Giant Magnetostrictions of Tb-Dy-Fe Polycrystals with < 110 > Axial Alignment

Preparing method and processing of Tb-Dy-Fe alloy samples with [110] axial orientation as well as their magnetostrictive properties have been studied. It has been found that the magnetostrictive strains of polycrystal samples with [110] axial orientation can reach (1550-1900) ×10~-6 in a low magnetic field less than 80 kA/m, which are equal to or somewhat better than that of the polycrystal samples with [112] axial orientation.

CYCLIC HARDENING BEHAVIOR OF POLYCRYSTALS WITH PENETRABLE GRAIN BOUNDARIES:TWO-DIMENSIONAL DISCRETE DISLOCATION DYNAMICS SIMULATION

A two-dimensional discrete dislocation dynamics （DDD） technology by Giessen and Needleman （1995）, which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocationpenetrable grain boundaries （GBs） on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

Direction-dependent functions of physical properties for crystals and polycrystals

Some physical properties of crystals differ in direction n because crystal lattices are often anisotropic. A polycrystal is an aggregate of numerous tiny crystallites. Unless the polycrystal is an isotropic aggregate of crystallites, the physical properties of the polycrystal vary with n. The direction-dependent functions （DDF） for crystals and polycrystals are introduced to describe the variations of the physical properties in direction n. Until now there are few papers dealing systematically with relations between the DDF and the crystalline orientation distribution. Herein we give general expressions of the DDF for crystals and polycrystals. We discuss the applications of the DDF in describing the physical properties of crystals and polycrystals.

A Numerical Study of Localized Plastic Deformationin Polycrystals

A finite element formulation which derives constitutive response from crystal plasticity theory was used to examine localized deformation in fcc polycrystals. The polycrystal model was an idealized planar array of 22 hexagonal grains. The constitutive description used is based on a finite strain kinematical theory that accounts for lattice rotations. Formation of shear bands was successfully modeled in both single crystal and polycrystals. Stress and strain distribution around triple junctions was also analyzed. Results show the distributions of stresses and strains are distinctly inhomogeneous. Stress and strain fields across grain boundaries are highly discontinuous. However, this discontinuity will be restrained when shear bands are fully developed.

Realization of Superplastic State of Polycrystals under Grain-boundary Diffusion

Creep behaviour of Mo and Fe polycrystals has been studied for a wide range of Strain rates in the presence of grain-boundary diffusion flux of Ni. The role of the impurity grain-boundary diffusion and grain-boundary migratin in the realization of the superplastic strain of metals under these conditions is examined

PREDICTION OF YIELD FUNCTIONS ON BCC POLYCRYSTALS

By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ model, where the volume average of the yield functions of all BCC crystallites in Ω is taken as the macroscopic yield function of the BCC polycrystal. In constructing the formula, we try to find the relationship among the macroscopic yield function, the orientation distribution function （ODF）, and the single BCC crystal＇s plasticity. An expression for the yield stress of a uniaxial tensile problem is derived under Taylor＇s model in order to compare the expression with that of the macroscopic yield function.

Mechanisms of the epitaxial growth of two-dimensional polycrystals

In the epitaxial growth of two-dimensional(2D)materials on substrates,2D polycrystals with various shapes have been broadly observed,but their formation mechanisms are still highly elusive.Here we present a complete study on the formation mechanisms of various 2D polycrystals.The structures of the 2D polycrystals are dependent on the symmetries of both the 2D material and the substrate.We build four complete libraries of 2D polycrystals for(i)threefold symmetric 2D materials on two-or six-fold symmetric substrates(i.e.,family-Ⅲ/Ⅱor-Ⅲ/Ⅵ),(ii)threefold symmetric 2D materials on fourfold symmetric substrates(i.e.family-Ⅲ/Ⅳ),(iii)fourfold symmetric 2D materials on three-or six-fold symmetric substrates(i.e.,family-Ⅳ/Ⅲor-Ⅳ/Ⅵ),and(iv)sixfold symmetric 2D materials on fourfold symmetric substrates(i.e.,family-Ⅵ/Ⅳ),respectively.The four libraries of 2D polycrystals are consistent with many existing experimental observations and can be used to guide the experimental synthesis of various 2D polycrystals.

