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	MODELLING AND SIMULATION
   IN MATERIALS SCIENCE AND ENGINEERING
            Vol. 1 No. 1

             ABSTRACTS
             =========
 
 
Developing Potentials for Atomistic Simulations
 
John R Smith, Physics Department, General Motors 
Research and Environmental Staff, Warren, MI 
48090-9055, USA
 
David L Srolovitz, Department of Materials Science 
and Engineering, University of Michigan, Ann 
Arbor, MI 48109-2136, USA
 
Abstract: 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 capabilties. The systematics of many-atom 
potential development is in its infancy, and 
progress appears to be rapid.
 
 
Crack Tip Fields in a Material with Three 
Independent Slip Systems: NiAl Single Crystal
 
Maryam Saeedvafa and James R Rice, Division of 
Applied Sciences, Harvard University, Cambridge, 
MA 02138, USA
 
Abstract: 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 {110} <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 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 sigma12 = 
+/-tau0 (1 is the cracking direction, 2 the crack 
plane normal, and tau0 the critical resolved shear 
stress). An elastic-ideally plastic asymptotic 
solution is derived in which the stress field has 
a ln tau type singularity, with a shearing 
discontinuity at 90 degrees. These features are 
verified and more fully quantified by a finite 
element solution. At the regions very near the 
tip, the field is divided into 5 angular sectors 
of which 2 are plastic regions (sigma12 =+/-tau0), 
one inclined ahead of the crack front and one 
behind. Further away from the tip, at distances 
greater than about 0.01 of the overall plastic 
zone size, the plastic region ahead of the crack 
front, in which  sigma12=tau0, disappears 
resulting in 3 angular sectors of which one is 
plastic, which persists for the rest of the 
plastic region. While the plastic zone covers a 
broad angular range, the deformation field within 
it is dominated by the shear discontinuity of 
displacement. The stress on other possible systems 
such as {110}<111>, which in NiAl becomes 
activated at higher temperatures, and on 
{112}<111> systems, which are the usual twinning 
systems for BCC crystals, is also investigated. 
The results show that zones over which the 
resolved shear stresses on these two sets of 
systems become of the order of critical resolved 
shear stress on the soft {110}<100> system, 
although very close to the crack tip the stresses  
on some of these systems become significant due to 
the stress singularity. Thus, even though the 
shear strength is much larger for these two 
systems than for the {110}<100> system, over 
regions extremely near the tip they may become 
activated and alleviate the stress singularity. It 
is also possible that cleavage will occur first at 
lower temperatures or higher strain rates, and 
such is consistent with the brittleness of NiAl.
 
 
Calculation of Adiabatic Barriers for Cation 
Diffusion in Li2O and LiCl Crystals
 
J L Gavartin, C R A Catlow and A L Shluger, The 
Royal Institution, 21 Albemarle Street, London, 
W1X 4BS, UK (J L Gavartin and A L Shluger, 
University of Latvia, 19 Rainis Boulevard, Riga, 
Latvia)
 
A N Varaksin and Yu N Kolmogorov, Ural 
Politechnical Institute, Ekaterinburg, Russian 
Federation
 
Abstract: We have studied the interstitialcy 
mechanism for interstitial cation diffusion in 
Li2O and LiCl. First, we 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 we 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 
we have shown that the interstitialcy mechanism is 
very effective because of the relatively small 
additional displacements of ions 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, we 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.
 
 
The Stability of SnO2 Surfaces
 
P A Mulheran and J H Harding, AEA Industrial 
Technology, Harwell Laboratory, Didcot, Oxon, OX11 
0RA, UK
 
Abstract: 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. We 
discuss the implications of these findings for the 
sensor applications of the oxide.
 
 
Mechanism of Lubrication by a Thin Solid Film on a 
Metal Surface
 
J A Nieminen, Department of Electrical 
Engineering, Tampere University of Technology, PO 
Box 527, SF-33101 Tampere, Finland
 
A P Sutton and J B Pethica, Department of 
Materials, University of Oxford, Oxford, OX1 3PH, 
UK
 
K Kaski, Department of Electrical Engineering, 
Tampere University of Technology, PO Box 527, 
SF-33101 Tampere, Finland
 
Abstract: 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 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.
 
 
Periodic ab initio Determination of Interatomic 
Potentials for Alumina
 
J D Gale and C R A Catlow, Davy Faraday Research 
Laboratory, Royal Institute of Great Britain, 21 
Albemarle Street, London, W1X 4BS, UK
 
W C Mackrod, ICI Chemical and Polymers plc, PO Box 
8, The Heath, Runcorn, Cheshire, WA7 4QD, UK
 
Abstract: 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 
parametrizations.
 
 
Exploring the Structure of Solids Through Magic 
Strains: Prediction of a New Metallic Phase of Si
 
Efthimios Kaxiras, Department of Physics and 
Division of Applied Sciences, Harvard University, 
Cambridge, MA 02138, USA
 
L L Boyer, Complex Systems Theory Branch, Naval 
Research Laboratory, Washington, DC 20375, USA
 
Abstract: 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 us 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.
 
 
Coupling of Microsegregation Models to Heat Flow 
Simulations in Casting
 
R Sasikumar and H E Exner, Technische Hochschule 
Darmstadt, Petersenstrasse 30, 6100 Darmstadt, 
Federal Republic of Germany
 
Abstract: Kinetic efforts 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. 
This 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 'interative' 
coupling is explored. This involves generating the 
thermal history by macroscopic calculations using 
an approximate relationship between the fraction 
of the solid, fs, and temperature, T, and then 
correcting the fs(T) relationship by a detailed 
calculation of solute redistribution using a 
kinetic model of microsegregation. The corrected 
fs(T) relationship including the kinetic effects 
is then input to the macroprogram and the 
interation 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.
 
 
Simulation of Dislocation Mocrostructures in Two 
Dimensions: I. Relaxed Structures
 
A N Gulluoglu and C S Hartley, Department of 
Mechanical Engineering, College of Engineering, 
Florida Atlantic University, Boca Raton, FL 33431, 
USA
 
Abstract: 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.
 
 
Atomic Simulation Studies of Hole Formation and 
Ca-Doping in YBa2Cu4O8
 
X Chang and C R A Catlow, Davy Faraday Research 
Laboratory, The Royal Institution, 21 Albemarle 
Street, London, W1X 4BS, UK
 
Abstract: Hole formation and Ca-doping in 
YBa2Cu4O8 have been studied using atomistic 
simulation techniques. The resluts predict that it 
is energetically favourable for Ca ions to enter 
the Ba sites suggesting that the increase in Tc 
upon Ca-doping cannot be explained by an increase 
in hole concentration. We 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 Tc in YBa2Cu4O8 
upon Ca-doping or due to pressure may have the 
same root cause.