On the core structure and mobility of the 〈100〉{010} and 〈100〉{01^-1} dislocations in B2 structure YAg and YCu

Dislocations are thought to be the principal mechanism of high ductility of the novel B2 structure intermetallic compounds YAg and YCu.In this paper,the edge dislocation core structures of two primary slip systems 〈100 〉{010} and 〈100 〉 {011} for YAg and YCu are presented theoretically within the lattice theory of dislocation.The governing dislocation equation is a nonlinear integro-differential equation and the variational method is applied to solve the equation.Peierls stresses for 〈100 〉 {010} and 〈100 〉 {011} slip systems are calculated taking into consideration the contribution of the elastic strain energy.The core width and Peierls stress of a typical transition-metal aluminide NiAl is also reported for the purpose of verification and comparison.The Peierls stress of NiAl obtained here is in agreement with numerical results,which verifies the correctness of the results obtained for YAg and YCu.Peierls stresses of the 〈100 〉 {011} slip system are smaller than those of〈100 〉 {010} for the same intermetallic compounds originating from the smaller unstable stacking fault energy.The obvious high unstable stacking fault energy of NiAl results in a larger Peierls stress than those of YAg and YCu although they have the same B2 structure.The results show that the core structure and Peierls stress depend monotonically on the unstable stacking fault energy.     

Elastic interaction between a dilatation centre and split dislocations in b.c.c. metals

The elastic interaction energy between a dilatation centre and 1/2[111] dislocation split on {110} planes in b.c.c. metals is calculated for sessile and planar splittings of screw dislocation and for planar splitting of edge dislocation; the splitting is considered as a model of the dislocation core. It is concluded that the interstitial impurity atom has three or four possible minimum energy positions in the dislocation core. A hypothesis on the impurity jumps in the dislocation core is proposed.On leave from theInstitute of Physics, Hanoi, VDR.     

bcc Fe中刃型位错的结构及能量学研究

基于位错理论，利用分子动力学方法建立了〈100〉{010}，〈100〉{011}，1/2〈111〉{011}和1/2〈111〉{112}刃型位错的芯结构，并计算了这四种刃型位错的形成能、位错芯能量和芯半径.计算结果表明：〈100〉{010}和〈100〉{011}刃型位错的形成能比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的要高，这表明〈100〉刃型位错比1/2〈111〉刃型位错更难形成.而〈100〉{010}和〈100〉{011}刃型位错的芯半径比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的小，这说明在1/2〈111〉刃型位错中位于奇异区的原子数多于〈100〉刃型位错，而这些原子要比完整晶体中的原子具有更大的活性.可见，1/2〈111〉刃型位错比〈100〉刃型位错更易运动，且〈100〉刃型位错在bcc Fe中难以形成. 关键词： bcc Fe 刃型位错 分子动力学模拟     

Sessile splitting of screw dislocations in b.c.c. metals

The energies of six different sessile splittings of screw dislocation 1/2 [111], which can occur in b.c.c. metals assuming the stacking faults on {110} and {112} planes according to the hard sphere model, are compared. The regions of the values of stacking fault energies on {110} and {112} planes, in which a given splitting is favoured over the other five, are then shown.     

Elastic scattering of two ground-state N atoms

An interaction potential for an N₂(X¹σ_g⁺) molecule is constructed by using the highly accurate valence internally contracted multireference configuration interaction method and the largest basis set, aug-cc-pV6Z, in the valence range. The potential is used to investigate the elastic scattering of two N atoms at energies from 1.0× 10^-11 to 1.0× 10^-4 a.u. The derived total elastic cross sections are very large and almost constant at ultralow temperatures, and the shape of total elastic cross section curve is mainly dominated by the s-partial wave at very low collision energies. Three shape resonances are found in the total elastic cross sections. Concretely, the first one is very sharp and strong. It results from the g-partial-wave contribution and the resonant energy is 3.645× 10^-6 a.u. The second one is contributed by the h-partial wave and the resonant energy is 1.752× 10^-5 a.u. This resonance is broadened by those from the d- and f-partial waves. The third one comes from the l = 6 partial wave contribution and the resonant energy is 3.522× 10^-5 a.u. This resonance is broadened by those from the g- and h-partial waves. The N₂(X¹σ_g⁺) molecular parameters, which are determined at the current theoretical level, achieve very high accuracy due to the employment of the largest correlation-consistent basis set in the valence range.     

