Table of contents

The self-organization of dislocation structures produced by rearrangement under mutual interaction forces has been simulated by a computer technique based on molecular dynamics. The model employed is a two-dimensional array of straight, mixed dislocations on orthogonal slip planes. Equations of motion are developed that account for viscous drag in glide and climb motion, inertial forces, externally applied stresses and interaction forces due to other internal defects. Relaxation of an initially random dislocation structure by glide and by a combination of glide and climb has been studied. Results are expressed in the form of a two-dimensional distribution function, which reveals the degree and nature of the self-organization of the structure. Kinetics of the reduction of dislocation density by annealing are found to agree with a similar model based on mutual annihilation of dislocations in cell walls.

Kinetic effects such as coarsening and back diffusion affect the solidification rate and hence the rate at which latent heat of fusion is liberated. Neglecting these effects can bring about errors in macroscopic heat flow simulation. The paper investigates whether these errors are large enough to necessitate interactive coupling of the heat flow simulation to kinetic models of microsegregation. An approach of 'iterative' coupling is explored. This involves generating the thermal history by macroscopic calculations using an approximate relationship between the fraction of solid, f_S, and temperature, T, and then correcting the f_S(T) relationship by a detailed calculation of solute redistribution using a kinetic model of microsegregation. The corrected f_S(T) relationship including the kinetic effects is then input to the macroprogram and the iteration is continued until the predicted microsegregation parameters converge. For Al-4.8% Cu alloy and Al-54% Zn alloy the predictions converge within a single iteration. This shows that the coupling between the heat flow calculations and the solute redistribution calculations is weak.

The authors have studied the interstitialcy mechanism for interstitial cation diffusion in Li₂O and LiCl. First, the authors have checked and validated the applicability of the ionic approximation and the reliability of the pair-potential technique for the investigation of interstitial cation diffusion in ionic crystals. For these purposes the authors calculated the electronic structure and optical absorption energies for these ions, and compared the atomic structure calculated quantum-chemically with that calculated employing the pair-potential technique. Secondly the authors have shown that the interstitialcy mechanism is very effective because of the relatively small additional displacements of ions on moving from the initial configuration to the barrier point in compact sublattices, such as the cation sublattice in Li₂O or in LiCl. This results from the favourable lattice distortion around the interstitial ion. In addition the authors report a comparison of results obtained using the MOLSTAT Mott-Littleton code on CaF₂ with earlier calculations that employed the HADES program, and obtain a satisfactory measure of agreement between the two sets of calculations.

A number of crystallographic surfaces of SnO₂ have been modelled on the atomic scale using standard computational techniques. The calculated excess surface energies per unit cell are dominated by the electrostatic contribution and are ordered in the same way as the areas of the surface unit cells. In contrast the thermodynamic quantity of excess energy per unit area is dominated by the relaxation energy of the surfaces and does not follow any simple ordering. The authors discuss the implications of these findings for the sensor applications of the oxide.

Hole formation and Ca doping in YBa₂Cu₄O₈ have been studied using atomistic simulation techniques. The results predict that it is energetically favourable for Ca ions to enter the Ba sites suggesting that the increase in T_c upon Ca doping cannot be explained by an increase in hole concentration. The authors also show that the structural changes in YBa₂Cu₄O₈ upon Ca doping are similar to those induced by high pressure, which implies that the enhancement of T_c in YBa₂Cu₄O₈ upon either Ca doping or due to pressure may have the same root cause.

The crack tip stress and deformation field is analyzed for an ideally plastic ordered NiAl single crystal of B2 (BCC type) structure which has only three independent slip systems at room temperature, the (100)(100) systems. For the crack on the (010) plane growing in the (101) direction, only one of these systems is capable of sustaining considerable plane plastic flow. It involves slip planes lying parallel to the crack tip, making an angle of 90 degrees with the crack plane, and has the yield condition sigma ₁₂=+or- tau ₀ (1 is the cracking direction, 2 the crack plane normal, and tau ₀ the critical resolved shear stress). An elastic-ideally plastic asymptotic solution is derived in which the stress field has a In r type singularity, with a shearing discontinuity at 90 degrees . These features are verified and more fully quantified by a finite element solution.

Periodic ab initio Hartree-Fock calculations are used to obtain two-body potential parameters for alpha -Al₂O₃. The approach, which is generally applicable to crystalline solids, yields potential models that are superior to empirical parameterizations.

Molecular dynamics simulations of frictional sliding of a tip over a flat substrate in the presence of a lubricating solid film of one or two atomic layers thickness are presented. The role of adhesion in promoting wear is discussed by reference to earlier simulations of frictional sliding in the absence of a lubricating layer. Two conditions for lubrication are identified. First, the shear strength of the interface between the tip and the film must be lower than the shear strength of the tip or the underlying substrate. Secondly, the film must have a sufficient strength in compression to prevent penetration of the film by the tip. The importance of lubrication in obtaining images from the frictional force microscope is also discussed.

A new approach that allows thorough exploration of low-energy structures with arbitrary symmetry for any material is presented, along with an application to silicon. This approach, coupled with first-principles total-energy calculations, has led the authors to discover a previously unknown metastable structure of Si. The physical properties of this structure and the insight they afford to metallic versus covalent bonding and the nature of the amorphous and liquid phases are discussed.

A small group of researchers met recently to review the new and rapidly growing field of many-atom potentials for solids. The workshop was held on 25-27 September 1991, in Ann Arbor, MI, and was commissioned by the Air Force Office of Scientific Research. Some classes of materials are being treated well by many-atom potentials, while others are only now being considered. Combinations of materials including more than one type of bond seem clearly beyond our present capabilities. The systematics of many-atom potential development is in its infancy, and progress appears to be rapid.