The strength of two-phase ceramic/glass materials

The strengths of various glasses, with a range of expansion coefficients, containing 10 vol % thoria spheres, of diameter 50 to 700m, have been measured. Stresses occur around the spheres, due to differences in the expansion coefficients of the glass and the spheres, on cooling from the fabrication temperature. Stress magnification occurs near the spheres, due to differences in elastic properties, in the presence of an applied stress. When the expansion coefficient of the sphere is greater than that of the glass, circumferential cracks form around the spheres but only when the sphere diameter is greater than a critical value. An approximate value for the critical diameter may be obtained by an energy balance criterion. Cracks may form around spheres smaller than the critical diameter under application of applied stress at stresses below the macroscopic fracture stress. In these cases the strength is governed by a Griffith relationship with the crack size equal to the sphere diameter. When the expansion coefficients of the spheres and glass are similar, the strength of the glass is reduced only when large spheres (300m diameter) are present. When the expansion coefficient of the spheres is less than that of the glass, linking radial cracks form between the spheres and the material has very low strength.     

On grain growth kinetics in two-phase polycrystalline materials through Monte Carlo simulation

Monte Carlo Potts model simulation was carried out on a 2D square lattice for various surface fractions of second phase particles for over 50,000 iterations. The observations are in good agreement with known theoretical and experimental results with respect to both growth kinetics as well as grain size distribution. Further, the average grain size and the largest grain size were computed for various surface fractions which have indicated normal grain growth and microstructure homogeneity. The surface fraction of the second phase particles interacting with the grain boundaries (Φ), hitherto not computed through the simulation route, is shown to vary inversely as the average grain size due to Zener pinning.     

Thermodynamic nature of exponential empirical time dependences of strength characteristics and the failure of materials. Report 1. Creep and long-term strength

It is shown that the correlation between two parameters describing the power law of creep or the long-term-strength curve, which can be demonstrated on the basis of numerous experimental data, is governed by the thermodynamics of the processes of steady-state creep or creep-induced failure. Analytical expressions describing this correlation are obtained, and values of the parameters entering into this relationship are explained on the assumption of the logarithmic character of the dependence of the activation energy of the process on deforming stress. An independent method of evaluating the cohesive energy of the material from results of creep tests is proposed. A simple formula is presented for the a priori estimation of the slope of the long-term strength curve from the material's melting point.The paper is written as an outgrowth of a study performed by the authors, which was published in the journal Problemy Prochnosti, No. 6, 3–9 (194).Translated from Problemy Prochnosti, No. 2, pp. 5–24, February, 1996.     

Angular and temperature dependences of ion-induced electron emission of polycrystalline graphite

The dependences of ion-induced electron emission yields γ for MPG-8 graphite under high fluence 30 keV N₂⁺ and Ar⁺ ion irradiation on target temperature at different incidence angles (0-80°) have been measured. At normal incidence a step-like increase of the electron yield has been found at the defect annealing temperature T_a≈190°C. This effect and the changes of γ(T) with incidence angle are discussed in terms of electron path length rise due to decrease of the degree of amorphisation as a result of a phase-transition type of re-crystallisation and radiation damage annealing.     

An approach for simulating microstructures of polycrystalline materials

In this paper, an approach is identified using concepts in molecular dynamics (MD) and discrete element method (DEM) to generate the microstructure of polycrystalline materials. Using the proposed methods, different types of particles with different grain size and volume fraction in the real material, can be easily generated. It is assumed that the particles can be randomly packed together into a simulation region, by defining artificial interaction forces among them. Such forces may be either adopted from Van der Waals potential energy, or Hooke pair and gravity forces. The proposed method has proved to be fast due to the fact that the algorithm has been implemented on graphical processing units (GPU). Utilizing the Voronoi tessellation method, the set of the generated discrete grains have been altered to space-filling, adjoining polyhedrons with respect to the real geometry. Moreover, as an advantage, the boundary and the interface region of the microstructures were modeled.     

The temperature-dependence of the strength of polycrystalline MgO

The temperature-dependence of the strength of a range of polycrystalline magnesias has been studied. Fracture occurs either by the extension of inherent flaws or is initiated by plastic flow. It has been possible to give a quantitative account of the effects of temperature, grain-size, porosity and surface condition on the strength of polycrystalline magnesia.     