bcc Fe的刃型位错中氦-空位团的稳定性

 采用周期性原子阵列方法建立bcc Fe中的刃型位错,利用分子动力学计算了0 K时bcc Fe的位错芯里氦-空位团的稳定性,并与理想Fe晶体里氦-空位团的稳定性进行比较发现,位错的作用导致氦-空位团不稳定。点缺陷（He、空位与自间隙Fe原子）与氦-空位团的结合能与团中氦-空位比例密切相关,当氦与空位数之比在3~6时,结合能趋于稳定。     

Orientation effects in interaction of screw dislocation with interstitial impurity atom in b.c.c. lattice

The cross-slip and pinning of a 1/2a〈111〉 screw dislocation in b.c.c. metals in the vicinity of an interstitial impurity atom are studied in dependence on crystal orientation. To this purpose, the interaction energy between the dislocation and an interstitial atom is calculated in an anisotropic elastic continuum and it is assumed that the screw dislocation moves microscopically on {112} or {110} planes between its stable configuration positions in b.c.c. lattice. It is found that the probability of induced cross-slip is orientation dependent. This result is used for discussion of orientation dependence of the change of CRSS due to increased carbon content which was experimentally determined for Fe-3.2% Si alloy single crystals in a previous paper (Blahovec J., Kade?ková S.: Czech. J. Phys.B 21 (1971), 846).     

On the anisotropy of electron backscattering and of the sticking coefficient of gases on single crystal surfaces

The influence of the anisotropy of backscattering of electrons with energies above the core hole ionization energy of selected Auger processes on the anisotropy of the Auger electron emission is studied using a cylindrical W single crystal. After the determination of the anisotropy of the elastic backscattering its contribution to the anisotropy of Auger electron emission from adsorption layers is obtained by postulating that all other anisotropy causes are negligible. When applied to an adsorbate with orientation-independent coverage such as Fe this leads to an orientation-dependent backscattering correction term which may be applied to adsorbates with orientation-dependent sticking coefficients such as O₂. In this manner Auger signals may be converted into true relative coverages (corrected for backscattering anisotropy). In the case of oxygen adsorption this correction is significant and leads to an orientation-independent initial sticking coefficient (of 1) over most of the 〈110〉 zone except in the neighbourhood of the {110} planes where it reaches its minimum value of 0.28.     

The energy dependence of the {111} and {100} tilt grain boundaries on disorientation angle in Ni<Subscript>3</Subscript>Al

Dependence of the energy of {111} and {100} boundaries on tilt angle θ is investigated by the molecular dynamics method using intermetallic Ni₃Al as an example. It is shown that the energy (per single grain dislocation) of grain boundaries in the {111} planes is higher than that of the grains located in the {100} planes. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 33–35, November, 2007.     

Role of strain-dependent surface energies in Ge/Si(100) island formation

Formation energies for Ge/Si(100) pyramidal islands are computed combining continuum calculations of strain energy with first-principles-computed strain-dependent surface energies. The strain dependence of surface energy is critically impacted by the presence of strain-induced changes in the Ge {100} surface reconstruction. The appreciable strain dependencies of rebonded-step {105} and dimer-vacancy-line-reconstructed {100} surface energies are estimated to give rise to a significant reduction in the surface contribution to island formation energies.     

The effect of shear stress on the screw dislocation core in bcc metals

The screw dislocation core structure in bcc metals under an external shear stress is investigated using the model of generalized splitting as an approximation of the Peierls-Nabarro model of screw dislocation dissociated on three {110} slip planes. The shear stress for which the core structure in this model is unstable is found.     

