Influence of a reduced mobility of triple points on grain growth in two dimensions

A vertex dynamics model is applied to the two-dimensional simulation of grain growth including a limited mobility of triple points. It is shown that a recent experiment on a triple node is well reproduced by the proposed model. This model is then applied to the grain growth of polycrystals to investigate the possible consequences of a reduced mobility of triple nodes. It is shown how this effect can be detected through a careful examination of the grain growth kinetics and grain size distribution function.     

Growth and Characterization of Multiphased Mixed Crystals of NaBr and KBr

Mixed crystals of NaBr and KBr were prepared from melt and physically characterized. Bulk composition of the crystals was estimated using the measured density values. X-ray diffraction analysis indicates the existence of two phases in the mixed crystals. Thermal parameters determined from the X-ray powder diffraction data are found to vary nonlinearly with bulk composition. DC and AC electrical measurements were carried out at various temperatures. The results indicate that the electrical parameters increase with the increase in temperature and vary nonlinearly with the bulk composition. Activation energies and mean jump frequencies were also determined.     

Symmetric and Asymmetric Rolling Pure Copper Foil: Crystal Plasticity Finite Element Simulation and Experiments

A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method(CPFEM),and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling(ASR) is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling(SR). For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.     

Dislocation structures in cyclically deformed nickel polycrystals

The effect of the grain orientation and the plastic strain amplitude _pa on the saturated dislocation structure was studied on individual grains of cyclically deformed nickel polycrystals by means of scanning electron microscopy using the electron back scattering pattern technique and the channelling contrast of back scattered electrons. The main features of the dislocation configuration in a grain were found to be essentially determined by the crystallographic axial orientation of the grain. A labyrinth-like dislocation pattern is typical for grains with axial orientations near [001], a patch pattern exists in grains with a loading axis (LA) near [011] and fragmented dislocation walls are dominant in grains with LA near [ 11]. Grains with axial orientations in the central part of the stereographic standard triangle contain a bundle arrangement of dislocation structures. All four types of dislocation structures, but mostly the bundle type, can occur together with the ladder structure of persistent slip bands. Cell patterns were found to be a result of a modification of the bundle and patch configuration at high deformation amplitudes. The mesoscopic dimensions of the dislocation patterns turned out to depend on _pa in the same way for all grain orientations: while the thickness of regions with high dislocation density is reduced with increasing _pa, the width of regions with low dislocation density remains roughly constant.     

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.     

Simulation of crystal plasticity under dynamic loading

Phenomenological constitutive equations of relaxation type have been constructed and applied to simulate plastic deformation of heterogeneous media. Both the dislocation kinetics and the viscous model with function of relaxation times were used to calculate the plastic strain rate. The deformation at the meso scale level of polycrystals subjected to dynamic loading (including shock waves) has been numerically investigated. The results obtained are reported and discussed.     

Strengthening mechanism in micro-polycrystals with penetrable grain boundaries by discrete dislocation dynamics simulation and Hall–Petch effect

The grain-size effect on the yield stress and the flow strength in micro-polycrystals relates closely to the penetrability of grain boundary (GB) to dislocations. To simulate the dislocation transmission across grain boundary, a dislocation–grain boundary penetration model is proposed and then integrated into the two-dimensional discrete dislocation dynamics (DDD) framework by Giessen and Needleman (1995). By this extended DDD technology, the Hall–Petch effect in micro-polycrystals and the strengthening mechanism are computationally studied, with the main focus on the significant influence of the dislocation transmission across grain boundary that is not fully considered formerly. Results indicate that the Hall–Petch type relation is still applicable, but depends strongly on the GB-penetrability to dislocations, especially for the flow strength at large offset strains. The fitting values of Hall–Petch grain-size sensitive exponents n for initial yield stress and flow stress basically agree with experimentally measured data in published literatures.     

Interplay of martensitic phase transformation and plastic slip in polycrystals

We study the interplay between martensitic phase transformation and plastic slip in polycrystalline media. The work is motivated by the phenomenon of superelasticity – the ability of the material to recover strains beyond their apparent elastic limit – observed in shape-memory alloys. Often the recovery is not perfect with residual strain after a deformation and recovery cycle, and the stress–strain curve changes with cycling. We develop a mesoscale model at the single crystal level, and use it to study polycrystals. The model is able to reproduce various observations and provide important insight into the interplay. In particular, we show that transformation and plasticity can occur synergistically, with plasticity providing a mechanism for bridging across poorly oriented and thus non-transforming grains.     

Electrical activity of grain boundaries in silicon bicrystals and its modification by hydrogen plasma treatment

Electrical activity of grain boundaries (GBs) and its transformation under the influence of low-energy hydrogen plasma treatment in p-type silicon bicrystalline samples cut from EFG silicon polycrystals were investigated. Comprehensive studies have enabled one to investigate the electrical activity of GBs relative to the minority (MIC) and majority (MAC) carriers and to demonstrate the possibility of controlling this activity by different processing methods. These studies allowed for establishing the correlation between the type, structure and individual electrical activity of GBs and also thermal pre-history of samples. Among the tested modes, hydrogenation was found to be the most radical method of electrical activity modification for all types of GBs. In the process, results on hydrogenation of GBs in EFG silicon crystals depend essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects.