空位位错环的形成

本文研究了淬火面心立方金属内位错环和空洞的形核和长大问题。两种核都能形成,但是位错环的长大速度比空洞的长大速度大很多倍,故电子显微镜能看清的大缺陷都是位错环,而不是空洞。     

Closed and open-ended stacking fault tetrahedra formation along the interfaces of Cu–Al nanolayered metals

Stacking fault tetrahedra (SFTs) are volume defects that typically form by the clustering of vacancies in face-centred cubic (FCC) metals. Here, we report a dislocation-based mechanism of SFT formation initiated from the semi-coherent interfaces of Cu–Al nanoscale multilayered metals subjected to out-of-plane tension. Our molecular dynamics simulations show that Shockley partials are first emitted into the Cu interlayers from the dissociated misfit dislocations along the Cu–Al interface and interact to form SFTs above the triangular intrinsic stacking faults along the interface. Under further deformation, Shockley partials are also emitted into the Al interlayers and interact to form SFTs above the triangular FCC planes along the interface. The resulting dislocation structure comprises closed SFTs within the Cu interlayers which are tied across the Cu–Al interfaces to open-ended SFTs within the Al interlayers. This unique plastic deformation mechanism results in considerable strain hardening of the Cu–Al nanolayered metal, which achieves its highest tensile strength at a critical interlayer thickness of ~4 nm corresponding to the highest possible density of complete SFTs within the nanolayer structure.     

Comments on the structures observed for atomic adsorption on (110) surfaces of face-centred cubic metals

The hybridisation model for face-centred cubic metals, proposed by Altmann, Coulson and Hume-Rothery, is used for analysing some aspects of the surface structures reported by LEED crystallography for S adsorbed on the (110) surfaces of nickel and rhodium, and for O adsorbed on the (110) surfaces of nickel and iridium. The main objectives are to rationalise the different adsorption sites adopted by S and O, and to assess the measured bond lengths. For the latter, use is made of a bond length-bond order relationship given by Pauling.     

Zur Temperaturabhängigkeit der elastischen Konstanten kubischer Gitter

An equation involving the temperature variations of the elastic constants of cubic metals is implicitly contained in the Bhatia model for cubic metals. This result is of interest because it is closely connected with the question of the equilibrium condition in lattice dynamics. Recent experimental results on the elastic constants and thermal expansion coefficients of copper and aluminum are compared with a formula derived from the Bhatia theory. The experimental results are not yet sufficiently accurate to permit a detailed comparison with the theory though rough agreement is found.     

Thermal stability of nanotwinned and nanocrystalline microstructures produced by cryogenic shear deformation

Nanotwinned microstructures are of significant interest due to their high strength and enhanced thermal stability, attributed to the presence of a dense network of coherent twin interfaces. Propensity for twinning during deformation is known to increase at low temperature and/or high-strain-rate. In this study, we use high-strain-rate (~10^3?s^?1) shear deformation in cutting over a range of strains (γ ~1–5) and temperatures (cryogenic to ambient) to engineer a variety of microstructures in three face-centred cubic (FCC) metals – copper, brass and aluminium. The microstructures include nanocrystalline-equiaxed and densely (nano) twinned types of controllable domain size. The effects of low-temperature deformation and stacking fault energy on the resulting microstructure, hardening, stored energy and associated recrystallization kinetics are established. For copper, the nanotwinned microstructures are found to be thermally more stable and stronger than the equiaxed counterparts comprised of random high-angle grain boundaries. This enhanced effect of nanotwins on microstructure stability is, however, not observed in brass, while aluminium did not show any indications of twinning over the investigated range of deformation conditions.     

On high temperature susceptibilities of Heisenberg model ferromagnetics and anti-ferromagnetics

High temperature power series valid for general spin and including second-neighbour interactions are given for the physical susceptibility of the three cubic lattices and for the staggered susceptibility of some antiferromagnetic orderings of the body-centred and face-centred lattices. The critical ordering temperatures corresponding to these series are discussed and compared with the results of molecular field theory.     

Face-centred cubic to body-centred cubic phase transformation under [1 0 0] tensile loading

Molecular dynamics simulation was used to verify a speculation of the existence of a certain face-centred cubic (FCC) to body-centred cubic (BCC) phase transformation pathway. Four FCC metals, Ni, Cu, Au and Ag, were stretched along the [1?0?0] direction at various strain rates and temperatures. Under high strain rate and low temperature, and beyond the elastic limit, the bifurcation of the FCC phase occurred with sudden contraction along one lateral direction and expansion along the other lateral direction. When the lattice constant along the expansion direction converged with that of the stretched direction, the FCC phase transformed into an unstressed BCC phase. By reducing the strain rate or increasing the temperature, dislocation or ‘momentum-induced melting’ mechanisms began to control the plastic deformation of the FCC metals, respectively.     

X-ray diffraction evidence of the single solid solution character of bi-metallic Pt-Pd catalyst particles on an amorphous SiO2 substrate

An X-ray diffraction study was carried out on powders of a series of catalysts prepared from aqueous solutions of H₂PtCl₆ and PdCl₄ and amorphous SiO₂ with different concentrations in weight of Pt and Pd at about 4% in overall metallic weight. Measurements of the position of high angle Bragg reflections in the diffractograms show evidence of the fact that the small catalyst particles are bi-metallic Pt-Pd crystals having a face-centred cubic Bravais lattice. The lattice constant of these crystals was found to change with the relative concentration of Pt and Pd by following the Vegard’s rule. This correlation leads to the conclusion that the bi-metallic catalyst particles are made of a single solid solution of Pt and Pd atoms in the whole range of relative concentrations. Relative concentrations of these metals in the samples under study were determined by using energy dispersive X-ray spectrometry and their values were found to be close to the stochiometric relative concentrations in weight of the metals in the precursor aqueous solution. An average size of about 96 Å was estimated for the bi-metallic particles from the full-width at half-maximum value measured for the (2 0 0) diffractometric curve.     

Strengthening at nanoscaled coherent twin boundary in f.c.c. metals

This paper analyses slip transfer at the boundary of nanoscaled growth twins in face-centred cubic (f.c.c.) metals for strengthening mechanism. The required stress for slip transfer, i.e. inter-twin flow stress, is obtained in a simple expression in terms of stacking fault energy and/or twin boundary (TB) energy, constriction energy and activation volume. For nanotwinned Al, Cu and Ni, inter-twin flow stress versus twin thickness remarkably shows Hall–Petch relationship. The Hall–Petch slope is rationalized for various reactions of screw and non-screw dislocations at the TB. Additionally, strengthening at the boundary of nanoscaled deformation twins in f.c.c. metals is analysed by evaluating required twinning stress. At small nanograin size, the prediction of deformation twin growth stress shows inverse grain-size effect on twinning, in agreement with recent experimental finding.     

Atomic configurations in ideally flat surfaces—I Construction of models of surfaces in face-centred and body-centred cubic crystals

A method is described for constructing ball models of atomically flat surfaces of any orientation in face-centred and body-centred cubic crystals.     

Dislocation structure and dynamics govern pop-in modes of nanoindentation on single-crystal metals

ABSTRACT

There are two types of pop-in mode that have been widely observed in nanoindentation experiments: the single pop-in, and the successive pop-in modes. Here we employ the molecular dynamics (MD) modelling to simulate nanoindentation for three face-centred cubic (FCC) metals, including Al, Cu and Ni, and two body-centred cubic (BCC) metals, such as Fe and Ta. We aim to examine the deformation mechanisms underlying these pop-in modes. Our simulation results indicate that the dislocation structures formed in single crystals during nanoindentation are mainly composed of half prismatic dislocation loops. These half prismatic dislocation loops in FCC metals are primarily constituted of extended dislocations. Lomer–Cottrell locks that result from the interactions between these extended dislocations can resist the slipping of half dislocation loops. These locks can build up the elastic energy that is needed to activate the nucleation of new half dislocation loops. A repetition of this sequence results in successive pop-in events in Al and other FCC metals. Conversely, the half prismatic dislocation loops that form in BCC metals after first pop-in are prone to slip into the bulk, which sustains plastic indentation process after first pop-in and prevents subsequent pop-ins. We thus conclude that pop-in modes are correlated with lattice structures during nanoindentation, regardless of their crystal orientations.     

A relativistic coupled-cluster interaction potential and rovibrational constants for the xenon dimer

An accurate potential energy curve has been derived for the xenon dimer using state-of-the-art relativistic coupled-cluster theory up to quadruple excitations accounting for both basis set superposition and incompleteness errors. The data obtained is fitted to a computationally efficient extended Lennard-Jones potential form and to a modified Tang–Toennies potential function treating the short- and long-range part separately. The vibrational spectrum of Xe₂ obtained from a numerical solution of the rovibrational Schrödinger equation and subsequently derived spectroscopic constants are in excellent agreement with experimental values. We further present solid-state calculations for xenon using a static many-body expansion up to fourth-order in the xenon interaction potential including dynamic effects within the Einstein approximation. Again we find very good agreement with the experimental (face-centred cubic) lattice constant and cohesive energy.     

