Mechanism of Strain Rate Effect Based on Dislocation Theory

Based on dislocation theory, we investigate the mechanism of strain rate effect. Strain rate effect and dislocation motion are bridged by Orowan＇s relationship, and the stress dependence of dislocation velocity is considered as the dynamics relationship of dislocation motion. The mechanism of strain rate effect is then investigated qualitatively by using these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion. The process of strain rate effect is interpreted and some details of strain rate effect are adequately discussed. The present analyses agree with the existing experimental results. Based on the analyses, we propose that strain rate criteria rather than stress criteria should be satisfied when a metal is fully yielded at a given strain rate.     

Bridging Atomistic/Continuum Scales in Solids with Moving Dislocations

We propose a multiscale method for simulating solids with moving dislocations. Away from atomistic subdomains where the atomistic dynamics are fully resolved, a dislocation is represented by a localized jump profile, superposed on a defect-free field. We assign a thin relay zone around an atomistic subdomain to detect the dislocation profile and its propagation speed at a selected relay time. The detection technique utilizes a lattice time history integral treatment. After the relay, an atomistic computation is performed only for the defect-free field. The method allows one to effectively absorb the fine scale fluctuations and the dynamic dislocations at the interface between the atomistic and continuum domains. In the surrounding region, a coarse grid computation is adequate.     

Theoretical Strength of Face-Centred-Cubic Single Crystal Copper Based on a Continuum Model

The constitutive relation of single crystal copper based on atomistic potential is implemented to capture the nonlinear inter-atomic interactions. Uniaxial loading tests of single crystal copper with inter-atomic potential finite-element model are carried out to determine the corresponding ideal strength using the modified Born stability criteria. Dependence of the ideal strength on the crystallographic orientation is studied, and tension- compression asymmetry in ideal strength is also investigated. The results suggest that asymmetry for yielding strength of nano-materials may result from anisotropic character of crystal instability. Moreover, the results also reveal that the critical resolved shear stress in the direction of slip is not an accurate criterion for the ideal strength since it could not capture the dependence on the loading conditions and hydrostatic stress components for the ideal strength.     

A Simple Theoretical Method to Predict the Hardness of Pure Metal Crystals

Design of superhard bulk materials requires predicting their hardness, challenging current theories for material design. By introducing a concept of condensing force （CF）, it is shown via ab initio calculations for fcc （Ni, Cu, Al, Ir, Rh, Au, Ag, Pd） and hcp Re crystals that materials with larger CF can have greater hardness. Since the calculation of CF is easy, this method might prove a convenient way to evaluate the hardness of newly designed materials.     

Temperature Dependence of Elastic Properties for Amorphous SiO2 by Molecular Dynamics Simulation

Large-scale and long-time molecular-dynamics simulations are used to investigate the temperature dependences of elastic properties for amorphous SiO2. The elastic moduli increase in a temperature range up to 1600 K and decrease thereafter. The anomalous behaviour in elasticity is explained by analysing the changes of atomic-scale structure with respect to increment of temperature. The mechanism originates predominantly from distortion of the SiO4 tetrahedra network in low-temperature ranges. At an elevated temperature range, thermal-induced Si-O bond stretching dominates the process and leads to normal temperature dependence of elastic properties.     

Ab initio study of Nb2SC and Nb2S2C: Differences in coupling between the S and Nb-C layers

Using ab initio calculations, we have studied the structurally related compounds Nb₂SC and Nb₂S₂C. In Nb₂S₂C (space group , prototype Bi₂Te₃), S atoms are nearest neighbours, while in Nb₂SC (space group P6₃/mmc, prototype Cr₂AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6-3.7% to previously-published experimental data and the bulk modulus-to-c₄₄ ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb₂S₂C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb₂SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.     

Antisite Defects of the L12 Structure Determined by the Phase Field Microelasticity Model

A phase field microelasticity simulation is performed to examine the antisite defect of L12-Ni3Al in Ni75Al5.3 V19.7 ternary alloy. Combinimg strain energy with the phase field model leads to an atom configuration change as time proceeds. For the Ni sublattice, the antisite defect AlNi, the equilibrium occupancy probability （OP） of which declines, precedes NiNi and VNi in reaching equilibrium; subsequently, NiNi and VNi present a phenomenon of symmetrical rise and decline individually. Similarly, for the Al sublattice, the antisite defect NiAl, the OP of which eventually rises, takes fewer time steps than AlAl and VAl to attain equilibrium. Thereafter, AlAl rises while VAl declines symmetrically at the axes of the NiAl curve. Furthermore, the OP for the Al sublattice is much more sensitive to strain energy than that for the Ni sublattice.     