Phase-field study of crystallographic texturing in piezoelectric polycrystals

Crystallographic texturing enables the design of piezoelectric polycrystals that outperform traditional random polycrystals by exhibiting outstanding piezoelectric properties.In this work,phase-field modeling and computer simulation were employed to study the effect of crystallographic texture on the piezoelectric properties of ferroelectric polycrystals at the domain level.Domain evolutions for single crystal,random polycrystal,and textured polycrystal are systematically simulated.The simulations reveal that the[001]-textured polycrystal can fully exploit the intrinsic anisotropic properties of piezoelectric materials by exhibiting a piezoelectric coefficient that is as large as that of single crystal while being much larger than that of random polycrystal.To better understand the mechanism of piezoelectricity enhancement by crystallographic texturing,a theoretical analysis based on Landau theory is provided.In comparison with random polycrystal,the textured polycrystal manifests a flatter energy landscape and thus possesses a higher piezoelectric coefficient.

Finite element analysis using elasto-visco-plastic self-consistent polycrystal model for E-form Mg sheet subjected to bending

The mechanical behavior of a magnesium alloy E-form under bending was investigated using the elasto-visco-plastic polycrystal model(ΔEVPSC) and its finite element(FE) implementation(ΔEVPSC-FE) developed in Jeong et al. and Jeong and Tomé. The crystallographic orientation distribution(COD) obtained from X-ray diffraction was used to represent the initial texture, and the Voce hardening parameters were calibrated by fitting the uniaxial tension and the compression flow stress curves. A quasi-static FE analysis of a miniaturized V-bending operation was conducted using the ΔEVPSC-FE model. The bending induced an inhomogeneous stress response along the through-thickness and the lateral directions, which was well captured by the model. Moreover, the predictive capability of the model was validated by comparing with various experimental results such as(1) force vs. displacement curves;(2) the through-thickness variations in the twin volume fraction;and(3) the changes in crystallographic texture as a function of displacement. Additional bending simulation was performed using an isotropic texture, the result of which suggests that the potential improvement in bendability of the magnesium alloy is attainable by weakening the initial texture. Moreover, the simulation results imply that the crystallographic texture, which may affect the twin activation across the thickness direction, plays a significant role in the shifting direction of the neutral layer.

Preparation and Mechanical Properties of Zirconia Ceramics Doped with Different Y_(2)O_(3)Contents

The sintering behavior and mechanical properties of zirconia doped with 2.0mol%-3.0mol%Y_(2)O_(3)were studied by pressure-less sintering.The experimental results show that the densification temperature of zirconia ceramics increases gradually with the decrease of Y_(2)O_(3)doping content by which decreases the sintering driving force due to the lower oxygen vacancy concentration of the systems.Furthermore,the bending strength and fracture toughness of the prepared zirconia ceramics increase with the decrease of Y_(2)O_(3)doping content.It can be attributed to the fact that the phase stability of tetragonal zirconia decreases with the decrease of Y_(2)O_(3)doping content,which is easier to induce"phase transformation toughening"and dissipate impact energy.The relative density,bending strength and fracture toughness of 2.0 mol%Y_(2)O_(3)doped zirconia ceramics(2.0Y-ZrO_(2))sintered at 1525℃are 99.00%,1256.65±20.82 MPa and 9.85±0.13 MPa·m^(1/2),respectively.