Porous inorganic materials

The field of porous, inorganic materials is experiencing explosive growth, as is shown by more than 6000 literature citations since 1994 along with numerous recent symposia and workshops that have been devoted to this topic. Much of the recent interest has been fueled by new synthetic strategies, such as ‘supramolecular templating’, that have enabled precise engineering of pore size, shape, and connectivity on the mesoscopic scale. In general, template-based approaches involving the cooperative organization of organic—inorganic assemblies as intermediates are emerging as a promising conceptual basis for future developments in the field of porous inorganic materials, such as the synthesis of hierarchical morphologies that mimic the intricate structures found so often in nature.     

Failure modes and effective strength of two-phase materials determined by means of numerical limit analysis

Summary In the most general case, composites composed of two materials, exhibiting a matrix-inclusion morphology, can be described by three phases: the matrix phase, the inclusion phase, and the interface between the inclusion and the matrix. In order to relate effective material properties to the matrix-inclusion behavior and the morphology, i.e., to the arrangement of the inclusions within the matrix phase, analytical and/or numerical schemes may be employed. Regarding effective strength properties, averaging schemes used, e.g., for upscaling elastic and viscoelastic properties are not able to capture the localized mode of material failure and do not provide information about the failure modes within the material. In this paper, the application of limit analysis to two-phase materials subjected to uniaxial/biaxial loading is proposed, giving access to the respective material strength and the corresponding failure modes. Based on a discretized form of limit analysis, different strength properties are assigned to the matrix, the inclusion, and the interface. The solution of the underlying optimization problem arising from the respective upper- and lower-bound formulation is based on second-order-cone-programming (SOCP). The presented upscaling scheme is used to illustrate the finer-scale origin of frequently observed failure and degradation scenaria in matrix-inclusion materials, highlighting the effect of strength properties, morphology, and interface degradation on the effective strength of the composite material. Dedicated to Professor Franz Ziegler on the occasion of his 70th birthday     

Creep in transforming polycrystalline materials

An extension, taking recovery into account, has been made of Poirier's (1982) theory of transformation plasticity, that is, the acceleration of creep due to the introduction of accommodation dislocations in the plastic relaxation of the misfit of domains of new phase formed during phase transformation. This enables more realisticc distinctionsto be made between situations where transformation plasticity may be a significant effect in the Earth and where it may not.     

Thermal stability and strength of polycrystalline nanowires

We considered a polycrystalline cylindrical nanowire with initial radius R₀ composed of identical cylindrical grains of the length L₀, strained uniaxially by an external stress P. At the temperatures at which some surface and grain boundary diffusion are allowed the thinning of the nanowire in the vicinity of grain boundaries occurs due to the phenomenon of grain boundary grooving. We calculated the equilibrium shapes of the nanowire achieved after long annealing times. Our calculations demonstrated that for any given L₀/R₀ ratio some critical value of the applied stress exists above which the nanowire is unstable and breaks down into the string of isolated spherical particles, in full analogy with the Rayleigh instability of long cylinders. The kinetics of the shape change was calculated numerically. It was shown that the rate of thinning of unstable nanowires diverges as the moment of breakdown is approached. We also demonstrated that the breakdown may occur even for nominally stable nanowires “on the way” to achieving their equilibrium shape. Therefore, the stability of nanowire is determined by a combination of geometric (L₀/R₀), thermodynamic (grain boundary energy), and kinetic (ratio of grain boundary and surface diffusivities) parameters. An application of external tensile stress accelerates the breakdown of nanowires.     

On the dynamic plasticity and strength of polycrystalline beryllium

Dynamic deformation and failure behavior of polycrystalline beryllium under uniaxial strain conditions within impactor velocity range of 16–285 m/s are studied by using a light gas gun facility. In the majority of experiments the thicknesses of target and impactor were adjusted to provide spallation. It is found that dynamic failure in tension during spallation is strongly dependent on the plastic instability threshold under compression at the front of compressive pulse.     

Method for determining the elastic modulus at grain boundaries for polycrystalline materials

Abstract

On the basis of the model of non-equilibrium grain boundary segregation induced by tensile stress, a set of kinetic equations is derived to formulate this process. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. In the present paper, based on such a widely approved model, a new approach is proposed to quantify the elastic modulus at grain boundaries for polycrystalline materials. Using the observation of Misra, the grain boundary elastic modulus E _gb = 2.03 × 10⁹ Pa at 883 K for Cr-Mo-V-2.6Ni steel is obtained for the first time. This result shows excellent agreement with the local elastic constants simulated theoretically by Kluge et al., and indicates that the grain boundary elastic modulus for a polycrystalline material is much lower than the commonly assumed value.     

Advanced inorganic materials for hard magnetic media

A review of magnetic materials used in modern hard magnetic media is presented. Advanced materials and technologies, along with conventional ones, are described. The capabilities and prospects of further development of data recording and storage systems are discussed.