Building the pyramids: perfect bubble crystals

Abstract

The use of appropriate flat-sided pyramidal containers to grow ordered foams allows single crystals to be formed. In the case of face-centred cubic crystals, these have been prepared with up to 500 bubbles. Strained and deliberately defective crystals can also be grown. The growth of simple cubic and body-centred cubic crystals is limited by instability; preliminary results are presented for these, as well as ordered bidisperse foams.     

面心立方金属的空穴松弛能和空穴形成能的计算

本文利用纯金属的Morse势计算了五种面心立方金属(Pb,Ag,Ni,Cu,Al)的空穴松弛能和空穴形成能。计算松弛能时,除了考虑原子重新分布外,还特别考虑了电子云重新分布所引起的效应。如此求得的松弛能分别为1.27—1.36,>1.73,1.93—2.29,1.52—1.84,>1.09eV,比没有考虑电子云重新分布求得的松弛能大一个多电子伏。这表明电子云重新分布对松弛能的贡献是很重要的。由于求得了比较合理的松弛能,因而找到了一种模型比较简单,充分考虑到松弛效应的、适合于计算所有立方金属空穴形成能的方法。最后,求得上述金属的空穴形成能分别为0.64—0.74,<1.22,1.78—2.15,1.52—1.85,<1.67eV,比实验值大一些;它给出了上述实际金属的合理的理论上限值。     

Investigation of the activation-parameters of low-temperature slip in cubic metals

The macroscopic critical resolved shear stress (CRSS)τ of 9 body-centred cubic (BCC) and 5 face-centred cubic (FCC) metals has been found to vary with temperatureT in the range 0 to 300 K as given by: lnτ=A − BT, whereA andB are positive constants. Theτ−T data have been analysed within the framework of a kink-pair nucleation (KPN) model of plastic flow in crystals. The microscopic parameters of the unit activation process of yielding, e.g. the initial length of the glide dislocation segment, the critical height of the kink-pair nucleated in it, the activation volume associated with the CRSS, and the binding energy per interatomic spacing along the glide dislocation in the slip plane etc., have been evaluated. A consistent picture of the dislocation kinetics involved in the yielding of BCC and FCC metals emerges, which is adequately described by the KNP model of plastic flow in crystals.     

Nanoscale thermodynamics of multicomponent,elastic, crystalline solids: diamond,silicon, and silicon carbide

This paper extends the thermodynamic behaviour of two-dimensional and simple three-dimensional crystalline solids developed by Oh et al. and Slattery and Lagoudas to more complex, multicomponent, three-dimensional, elastic, crystalline solids. The analysis recognizes that the Helmholtz free energy is an explicit function of the lattice vectors defining the crystalline structure. From this theory, we obtain the stress-deformation behaviour and the elastic properties of diamond, silicon, and silicon carbide, which are face-centred, cubic, crystal structure. These are compared with available experimental values.     

Cross-slip in face centred cubic metals: a general full stress-field dependent activation energy line-tension model

Cross-slip is a thermally activated process by which a screw dislocation changes its slip plane. Understanding and modelling the activation barrier of the cross-slip process as a free-energy barrier that depends on the stress conditions at the vicinity of the dislocation is crucial. In this work, we employ the line-tension model for the cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier when both Escaig stresses are applied on the primary and cross-slip planes and Schmid stress is applied on the cross-slip plane. We propose a closed-form expression for the activation energy for cross-slip in a large range of stresses, without any fitting parameters. The results of the proposed model are in good agreement with previous numerical results and atomistic simulations. We also show that, when Schmid stress is applied on the cross-slip plane, the energy barrier is decreased, and in particular, cross-slip can occur even when the Escaig stress in the primary plane is smaller than that on the cross-slip plane. The proposed closed-form expression for the activation energy can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality. This will allow cross-slip to be more accurately related to macroscopic plasticity.     

The sixth frequency moment of the relaxation-shape function for a Heisenberg paramagnet

A correction to the hitherto published expression for the sixth frequency moment of the relaxation-shape function of a Heisenberg paramagnet at elevated temperatures is presented. The results are in accord with a computer calculation for a face-centred cubic lattice with nearest- neighbour exchange interactions.