Twin-transformed lattice defects in γ-TiAl

We study the transformation of lattice defects in γ-TiAl upon intersection by deformation twins. A faulted dipole is chopped forming a dislocation loop faulted on the cube plane of the twin; faulted dipoles and twin loops may shrink at different rates. Dislocations with 1/2<110] and <011] Burgers vectors gliding on octahedral planes are transformed into configurations with low mobility on {010} plane. Elementary mechanisms involved in the transformation are considered and a pole mechanism for twin growth at forest dislocations is proposed.     

Computer simulation of symmetrical tilt grain boundaries in noble metals with MAEAM

This paper reports that an atomic scale study of [\bar {1}10] symmetrical tilt grain boundary (STGB) has been made with modified analytical embedded atom method (MAEAM) for 44 planes in three noble metals Au, Ag and Cu. For each metal, the energies of two crystals ideally joined together are unrealistically high due to very short distance between atoms near the grain boundary (GB) plane. A relative slide between grains in the GB plane results in a significant decrease in GB energy and a minimum value is obtained at specific translation distance. The minimum energy of Cu is much higher than that of Ag and Au, while the minimum energy of Ag is slightly higher than that of Au. For all the three metals, the three lowest energies correspond to identical (111), \mbox(113) and \mbox(331) boundary successively for two translations considered; from minimization of GB energy, these boundaries should be preferable in [\bar {1}10] STGB for noble metals. This is consistent with the experimental results. In addition, the minimum energy increases with increasing reciprocal planar coincidence density \Sigma, but decreases with increasing relative interplanar distance d /a.     

Fe中<100>{010}刃位错芯结构的各向异性修正

在位错晶格理论基础上,采用改进的Peierls-Nabarro方程研究了Fe中<100>{010}刃位错在各向异性近似下的芯结构和Peierls应力. 各向异性近似下的晶格离散效应、切变模量和能量因子的表达式都已确切给出. 在这三个各向异性因素中,晶格离散效应和能量因子可以使位错宽度变窄,切变模量可以使位错宽度变宽. 相比于各向同性近似,各向异性近似下的位错宽度变窄了近20%,并且各向异性近似下的位错宽度与数值计算的结果相一致. 更为重要的是,各向异性使位错的Peierls应力数值几乎加倍,数量级也由 变成了 ,而这些都会显著影响位错的运动机制. 因此,各向异性对于位错来说非常重要,在研究位错芯结构以及运动机制时需要考虑各向异性的影响.     

Transition energy and dipole oscillator strength for 1s22p-1s2nd of Cr2l+ ion

The transition energies, wavelengths and dipole oscillator strengths of 1s^22p-1s^2nd （3 ≤ n ≤ 9） for Cr^21＋ ion are calculated. The fine structure splittings of 1s^2nd （n ≤ 9） states for this ion are also calculated. In calculating energy, we have estimated the higher-order relativistic contribution under a hydrogenic approximation. The quantum defect of Rydberg series 1s^2nd is determined according to the quantum defect theory. The results obtained in this paper excellently agree with the experimental data available in the literature. Combining the quantum defect theory with the discrete oscillator strengths, the discrete oscillator strengths for the transitions from initial state 1s^22p to highly excited 1s^2nd states （n ≥ 10） and the oscillator strength density corresponding to the bound-free transitions are obtained.     