Influence of Al on the magnetostriction of Fe-Ga polycrystal alloys under compressive stress

Fe80Ga20-xAlx （x = 0, 6, 9, 14） ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the partial substitution of Ga in Fe-Ga alloys with A1 on their magnetostrictive properties was investigated, and the effects of different heat treatment conditions on the magnetostriction and microstructure of the alloy rods were also examined. The saturation magnetostriction value of FesoGa2o can reach to 240 x 10-6 under a compressive stress of 20 MPa. The Fe80GallA19 alloy has many good properties, such as low hysteresis, high linearity of the magnetostriction curve, and low saturated magnetic field, which make it a potential candidate for magnetostrictive actuator and transducer applications. It is found that subgrains have little influence on the magnetostriction of Fe-Ga alloys.

Enhanced thermoelectric properties of Co_(1-x-y)Ni_(x+y)Sb_(3-x)Sn_x materials

Co1-x-yNix＋ySb3-xSnx polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite CO1-xNixSb3-xSnx was investigated. A leap of electrical conductivity from the Co0.93Ni0.07Sb2.93Sn0.07 sample to the Co0.88Ni0.12Sb288Sn0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Coo.88Nio.12Sb2.ssSno.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co0.81Ni0.09Sb2.88Sn0.12 reaches 3.0 mW-m-1·K-2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co0.81Ni0.19 Sb2.88Sn0.12,

Observation of Multi-layer Film on Si Substrate

The microstructures of Si substrate/amorphous SiO2 layer/SOI Si layer / polycrystal Si layer / amorphous SiO2 layer / coarse grain Al electrode were studied by using cross-section transmission electron microscopy, scanning electron microscopy, and high resolution electron microscopy. The straightness of interface between amorphous SiO2 layer and Si substrate is in 2-3 atomic layer. The defects were seldom found in the active area of SOI Si layer, and can be effectively confined in defect entrainment regions. The defects observed in defect entrainment regions are subgrain boundaries, large angle boundaries and dislocations. The dislocation loops which are parallel to the interface and have images with lines of no contrast under two beam condition have been observed in the side of SOI Si layer. Polycrystal Si layer consists of columnar and fine grains without preferential orientation. An ultra-thin amorphous SiO2 layer exists between SOI Si layer and polycrystal Si layer.

SOME MISUNDERSTANDINGS ON ROTATION OF CRYSTALS AND REASONABLE PLASTIC STRAIN RATE

It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if it is directly expressed with amounts of velocity of slips in glide systems such as (gamma) over dot upsilon circle times n. The geometrical figure of crystal lattices does nor change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rare induced by slips is proposed. Constitutive equations are recommended.

Anomalies in Young's modulus behavior after annealing in polycrystalline SmS

In this paper,the dependencies of Young's modulus and attenuation decrement on samarium sulfide polycrystals(SmS)under various annealing temperatures are studied by the piezoelectric ultrasonic composite oscillator technique at a frequency of 100 kHz in the temperature range of 80-300 K.A decrease in Young's modulus with an increase of the annealing temperature due to the texturing of the material was revealed.At the same time,attenuation peaks were observed at temperatures about 90 and 125 K,presumably due to Niblett-Wilks and Bordoni relaxations.

Constitutive relation of an orthorhombic polycrystal with the shape coefficients

An orthorhombic polycrystal is an orthorhombic aggregate of tiny crystallites. In this paper, we study the effect of the crystalline mean shape on the constitutive relation of the orthorhombic polycrystal. The crystalline mean shape and the crystalline orientation arrangement are described by the crystalline shape function （CSF） and the orientation distribution function （ODF）, respectively. The CSF and the ODF are expanded as an infinite series in terms of the Wigner D-functions. The expanded coefficients of the CSF and the ODF are called the shape coefficients s^lm0 and the texture coefficients c^lmn respectively. Assuming that Ceff in the constitutive relation depends on the shape coefficients s^lm0 and the texture coefficients c^lmn by the principle of material frame-indifference we derive an analytical expression for C^eff up to terms linear in s^lmo and c^lmn and the expression would be applicable to the polycrystal whose texture is weak and whose crystalline mean shape has weak anisotropy. C^cff contains six unspecified material constants （λ, μ, c, s1, s2, s3）, five shape coefficients （s^2 00, s^2 20, s^4 00, s^4 20, s^4 40）, and three texture coefficients （c^4 99,c^4 20, c^4 40）, The results based on the perturbation approach are used to determine the five material constants approximately. We also find that the shape coefficients 2 and a s^2mo and s^4m0 are all zero if the crystalline mean shape is a cuboid. Some examples are given to compare our computational results.

