Effect Of Domain Boundaries Research Articles

A Monte–Carlo approach to domain boundary precipitation in binary alloys

A Monte–Carlo approach on the second phase precipitation in polycrystalline binary alloys is developed. The approach starts from the kinetic spin-exchange Ising model and the Q-state Potts model, and a coupling algorithm for simulation of both phase precipitation and domain growth is developed. The simulation on a simplified system with slow domain boundary migration reveals precipitation phenomena on the domain boundaries. The effect of domain boundaries on the morphology and kinetics of the second phase precipitates is investigated. It is shown that the scaling concept and the Lifshitz–Slyozov–Wagner law are broken in the present system.

Role of interdomain and interplatelet boundaries on magnetic flux penetration in melt grown YBCO

The microstructure of a melt processed YBCO sample reveals the presence of random domains with aligned platelets separated by microcracks inside the domains. The lower critical field (H/sub c1/) and critical current density (J/sub c/) of these boundaries will be less than that of the grains, and their role in determining the bulk properties cannot be neglected. We have employed a sensitive lock-in flat-band detection technique and recorded the low field ac M-H loops at 33 Hz and 77 K and derived the flux profiles. The results show an enhancement in H/sub c1/ due to melt processing. Inclusion of Y/sub 2/BaCuO/sub 5/, and Ag enhances the H/sub c1/ further. The flux profiles show distinct slope changes at certain fields which reflect different coupling strengths of weaklinks that become operative for flux entry at different applied fields. While the low field flux profiles are found to reflect the effect of residual weaklinks (like liquid phase and micropores) and domain boundaries, those at higher fields might reflect the effect of platelet boundaries.

Theory of domain boundary effects in a phase-separated mixture of polymer and liquid crystal

We calculate the end results of a phase-separation process in which a nematic liquid crystal is expelled from solution in a polymerizing solvent. Our model combines the essential features of the Flory–Huggins, Maier–Saupe, and Hillert models, and contains terms related to the nature of the anchoring process at the polymer–liquid–crystal interface. We find that there is a minimum size of droplet below which no nematic ordering can occur, and that in the absence of the effects due to curvature of the interface, homeotropic anchoring gives rise to a lower surface free energy than does planar anchoring.

Correlation between normal flux transfer from lamination to lamination during the magnetization process with static domain structures in grain oriented 3% silicon iron

Grain oriented 3% silicon iron is widely used as magnetic core material in power devices. It is usually assessed on the basis of its magnetic losses when magnetized as single strips or in the Epstein square. When material is built into cores and magnetized, some flux passes normal to the plane of the laminations and increases the losses of transformers and rotating machines. Pairs of strips were placed together and magnetized at 50 and 60 Hz at ac longitudinal flux densities from 1.0 to 1.8 T and the normal flux density, BN transferred between them was calculated from the emfs induced in search coils located between the strips. Static domain structures were observed using a colloidal technique. Values of BN have been correlated with the domain structures, grain size, and boundary effects. BN was found to be dependent on the static wall spacing in adjacent grains and varied between 0.5 and 3.0 mT with a repeatability better than ±5%. Most large grains had low surface magnetic charge at their boundaries and BN was low. The opposite was found where the flux was transferred between small, not so well oriented, grains. This large normal flux transfer between poorly oriented small grains causes small additional losses in laminated cores.

Metastable charge-density-wave states in NbSe3 studied by magnetotransport.

Magnetoquantum oscillations in ${\mathrm{NbSe}}_{3}$ have been used to study metastable electronic states produced by impurity pinning of charge-density waves (CDW's). Simultaneous measurements of the oscillations in separate sections of the crystal have provided information on the distribution of the metastable domain configurations. A model calculation of the magnetic-breakdown network responsible for the magnetoresistance oscillations was used to generate the various observed frequency distributions. The effects of domain boundaries on the oscillatory components of the magnetoresistance and Hall resistance were evaluated from a simple model. Metastable CDW states are shown to be directly related to the volume impurity pinning of the CDW; they also determine the threshold electric fields required for CDW collective motion.