BiOCl{001}表面原子与电子结构的第一性原理研究

基于密度泛函理论的第一性原理方法研究了BiOCl{001}的三种不同终端面({001}-1Cl, {001}-BiO 和{001}-2Cl)的表面弛豫、能带结构、电子态密度和表面能. 计算结果表明: {001}-1Cl, {001}-BiO和{001}-2Cl表面均发生明显弛豫, 而在双Cl原子层处的层间距变化较大, 但未出现振荡弛豫现象, 其中{001}-1Cl表面弛豫较小. 与体相BiOCl电子结构相比, BiOCl{001}面具有较窄的带隙宽度, 并呈现较强局域性:对于{001}-BiO表面, 其导带与价带均往低能方向发生较大移动, 并且在导带底部出现表面态; 而{001}-2Cl表面的表面态主要出现在价带顶; {001}-1Cl表面的带隙中则无表面态产生; 表面态的出现导致{001}-BiO面和{001}-2Cl面带隙明显减小. BiOCl{001}三种终端表面的表面能分析结果表明, {001}-1Cl表面的表面能最小(0.09206 J·m^-2), 结构最稳定, 而{001}-BiO表面和{001}-2Cl表面的表面能分别为2.392和2.461 J·m^-2. 理论预测{001}-BiO表面和{001}-2Cl表面具有较高的活性, 但在BiOCl晶体生长过程中不易暴露. 本文计算结果为实验获得BiOCl高活性面{001}给予了基础理论解释, 进一步为BiOCl新型光催化材料的应用研究提供理论指导. 关键词： BiOCl{001}表面 表面弛豫 表面能 第一性原理     

Transition parameters of Li-like ions(Z = 7–11) in dense plasmas

The energy levels, transition energies, transition probabilities, weighted oscillator strengths, and line strengths of Lilike ions(Z = 7–11) in dense plasmas are investigated in this work. The relativistic effects and electron correlation effects are described by the MCDHF method. The ion sphere model is applied to include the dense plasma screening effect. The ground configuration 1 s~22 s and the excited 1 s~22 p, 1 s~23 l(l = 0–2) are considered. The configuration sets are enlarged until n = 7 where the calculated energy levels have converged. The critical free electron densities of 1 s~23 d states are estimated.Except for 1 s~23 s–1 s~23 p transitions, the transition energies for Δn = 0 increase, and for Δn ≠ 0 decrease with increasing free electron densities. For 1 s~23 s–1 s~23 p transitions, the spectra show blue-shift at lower free electron densities and red-shift at higher free electron densities, and the energy level crossing phenomens are observed at higher free electron densities.     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

Dislocation modeling in bcc lithium: A comparison between continuum and atomistic predictions in the modified embedded atoms method

In this study, the modified embedded-atom method (MEAM) was applied to compare the predictions of dislocation core properties obtained by molecular statics with the continuum predictions obtained in the framework of the simplified 1D-Peierls–Nabarro model. To this end, a set of four fictive Li potentials in the MEAM framework was proposed with the condition that all four potentials reproduce the same elastic constants, the same transition energies between bcc and fcc crystal structures, and between bcc and hcp crystal structures, while the unstable stacking fault energy on the plane {110} in the direction <111> was varied around the value predicted by first-principles. Within these potentials, direct atomistic calculations were performed to evaluate dislocation core properties such as dislocation half width and Peierls stress and the results were compared with continuum predictions. We found that the trends predicted by the Peierls–Nabarro model, i.e. (i) a decrease of the dislocation half width with increasing unstable stacking fault energy, and (ii) an increase of the Peierls stress with increasing the magnitude of the unstable stacking fault energy, were recovered using atomic calculations in the MEAM framework. Moreover, the magnitude of the dislocation half width and the Peierls stress calculated in the MEAM framework are in good agreement with the Peierls–Nabarro predictions when the dislocation half width is determined using a generic strategy. Specifically, the dislocation half width is defined as the distance for which the disregistery is included between b/4 and 3b/4. It was, therefore, demonstrated herein that the set of fictive potentials could be parameterized in the MEAM framework to validate or to disprove the continuum theory using atomistic methods.     

Dislocations in Fe-3% Si alloy single crystals deformed at a higher rate

The method of etching dislocations is used to study the distribution of dislocations and twins in Fe-3% Si alloy single crystals prepared from the melt after plastic deformation with higher speed. The crystals are deformed by twinning in the 〈111〉 directions along the {112} planes and by slip in the 〈111〉 directions along the {110} planes. The results prove that the dislocations causing plastic deformation move in the {110} planes during both fast and slow deformation. The difference in the slip surfaces during fast and slow deformation is explained by the different number of cross slips per unit dislocation path.