Influence of grain boundary on melting

The temperature behaviour of an Al bicrystal with surfaces consisting of (110) and (111) crystals is simulated using molecular dynamics. The result shows that the (110) crystal losses its crystalline order at 820K, whereas the disorder does not propagate through the (111) crystal at this temperature. Instead, some disordered atoms are recrystallized into the (111) crystal and the initial grain boundary changes into a stable order-disorder interface. Thus, it was discovered that at a temperature near its melting point, the (111) crystal grew and obstructed the propagation of disorder. Such an obstruction is helpful for understanding melting.

The Influence of Grain Size and Temperature on the mechanical Deformation of Nanocrystalline Materials: Molecular Dynamics Simulation

Nanocrystalline (nc) materials are characterized by a typical grain size of 1-100nm. The uniaxial tensile deformation of computer-generated nc samples, with several average grain sizes ranging from 5.38 to 1.79nm, is simulated by using molecular dynamics with the Finnis-Sinclair potential. The influence of grain size and temperature on the mechanical deformation is studied in this paper. The simulated nc samples show a reverse Hall-Petch effect. Grain boundary sliding and motion, as well as grain rotation are mainly responsible for the plastic deformation. At low temperatures, partial dislocation activities play a minor role during the deformation. This role begins to occur at the strain of 5%, and is progressively remarkable with increasing average grain size. However, at elevated temperatures no dislocation activity is detected, and the diffusion of grain boundaries may come into play.

Dislocations And Twins In Nanocrystalline Ni After Severe Plastic Deformation: The Effects Of Grain Size

Deformation microstructures have been investigated in nanocrystalline (nc) Ni with grain sizes in the 50-100 nm range. It was found that deformation twinning started to occur in grains of similar to 90 nm, and its propensity increased with decreasing grain size. In most of the nc grains dislocations were observed as well, in the form of individual dislocations and dipoles. It is concluded that dislocation-mediated plasticity dominates for grain sizes in the upper half, i.e. 50-100 nm, of the nanocrystalline regime. (C) 2007 Published by Elsevier B.V.

A finite deformation analysis of polycrystal by considering the influence of grain boundary

By combining grain boundary (GB) and its influence zone, a micromechanic model for polycrystal is established for considering the influence of GB. By using the crystal plasticity theory and the finite element method for finite deformation, numerical simulation is carried out by the model. Calculated results display the microscopic characteristic of deformation fields of grains and are in qualitative agreement with experimental results.

Mechanical modeling of grain boundary sliding of polycrystals

Using a variational method, a general three-dimensional solution to the problem of a sliding spherical inclusion embedded in an infinite anisotropic medium is presented in this paper. The inclusion itself is also a general anisotropic elastic medium. The interface is treated as a thin interface layer with interphase anisotropic properties. The displacements in the matrix and the inclusion are expressed as polynomial series of the cartesian coordinate components. Using the virtual work principle, a set of linear algebraic equations about unknown coefficients are obtained. Then the general sliding spherical inclusion problem is accurately solved. Based on this solution, a self-consistent method for sliding polycrystals is proposed. Combining this with a two-dimensional model of an aggregate polycrystal, a systematic analysis of the mechanical behaviour of sliding polycrystals is given in detail. Numerical results are given to show the significant effect of grain boundary sliding on the overall mechanical properties of aggregate polycrystals.

Effect of grain size on slow fatigue crack propagation and plastic deformation near crack tip

Fatigue crack growth and its threshold are investigated at a stress ratio of 0.5 for the three-point bend specimen made of Austenitic stainless steel. The effect of grain size on the crack tip plastic deformation is investigated. The results show that the threshold value Δkth increases linearly with the square root of grain size d and the growth rate is slower for materials with larger grain size. The plastic zone size and ratio for different grain sizes are different at the threshold. The maximum stress intensity factor is kmax and σys is the yield strength. At the same time, the characteristics of the plastic deformation development is discontinuous and anti-symmetric as the growth rate is increased from 2·10—8 to 10−7 mm/cycle.

Dislocations and twins in nanocrystalline Ni after severe plastic deformation: the effects of grain size

Deformation microstructures have been investigated in nanocrystalline (nc) Ni with grain sizes in the 50-100 nm range. It was found that deformation twinning started to occur in grains of similar to 90 nm, and its propensity increased with decreasing grain size. In most of the nc grains dislocations were observed as well, in the form of individual dislocations and dipoles. It is concluded that dislocation-mediated plasticity dominates for grain sizes in the upper half, i.e. 50-100 nm, of the nanocrystalline regime. (C) 2007 Published by Elsevier B.V.

The Effects of Grain Size Distribution and Structure on Mechanical Behavior of Silt

Abstract: In order to investigate the effects of the grain size distribution and the micro-structure of soils on the mechani- cal characteristics, some static triaxial compression tests were carried out, and then the relationship of stress-strain and the strength behavior of silty sand were compared among undisturbed samples with different grain size distribution, undis- turbed and remolded samples with the same grain size distribution, and reconstituted samples (or called mixed samples) with different grain size distribution. The effects of grain size distribution and structure on the mechanic behavior of silty sands were mainly analyzed. It is shown that the obvious differences of the mechanical characteristics between undis- turbed soils and remolded soils are caused by the differences of soil structures. Although the grain size distribution are different between two soil samples, their mechanical characteristics may be close to each other, or may have obvious differences because of the effects of micro-structure.

Effects of grain size on the mosaic tilt and twist in InN films grown on GaN by metal-organic chemical vapor deposition

The structural property of InN films grown on Ga-face GaN layers by metal-organic chemical vapor deposition has been studied by high-resolution x-ray diffraction. The mosaic tilt and twist are found to be strongly dependent on the surface lateral grain size. The twist decreases with increasing grain size and finally approaches to a constant level. On the other hand, the mosaic tilt increases substantially when the grain size becomes large enough and exceeds the width of step terraces on the GaN surface, showing an important mechanism for the defect generation in the InN/GaN system with large out-of-plane lattice mismatch. (c) 2006 American Institute of Physics.

Storage of photoexcited electron-hole pairs in an AlAs/GaAs heterostructure created by electron transfer in real and kappa spaces

The storage of photoexcited electron-hole pairs is experimentally carried out and theoretically realized by transferring electrons in both real and k spaces through resonant Gamma - X in an AlAs/GaAs heterostructure. This is proven by the peculiar capacitance jump and hysteresis in the measured capacitance-voltage curves. Our structure may be used as a photonic memory cell with a long storage time and a fast retrieval of photons as well.

Synthesis and application on parallel image storage of bisfurylfulgide

A bisfurylfulgide, E, E-3,4-bis[1-(2,5-dimethyl-3-furyl)ethylidene]-3,4-dihydrofuran-2,5-dione, is synthesized by Stobbe condensation reaction. The molecular structure of target compound is confirmed by single crystal X-ray crystallography analysis. It shows that the distances between two possible reaction sites of molecule are 0.3394 and 0.3406 nm respectively, which is favorable to photocyclization. The photochromic properties of this compound in different solvents are investigated, and the result shows that the compound exhibits excellent photochromic behavior. The primary result of applied research on parallel image storage is also presented.