The effective diffusivity in polycrystalline material in the presence of interphase boundaries

The problem of calculating the long-time-limit effective diffusivity in stable two-phase polycrystalline material is addressed for the first time. We make use of a phenomenological model where the high-diffusivity interphase boundaries are treated as connected ‘coatings’ of the individual grains. The derivation of expressions for the effective diffusivity with segregation is along the lines of the analysis by Maxwell in . Monte Carlo computer simulation using lattice-based random walks on a very fine-grained mesh is employed to test the validity of the expressions. It is shown that, for the specific cases analysed, the derived expressions for the effective diffusivity are in very good agreement with results from the simulations. Since the pattern of behaviour is not entirely clear at present, it is difficult to guide the choice for the best expression in a given case. The equivalent of the Hart equation for this problem is also derived. This equation is shown to be invariably in poor agreement with simulation results.     

Fractal dimension of cluster boundaries in porous polycrystalline HTSC materials

The fractal dimension of the boundaries of clusters formed by pores and granules in polycrystalline materials is shown to be determined by the sample density and crystallite sizes. The dependence of the fractal dimension on the density has a maximum. It is shown that the maximum diamagnetic response can be obtained in a porous high-temperature superconductor with a porosity of 50?C60% and small crystallite sizes.     

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB₂, LaB₆). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 52–62, October, 1993.     

具有六方晶系结构的多晶体材料弹性常数——Y弹性常数

利用最近提出的新的物理参量——Ｙ弹性常数,将其应用于具有六方晶系结构的多晶体材料.推导了六方晶系结构的多晶体材料之Ｙ弹性常数,通过算例与具有六方晶系结构的多晶体材料之X射线弹性常数进行了比较.运用这个Ｙ弹性常数进一步推导出的多晶体材料整体之机械弹性常数的表达式与Kneer的研究结果中的表达式虽然形式不同,但针对具体材料所计算的结果却完全符合. 关键词： Y弹性常数 六方晶系 多晶体材料     

Grain boundaries and elastic properties of aluminum-oxide and stainless-steel-based cermets

Elastic properties and thermal-phonon scattering are investigated in Al₂O₃+0.3% MgO ceramics and cermets of different porosities based on them. The cermets, reinforced with a metallic frame of the steel 12X18H9T, are obtained by dry compaction followed by sintering. It is shown that the elastic moduli of cermets are determined by their porosity and that the grain boundaries can be investigated in detail by a nonequilibrium-phonon propagation method.     

具有立方晶系结构的多晶体材料的弹性常数——Y弹性常数

提出了一个新的物理参量“Ｙ弹性常数”,并阐述了其物理含义.并将其应用于具有立方晶系结构的多晶体材料,推导了立方晶系结构的多晶体材料的Ｙ弹性常数,通过算例与具有立方晶系结构的多晶体材料的X射线弹性常数进行了比较.运用这个Ｙ弹性常数进一步推导出的多晶体材料整体的机械弹性常数的表达式与Krner的研究结果完全符合. 关键词： Y弹性常数 立方晶系 多晶体材料     

Micro-experimental study for the stress effects on the interphase of composite materials

Digital speckle micro-metrology system, which is a combination of a long-focus microscope and a digital image-measuring device, is developed for studying the micro-mechanics behavior of the interphase of bimaterials. The natural texture of a specimen's surface is thought as a carrier of deformation's information and is analyzed to obtain the displacement field in each step, strain field and their real-time variation of the interphase. The resolution of the micro-metrology system is 10 nm for in-plane displacements. In this paper, the micro-metrology system is employed to investigate the micro-mechanics behavior of the interphase under thermal impulsing. The experimental results show that the interphase is the main factor affecting the mechanical characteristic of the whole composite structure.     

Homogenization of random anisotropy properties in polycrystalline magnetic materials

This paper is devoted to the determination of the equivalent anisotropy properties of polycrystalline magnetic materials, modelled by an assembly of monocrystalline grains with a stochastic spatial distribution of easy axes. The mathematical theory of $\Gamma \text{-}\mathrm{convergence}$ is applied to homogenize the anisotropic term in the Gibbs free energy. The procedure is validated focusing on the micromagnetic computation of reversal processes in polycrystalline magnetic thin films.     

Anomalies of elastic properties of layered materials

Acoustic investigations of layered crystals KY(MoO₄)₂ and glassy alloys Si₂₀Te₈₀ (with inclusions of nanocrystallites) are performed with the purpose of elucidating the character of binding forces in layered materials. The absorption and velocity of sound, as well as the spatial evolution of the spectrum of acoustic fluxes in various directions in wide ranges of temperatures (90–300 K), frequencies (14–1800 MHz), and intensities (0.04–100 W/cm²) of sound, are measured. Acoustooptical and pulse-echo methods were used for the measurements. A theoretical analysis of the data obtained has revealed anomalously large values of the nonlinear elastic coefficients and anharmonicity constants of longitudinal phonon modes that are determined by the anharmonicity of binding forces across the layers and at boundaries with nanocrystallites. It is shown that the anisotropy of the mechanical strength of layered crystals is to a large extent determined by the anharmonicity of binding forces.     

Crystalline electric field effects on the paramagnetic susceptibility of polycrystalline materials

The paramagnetic susceptibility of polycrystalline materials having large magnetic anisotropies is considered. It is shown that polycrystalline data in many case may not reflect strong crystalline electric field phenomena, even though these are clearly seen in single crystal data.     