Electronic structure of Sc2AC (A=Al, Ga, In, Tl)

Using ab initio calculations, we have studied Sc₂AC with A=Al, Ga, In and Tl. We show that C 2p and Sc 3d as well as A p and Sc 3d states are hybridized, but the antibonding states in the vicinity of the Fermi level weaken the overall bonding. In terms of the chemical bonding, the influence of the size of the A element is minute. Furthermore, the bulk modulus of the corresponding binary transition metal carbide is not conserved in these phases. Therefore, Sc₂AC can be classified as weakly coupled MAX phases according to Sun and co-workers [Z. Sun, D. Music, R. Ahuja, S. Li, J.M. Schneider, Phys. Rev. B 70 (2004) 092102]. It is our ambition that these calculations will stimulate experimental research on these compounds.     

Crystal Structure of β-La2Mo2O9 from First Principles Calculation

Arrangements of O ions in β-La2Mo2O9 are studied by first principles calculation with two different calculation schemes. All final structure configurations consist of MoO4-tetrahedra, MoO5-hexahedra, LaO8 and LaO7 polyhedra. Molybdenum polyhedra are isolated from each other, lanthanum polyhedra are connected together by sharing O ions. The occupancies of three crystallographic distinct O sites O（1）, O（2） and O（3） are 100%, 91.7% and 25%, respectively, consistent with experiments. All configurations are related to each other by one of 12 symmetry operations of P213 space group, suggesting that the structure observed experimentally may be interpreted as a time and spatial average of these local or inherent structures.     

Structural phase transition (zincblende-rocksalt) and elastic properties in AlY (Y=N, P and As) compounds: Pressure-induced effects

The present paper addresses the pressure-induced structural aspects of ZnS-type (B3) to NaCl-type (B1) structure in AlY (Y=N, P, As). An effective-interionic interaction potential (EIoIP) with long-range Coulomb and three-body interactions and the Hafemeister-and-Flygare-type short-range overlap repulsion extended up to the second-neighbour ions and the van der Waals (vdW) interaction is developed. Emphasis has been given on evaluating the vdW coefficients by the Slater-Kirkwood variational method, as both the ions are polarizable. The lattice model calculations have revealed reasonably good agreement with the available experimental data on the phase-transition pressures (P_t=16, 14, 7.5 GPa) and the elastic properties of AlY (Y=N, P, As). The equation of state curves (plotted between V(P)/V(0) and pressure) for both the B3 and B1 structures obtained are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also close to their observed data. Further, the variations of the second-order elastic constants with pressure follow a systematic trend that is almost identical to that exhibited by the observed data measured for other semiconducting compounds with B3→B1 structural phase transitions.     

An Accurate Image Simulation Method for High-Order Laue Zone Effects

A completely different formulation for simulation of the high order Laue zone （HOLZ） diffractions is derived. It refers to the new method, i.e. the Taylor series （TS） method. To check the validity and accuracy of the TS method, we take polyvinglidene fluoride （PVDF） crystal as an example to calculate the exit wavefunction by the conventional multi-slice （CMS） method and the TS method. The calculated results show that the TS method is much more accurate than the CMS method and is independent of the slice thicknesses. Moreover, the pure first order Lane zone wavefunction by the TS method can reflect the major potential distribution of the first reciprocal plane.     

Gold Nanobelt Reorientation by Molecular Dynamics Simulation

The embedded atom method is used to study the structure stability of gold nanobelt. The Au nanobelts have a rectangular cross-section with （100） orientation along the x^-,γ- and z-axes. Free surfaces are used along the x- and y-directions, and periodic boundary condition is used along z-direction. The simulation is performed at different temperatures and cross-section sizes. Our results show that the structure stability of the Au nanobelts depends on the nanobelt size, initial orientation, boundary conditions and temperature. A critical temperature exists for Au nanobelts to transform from initial （100） nanobelt to final （110） nanobelt. The mechanism of the reorientation is the slip and spread of dislocation through the nanobelt under compressive stress caused by tensile surface-stress components.     

Molecular dynamics simulation of the solidification of liquid gold nanowires

The solidification behavior of liquid gold nanowires with about 1.84 nm in diameter has been studied by using molecular dynamics simulation with an embedded atom potential. It is found the cooling rate has great effect on the final structure of the gold nanowires during solidification from liquid. With the decrease of cooling rates, the final structure of the gold nanowires varies from amorphous to crystalline via helical multi-shelled structure. The face-centered cubic structure of the gold nanowires is proven energetically the most stable form.     

A Modified Free Volume Model for Characterizing of Rate Effect in Bulk Metallic Glasses

We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses （BMGs）. A dimensionless Deborah number DeiD = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively.     