A two-dimensional automatic mesh generator for finite element analysis for random composites

In this paper a two-dimensional automatic mesh generation technique has been proposed for composites, in which the second phase consisting of particulates or unidirectional fibers are randomly dispersed in the matrix. The discretization is based on Dirichlet tessellation of the composite domain to yield an aggregate of convex polygons. These polygons, called Voronoi polygons or Dirichlet cells, will serve as the elements in the finite element analysis of a composite structure. Effects of arbitrary domain boundaries, shapes and sizes of the inclusions have been incorporated in the mesh generator.

Diffuse Phase Transition in Ferroelectric Polymers and Its Irradiation Effect

The ferroelectric phase transition was investigated in the VDF/TrFE copolymer films before and after irradiations by means of dielectric, X-ray diffraction, IR and gel content measurements. The diffuse phase transition was explained in terms of the Ginzburg-Landau theory, in which the domain boundary effect was taken into account. On irradiation, the peak temperature on the ε'-Tcurve shifted to lower temperatures, but did not decrease below 40°C even irradiated with more than 200 Mrad. On the other hand, the result of X-ray diffraction measurement exhibited a radiation-induced phase transition at room temperature after irradiation with 120 Mrad. This discrepancy may be attributed to the phase coexistence over a wide temperature range.

Effect of antiphase domain boundaries on microdiffraction computer simulation

The intensity distributions of microdiffraction patterns from thin crystals can be very complicated because of their sensitivity to the presence of faults and defects. The effects are illustrated by computer simulations using dynamical diffraction theory for microdiffraction patterns from antiphase boundaries of various types between ordered domains of binary alloys.

Domain Nucleation and Boundary Effects in Thin Uniaxial Plates

In high-mobility materials with large uniaxial anisotropy (K>2πMs2) the magnetostatic wall-energy model as applied to an infinite lamina apparently provides quantitative explanation of most observed domain structures. However, the presence of boundaries and nucleation energy barriers usually prevent the spontaneous development of the equilibrium (lowest energy)1 structures. To understand this, we argue that the confluence of any two domains (or a domain and an edge boundary), or the reverse process of separation, always involves an intermediate state of higher energy. Consequently, for example, the periodic stripe array should not be expected to spontaneously divide into a lattice of bubbles when the latter is a lower energy structure. Similarly, the repulsive interaction with the boundaries prevents the equilibrium field dependence of the lattice nearest-neighbor distance. Our observations on an epitaxial Ga–YIG film2 are shown to be entirely consistent with these qualitative considerations, while the observed domain critical fields, sizes, and spacings are quantitatively predicted by the wall-energy model. Calculations of the nucleation energy for radial growth were carried out for a semicircular (half bubble) domain at a crack and at a free edge. The latter process is even more costly energetically than volume nucleation while the former requires less than half the volume nucleation energy. At crack intersections the barrier is reduced still further. Our observations on the Ga–YIG film show that domain nucleation occurs readily at cracks and rarely at edges in agreement with the theoretical suggestions. A more complete discussion of this work will be published elsewhere.

Diffusion in the face centered cubic ordered alloys

Diffusion in the face centered cubic alloys of the CuAu type has been studied using Kirkwood's second moment approximation. Formulae have been derived for the vacancy concentration and the migration energy. Both of them have short range order dependent terms above the critical temperature. It is shown that the number of sublattices necessary to describe the alloy (four for an f.c.c. alloy, two for a b.c.c. alloy) produces some important consequences. First, terms linear in the long range order parameter S appear for the vacancy formation energy in an f.c.c. alloy thus causing the vacancy concentration to become unequal for different sublattices. Second, in an f.c.c. alloy jumps between sites on different possible pairs of sublattices should have unequal weights, so that diffusion cannot be a random walk process. The possible effects of back jumps and domain boundaries on diffusion in an f.c.c. alloy are discussed.