Peculiarities in the thermal and elastic properties of polycrystalline titanium near the electronic topological transition

Temperature dependences of the photoacoustic signal magnitude and the relative velocity variation of longitudinal ultrasound in polycrystalline titanium are measured. Anomalous behavior of these quantities is observed in the temperature region corresponding to the electronic topological transition in a titanium single crystal. The results of measurements are interpreted as an experimental verification of the possibility for the anomalies associated with the electronic topological transition to “survive” in a polycrystalline metal.     

Correlations among micro- and macromechanical properties of composite materials based on the interphase concept

In this paper an attempt was made to define microstructural properties of carbon fiber/PP composites, with respect to fiber surface chemistry and morphology. In order to define the effects of the fiber surface sizings and morphology on the polymer microstructure, the interphase and mechanical properties of the composites, carbon fibers with similar, but not identical surface chemistry (CH and CT) were used. Characterization was performed by several techniques: SEM, POM, reflection microscopy, DSC, FTIR, XPS, contact angle measurements. For microstructural analysis, the geometrical method, method of intercept and DIF method were used. It was found that both carbon fibers have a strong influence on the nucleation mechanism and crystallization as well as on the microstructural parameters in the model and macro composites. Nucleation efficiency of the fibers has been confirmed by the nucleation parameter Q, measured by Muchova–Lednicky method and by the interfacial energy parameters. Microstructural analysis based on the photographs obtained by POM, SEM and reflection microscopy has shown that in the CH/PP model and macrocomposites the sieve-grain network was formed, which indicates better mechanical properties. The results obtained for the macromechanical properties of PP composites reinforced with CH and CT have confirmed the prediction based on micostructural analysis.     

Interdiffusion in two-layer Pd/Ag films I. The effects of interphase boundaries

Interdiffusion processes in epitaxial single-crystal Pd/Ag thin films within the temperature range 20°–500°C are studied by transmission electron microscopy, electron diffraction and electrical resistance methods. Homogenization is investigated both during deposition and during annealing.The mass transfer kinetics is found to depend significantly on the original structure of the interphase boundary (dislocations present or not) and on its reconstruction due to interdiffusion. Regular networks of misfit dislocations at the interface can retard interdiffusion, while network failures lead to acceleration of homogenization process. The vacancy flows play a decisive role in these processes. The effects of interphase dislocations on the DIGM, DIR and RID processes are discussed.The numerical study of electrical resistance variations during annealing is carried out. The concentration profiles are plotted for a nontrivial dependence of interdiffusion coefficient on concentration. The numerical results are compared with the experimental data.     

3D reconstruction of grains in polycrystalline materials using a tessellation model with curved grain boundaries

A compact and tractable representation of the grain structure of a material is an extremely valuable tool when carrying out an empirical analysis of the material’s microstructure. Tessellations have proven to be very good choices for such representations. Most widely used tessellation models have convex cells with planar boundaries. Recently, however, a new tessellation model — called the generalised balanced power diagram (GBPD) — has been developed that is very flexible and can incorporate features such as curved boundaries and non-convexity of cells. In order to use a GBPD to describe the grain structure observed in empirical image data, the parameters of the model must be chosen appropriately. This typically involves solving a difficult optimisation problem. In this paper, we describe a method for fitting GBPDs to tomographic image data. This method uses simulated annealing to solve a suitably chosen optimisation problem. We then apply this method to both artificial data and experimental 3D electron backscatter diffraction (3D EBSD) data obtained in order to study the properties of fine-grained materials with superplastic behaviour. The 3D EBSD data required new alignment and segmentation procedures, which we also briefly describe. Our numerical experiments demonstrate the effectiveness of the simulated annealing approach (compared to heuristic fitting methods) and show that GBPDs are able to describe the structures of polycrystalline materials very well.     

Size effects on tensile and fatigue behaviour of polycrystalline metal foils at the micrometer scale

Tensile and fatigue properties of as-rolled and annealed polycrystalline Cu foils with different thicknesses at the micrometer scale were investigated. Uniaxial tensile testing results showed that with decreasing foil thickness the uniform elongation decreases for both as-rolled and annealed foils, whereas the yield strength and ultimate tensile strength increase for as-rolled foils, but decrease for the annealed foils. For both the as-rolled or annealed foils, bending fatigue resistance decreases with decreasing the foil thickness. Deformation and fatigue damage behaviour of the free-standing foils were characterised as a function of foil thickness. In addition, the fatigue strength of various small-scale Cu foils was compared to understand the physical mechanisms of size effects on mechanical properties of the metallic material at micrometer scales.     

Regional stretch method to measure the elastic and hyperelastic properties of soft materials

Characterizing the mechanical properties of soft materials and biological tissues is of great significance for understanding their deformation behaviors. In this paper, a regional stretching method is proposed to measure the elastic and hyperelastic properties of a soft material with an adhesive surface or with the aid of glue. Theoretical and dimensional analyses are performed to investigate the regional stretch problem for soft materials that obey the neo-Hookean model, the Mooney-Rivlin model, or the Arruda-Boyce model. Finite element simulations are made to determine the expressions of the dimensionless functions that correlate the stretch response with the constitutive parameters. Thereby, an inverse approach is established to determine the elastic and hyperelastic properties of the tested materials. The regional stretch method is also compared to the indentation technique. Finally, experiments are performed to demonstrate the effectiveness of the proposed method.