cubic lattice.Altai State Technical University. Translated from Izvestia Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 57–61, November, 1994.     

带有点缺陷的二维颗粒系统离散元模拟

首先用离散元方法研究了颗粒系统在各向同性挤压和纯剪切状态下粒子间力的分布情况,并与相同条件下的实验结果进行对比. 然后模拟了带有不同数目点缺陷的二维颗粒系统在各向同性挤压和纯剪切时粒子间力的分布情况,并与无缺陷的情况做了比较,发现了点缺陷对颗粒系统的影响规律. 关键词： 颗粒系统 离散元 点缺陷 力概率分布     

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.     

Influence of supersaturation by point defects on the shear modulus of nickel and nickel-boron alloy

The influences of supersaturation of samples by point defects on the shear modulus is investigated by the internal-friction method. It is found that, in the temperature range 0.25–0.5T_m, the shear modulus of such samples is less. A mechanism associated with increase in mobility of the dislocations on account of diffusional influx of the point defects formed on irradiation is proposed, and an expression is obtained for estimating the magnitude of the effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 53–56, January, 1984.     

Atomistic underpinnings for orientation selection in alloy dendritic growth

In dendritic solidification, growth morphologies often display a pronounced sensitivity to small changes in composition. To gain insight into the origins of this phenomenon, we undertake an atomistic calculation of the magnitude and anisotropy of the crystal-melt interfacial free energy in a model alloy system featuring no atomic size mismatch and relatively ideal solution thermodynamics. By comparing the results of these calculations with predictions from recent phase-field calculations, we demonstrate that alloying gives rise to changes in free-energy anisotropies that are substantial on the scale required to induce changes in growth orientations.     

Atomistic simulation of slow grain boundary motion

Existing atomistic simulation techniques to study grain boundary motion are usually limited to either high velocities or temperatures and are difficult to compare to realistic experimental conditions. Here we introduce an adapted simulation method that can access boundary velocities in the experimental range and extract mobilities in the zero driving force limit at temperatures as low as ～0.2T(m) (T(m) is the melting point). The method reveals three mechanistic regimes of boundary mobility at zero net velocity depending on the system temperature.     

Atomistic simulation of Mn-site substitution in multiferroic h-YMnO3

Atomistic simulation has been performed to systematically investigate the Mn-site doping of h-YMnO(3) (hexagonal yttrium manganese oxide). It is found that tetravalent dopants are the most energetically favorable for incorporation into a crystal lattice. For divalent dopants, hole compensation is the more likely charge compensation mechanism, whereas for dopants with mixed valence, the divalent state is the energetically preferred form. Structural and local polarization changes caused by Mn-site doping are also investigated. The tilting angle of the MnO(5) trigonal bipyramid is suppressed for all of the dopants investigated, with the reduction dependent on the dopant ion radius, while the influence on buckling is closely related to the valence of the dopants. Our results reveal that both electrostatic and size effects play important roles in the ferroelectric polarization of h-YMnO(3).     

Atomistic simulation of the 60∘ dislocation mobility in silicon crystal

Dislocation velocity and mobility are studied via molecular dynamics simulation for a 60^∘ dislocation dipole in silicon crystal. The atomic interactions are described using the Stillinger–Weber potential and the external stress is applied by means of the Parrinello–Rahman algorithm. It is found that the dislocation begins to move when the applied stress is larger than the Peierls stress, and the calculated Peierls stress decreases as the temperature increases, which is in agreement with the Peierls–Nabarro model. The dislocation velocity at relatively low temperature is insensitive to variation of temperature. In fact, the velocity increases monotonically as the stress increases, and eventually approaches its plateau velocity which is about 2900 m/s. At higher temperature, however, the velocity no longer increases monotonically as the stress increases and the plateau velocity decreases as the temperature increases. In general, the dislocation velocity decreases as the temperature increases, which is consistent with the phonon drag model.     

Atomistic simulation of a superionic transition in UO2

The results of the atomistic simulation of a superionic transition and melting of uranium dioxide are presented. The temperature dependences of the concentration of defects in the oxygen sublattice and changes in the heat capacity and isothermal compressibility upon the superionic transition are calculated. It is shown that the curve of the superionic transition in the PT diagram can be described by the Ehrenfest’s equation. The possibility of describing the superionic transition within the framework of the theory of second- order phase transitions is discussed. Based on the results obtained, it is considered that this structural transformation can occur in other materials.     

Atomistic simulation of the surface structure of electrolytic manganese dioxide

Atomistic simulation methods were used to investigate the surface structures and stability of pyrolusite and ramsdellite polymorphs of electrolytic manganese dioxide (EMD). The interactions between the atoms were described using the Born model of Solids. This model was used to calculate the structures and energies of the low index surfaces {0 0 1}, {0 1 0}, {0 1 1}, {1 0 0}, {1 0 1} and {1 1 0} for both pyrolusite and ramsdellite. Pyrolusite is isostructural with rutile and similar to rutile the {1 1 0} surface is found to be the most stable with the relaxed surface energy 2.07 J m⁻². In contrast, for ramsdellite the {1 0 1} surface is the most stable with a surface energy of 1.52 J m⁻². Pyrolusite {1 0 0} and ramsdellite {1 0 0}_b surfaces have equivalent energies of 2.43 J m⁻² and 2.45 J m⁻², respectively and similar surface areas and hence are the likely source for the intergrowths. Finally, comparison of the energies of reduction suggests that the more stable surfaces of pyrolusite are more easily reduced.     

Atomistic simulation of the vacancy diffusion in (0 0 1) surface of MoTa alloy

The formation and diffusion of a single Mo or Ta vacancy in the (0 0 1) surface of the B₂-type MoTa alloy have been investigated by using modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface on the vacancy is only down to the sixth layer. It is easier for the vacancy to form in the first layer. Comparing the migration energy of the vacancy migrating in the intra-layer, to the upper layer and to the nether layer via 2NN jump, we find that the vacancy in the first or second layer is preferred to migrate in intra-layer, and that in the third or fourth layer is favorable to migrate to the upper layer. Although 1NN jump will result in an anti-site so that a disorder in the order alloy, it may also occur due to the much lower migration energy especially for the vacancy in the second and third layer to migrate to the first and second layer, respectively.