The Fine Structure of Spots in Electron Diffraction Resulting from the Presence of Planar Interfaces and Dislocations. III. Domain Boundaries

AbstractThe fine structure of electron diffraction spots caused by the presence of domain boundaries (δ‐boundaries) is derived theoretically. The same method is used as in part I, namely the Fourier transforms of the transmitted and scattered intensities are calculated and discussed. The predicted fine structure consists in a fourfold splitting of the scattered as well as of the transmitted beams in a direction perpendicular to the interface. It is shown that the relative intensities and the spacing of the spots in this quadruplet are strongly influenced by the anomalous absorption and by parameters such as e.g. Δs and s. The effect of domain boundaries on the multiplicity and nature of the splitting is discussed. Finally the relation between the constituent beams and the fringe pattern visible in transmission electron microscopy at domain boundaries is analysed. Excellent agreement is found with the pattern characteristics previously derived from the analytical discussion of the transmitted and scattered intensities.

Ultrasonic Attenuation and Velocity in Three Transformation Products in Steel

Ultrasonic attenuation measurements were made from 2 to 100 Mc/sec in the pearlitic-plus-ferritic, bainitic, and martensitic transformation products in SAE 4150 steel, a low-alloy, 0.5% carbon variety. Measurements were also made in the martensitic specimen after tempering. Ultrasonic velocity measurements were made at 10 Mc/sec in each case. To eliminate the question of grain size and grain size distribution, three specimens were treated identically through the austenitizing operation. Then they were cooled differently to produce the three transformation products. The attenuation can be expressed as Af4+Cf2, where f is frequency. The first term is Rayleigh scattering and the second may be from dislocation damping, atomic relaxations, or magnetic domain boundary effects. Both A and C are strong functions of microstructure. Both coefficients decrease in the order pearlite-plus-ferrite, bainite, martensite, tempered martensite. On tempering, A decreased in the ratio 3:2 while C decreased 3:1. In pearlite-plus-ferrite, A is larger by a factor of 225 than it is in tempered martensite, and C is larger by a factor of 10. The ratio CT/CL (T = transverse waves, L=longitudinal waves) was a constant independent of microstructure and equal to 2.4. This suggests either dislocation damping, an atomic relaxation, or a magnetic effect. The large change in C on tempering (with CT/CL = constant) indicates that the interstitial carbon is involved. The ultrasonic velocity measurements showed an increase in velocity on tempering the martensite. Pearlite-plus-ferrite has the highest velocities and density, while raw martensite has the lowest. The differences in velocities arise primarily from differences in the elastic moduli of the transformation products, not from the density differences.

Physical Acoustics and the Properties of Solids

The techniques of physical acoustics have been applied in determining the elastic properties and internal friction of polycrystal and single crystal metals, glasses, nonmetallic crystals, high polymer materials, and ceramics. They have been used as tools in investigating such solid state phenomena as grain and domain boundary effects in metals and ferromagnetic and ferroelectric materials, in the diffusion of atoms, molecules and vacancies through a solid, in the motion of imperfections such as dislocations and have even detected an interaction between the lattice sound vibrations and free electrons in metals at low temperatures. These effects are studied by techniques varying from the vibrations of torsional pendulums at frequencies below 1 cy to frequencies as high as 300 Mc by ultrasonic pulsing methods and have employed strain values from 10−8 to 10−2. Four examples of these techniques are discussed in the present paper. These include diffusions of nitrogen and carbon atoms in iron, the motion of domain walls in ferromagnetic nickel, the interaction of lattice vibrations with free electrons at low temperatures and the use of high strain values in studying internal friction and fatigue in metals. These examples were chosen because they result in definitely explained effects which are indicative of typical processes occurring in solids.