MICRO-EXPERIMENTAL STUDY OF LARGE PLASTIC DEFORMATION FOR HIGH-PURITY POLYCRYSTALLINE ALUMINUM

This paper aims at revealing various micro- deformation characteristics, such as crystalline slip and grain boundary slide, which are recorded under scanning electronic microscope for high-purity aluminum tensile specimen at room temperature. These experimental data provide us direct evidence for shear localization near the grain boundary network via multi- directional grain boundary slide. The nonuniform deformation induced in the grain interiors would have decisive effect on the plastic flow and failure of polycrystalline materials.

Experimental Study and Finite Element Polycrystal Model Simulation of the Cold Rolling Textures in a Powder Metallurgy Processed Pure Aluminum Plate

Ingot metallurgy (IM) aluminum has long been the subject and attracted the attention of many metallurgists and textural researchers of materials. Due to the introduction of large amounts of ex situ interfaces, however, the textures in powder metallurgy (PM) processed aluminum has been rarely reported. In this article, a pure aluminum plate was prepared via PM route. The starting billet was first produced with uni-axially cold compaction and flat hot-extrusion and then followed by cold rolling processes. The hot-extruded and cold rolling deformation textures of the pure PM aluminum at 50%, 80% and 90% cold rolling reductions were studied by orientation distribution functions (ODFs) analysis. The finite element polycrystal model (FEPM) was finally utilized to simulate the cold rolling textural evolution at various stages of cold rolling. In FEPM simulation, the initial hot-extruded textures were taken into account as inputs. The results showed that typical β-fiber texture formed in pure PM aluminum with the cold rolling reduction increased till 80%, and there was not much change after excessive cold rolling deformation. Homogeneous slip is not the only deformation mode in PM processed pure aluminum plate at over 80% cold rolling reduction. The experimental results were qualitatively in good agreement with the simulated ones.

Synthesis of AgGa1-xInxSe2 Polycrystalline Materials

AgGa1-xInxSe2 polycrystals were synthesized by the method of mechanical oscillation and temperature oscillation. X-ray diffraction spectra of polycrystal powder are conformable with the JCPDS cards. Lattice constants a and c calculated from the XRD were found to obey Vegard's law. The melting point of AgGa0.8In0.2Se2 obtained by means of differential scanning calorimetry (DSC) is 796.53 ℃. The DSC curve also show that there are no other transformation points below the melting point. The results indicate that polycrystalline materials synthesized by the method mentioned above are high-quality and can be used to grow single crystals by the vertical Bridgman technology.

Constitutive relation of weakly anisotropic polycrystal with microstructure and initial stress

Man （Nondestr Test Eval 15：191-214, 1999） derived the constitutive relation of a weakly-textured orthorhombic aggregate of cubic crystallites with effects of microstructure and initial stress. In this paper, a computational expression on the integration ∫SO（3） Q^× D^1m0dg is given. Then, by means of the computational expression, the general constitutive relation of a weakly-textured anisotropic polycrystal with the consideration of microstructure and initial stress is derived. As special cases of our general constitutive relation, two constitutive relations are given for an isotropic polycrystal and a weakly-textured anisotropic aggregate of cubic crystallites. The acoustoelastic tensor of the reference cubic crystal is derived to determine the material constants of the polycrystal. Two examples are given for understanding the physical meaning of the texture coefficients and the constitutive relations.