First principles calculations of structural phase transformation in CaTe at high pressure

First principles calculations of the total energy of CaTe as a function of unit cell volume have been carried out for the NaCl, MnP and CsCl structures on the basis of density functional theory (DFT). All these calculations are performed with the CRYSTAL06 program package. The sequence of high-pressure phases for CaTe transforms from NaCl phase to an intermediate state with a mixture of NaCl and MnP phases and then to the CsCl phase is obtained, which is in good agreement with the previous experimental results. Several structural properties (equilibrium lattice constant, bulk modulus, etc.) of NaCl structure have been calculated, which are also in agreement with the previous experimental results.     

Interaction of Point Defects with Twin Boundaries in Copper

The interaction between small vacancy clusters and twin boundaries in copper is studied by using many-body potential developed by Ackland et aL for fcc metals. The interaction energies of single-, di- and tri-vacancy clusters with （111） and （112） twin boundaries are computed using well established simulation techniques. For （111） twins the vacancy clusters are highly repelled when they are on the adjacent planes, and are attracted when they are away from the boundary. In the case of （112） twins, vacancy clusters are more attracted to the boundary when they are near the boundary as compared to away from it. Vacancy clusters on both the sides of the boundary are also investigated, and it is observed that the clusters energetically prefer to lie on the off-mirror sites as compared to the mirror position across the twin.     

Indentation Load Effect on Young＇s Modulus and Hardness of Porous Sialon Ceramic by Depth Sensing Indentation Tests

Depth sensing indentation （DSI） tests at the range of 200-1800mN are performed on porous sialon ceramic to determine the indentation load on Young＇s modulus and hardness values. The Young modulus and hardness （Dynamic and Martens） values are deduced by analysing the unloading segments of the DSI test load-displacement curves using the Oliver-Pharr method. It is found that Young＇s modulus ET, the dynamic hardness HD and the Martens hardness HM exhibit significant indentation load dependences. The values of Young＇s modulus and hardness decrease with the increasing indentation load, as a result of indentation load effect. The experimental hf /hm ratios lower than the critical value 0.7, with hm being the maximum penetration depth during loading and hf the final unloading depth, indicate that our sample shows the work hardening behaviour.     

Ab-initio calculation of stability and structural properties of cadmium chalcogenides CdS, CdSe, and CdTe under high pressure

The structural phase transformations of Cd chalcogenide compounds (CdS, CdSe, and CdTe) under high pressure are studied using the local approximation to the density functional theory, and the one-electron equations are solved by means of the full potential linear muffin-tin-orbital method. CdS, CdSe, and CdTe are found to have nearly similar behaviour of the structural properties under high pressure. In the CdS compound, thePmmn phase is predicted to be thermodynamically stable after the rock-salt structure, and theCmcm structure is found to be thermodynamically stable before the rock-salt structure in both CdSe and CdTe. The resulting structural properties of the zinc-blende, wurtzite, cinnabar, rock-salt,Pmmn, CsCl,Cmcm, and β-Sn phases for these considered compounds are found to be in agreement with the existing experimental data.     

基于多尺度分析的FCC金属应变率敏感性研究

 采用位错理论和分子动力学模拟研究了金属原子性质对其宏观应变率敏感性的影响。依据位错运动的Orowan关系,认为金属中位错速度对应力的依赖关系是此研究的关键,并分析提出研究金属原子性质与应变率敏感性关系的分析方法。构建了一个中等规模的二维分子动力学模型,应用此模型对单个位错在FCC金属中的运动进行模拟。综合位错理论分析和分子动力学模拟结果得出结论：影响金属应变率敏感性的原子性质是其原子量而不是其原子势。依据此结论分析得到的FCC金属应变率敏感性排序与试验结果相符。     

Electronic structure and site occupation for the intermediate phase of LaNi4.5Al0.5Hy

The crystal structure, electronic structure and hydrogen site occupancy of LaNi_4.5Al_0.5H_y intermediate phase (y=2.0, 3.0, 4.0) have been investigated using the full-potential linearized augmented plane wave (FLAPW) method. For the first time we analyzed the interstitial site occupation of hydrogen atoms. The H atoms were found to prefer the 6m, 3f and 12n sites, while no 4h sites were occupied. A narrowed Ni-d band is found due to the lattice expansion: the total DOS at E_F increases with y, which indicates that the compounds become less stable. The interaction between Al and Ni, with H plays a dominant role in the stability of LaNi_4.5Al_0.5H_y intermediate phase. The smaller the shift of E_F towards the higher energy region, the more stable the compounds will be. Our results are compared with experimental data and discussed in the light of previous works.