Arrangement Of Grain Boundaries Research Articles

A model revealing grain boundary arrangement-dominated fatigue cracking behavior in nanoscale metallic multilayers

Abstract

Investigation on mechanical performances of grain boundaries in hexagonal boron nitride sheets

In this work, molecular dynamic simulations were performed to investigate the effect of grain boundaries (GBs) on the mechanical properties and failure behaviors of hexagonal boron nitride (h-BN) nanosheets. It was confirmed that both the GB linear density and the detailed arrangements of GBs could affect the mechanical properties of h-BN sheets. The tensile tests of GB models were performed at different strain rates. Results showed that the ultimate tensile strength of GBs increased obviously with the increasing of strain rate. The effect of temperature on the mechanical properties of h-BN sheets was also discussed. Results showed that high temperature could reduce the ultimate tensile strength and Young's modulus of models containing high density of GBs. Molecular dynamics (MD) and density functional theory (DFT) calculation results indicated that the fracture behavior of GBs tended to begin at the bond with the lowest electron density. The information obtained in this work could provide evidence for further investigations of mechanical properties and failure mechanism of h-BN sheets with GBs.

Reaction-induced fracturing in a hot pressed calcite-periclase aggregate

The chemo-mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally. Dense calcite-periclase aggregates with <5% porosity and with a calcite to periclase ratio of 90/10 and 95/5 by volume were prepared by hot isostatic pressing. Subsequent hydration experiments were performed in a hydrothermal apparatus at temperatures of 580–610 °C and a pressure of 200 MPa for run durations of 5–60 min. The rate of the periclase to brucite transformation was primarily controlled by the access of fluid. Where fluid was present, the reaction was too fast for the associated positive volume increase of the solids of about 100% to be accommodated by creep of the calcite matrix, and fracturing was induced. The newly formed cracks greatly enhanced the access of fluid leading to a positive feedback between hydration and fracturing. Mostly the newly formed cracks follow pre-existing grain boundaries in the calcite matrix. Comparison of experimental results with numerical 2D discrete element modelling (DEM) of crack formation revealed that the geometry of the crack pattern around a reacting particle depends on the shape of the original periclase particle, on the mechanical strength of the particle-matrix interface and on the mechanical strength and arrangement of grain boundaries in the calcite matrix in the immediate vicinity of the swelling particle.

Energetics of vacancy segregation to symmetric tilt grain boundaries in hexagonal closed pack materials

Molecular static simulations of 190 symmetric tilt grain boundaries in hexagonal closed pack metals were used to understand the energetics of vacancy segregation, which is important for designing stable interfaces in harsh environments. Simulation results show that the local arrangements of grain boundaries and the resulting structural units have a significant influence on the magnitude of vacancy binding energies, and the site-to-site variation within each boundary is substantial. Comparing the vacancy binding energies for each site in different c/a ratio materials shows that the binding energy increases significantly with an increase in c/a ratio. For example, in the [12¯10] tilt axis, Ti and Zr with c/a = 1.5811 have a lower vacancy binding energy than the Mg with c/a = 1.6299. Furthermore, when the grain boundary energies of all 190 boundaries in all three elements are plotted against the vacancy binding energies of the same boundaries, a highly negative correlation (r = −0.7144) is revealed that has a linear fit with a proportionality constant of −25 Å2. This is significant for applications where extreme environmental damage generates lattice defects and grain boundaries act as sinks for both vacancies and interstitial atoms.

Engineering electrodeposited ZnO films and their memristive switching performance

We report the influence of zinc oxide (ZnO) seed layers on the performance of ZnO-based memristive devices fabricated using an electrodeposition approach. The memristive element is based on a sandwich structure using Ag and Pt electrodes. The ZnO seed layer is employed to tune the morphology of the electrodeposited ZnO films in order to increase the grain boundary density as well as construct highly ordered arrangements of grain boundaries. Additionally, the seed layer also assists in optimizing the concentration of oxygen vacancies in the films. The fabricated devices exhibit memristive switching behaviour with symmetrical and asymmetrical hysteresis loops in the absence and presence of ZnO seed layers, respectively. A modest concentration of oxygen vacancy in electrodeposited ZnO films as well as an increase in the ordered arrangement of grain boundaries leads to higher switching ratios in Ag/ZnO/Pt devices.

Microstructure evolution influenced by dislocation density gradients modeled in a reaction–diffusion system

Macroscopic properties of metallic materials are to a large extent dictated by the grain size and the presence and arrangement of grain boundaries. Plastic slip deformation will lead to the formation of dislocation pile-ups at grain boundaries, causing a heterogeneous distribution of dislocation density. This will on the macroscale manifest itself as a grain size dependence of the yield stress. Considering dynamic recrystallization, new grains will nucleate at sites of high stored energy. The existence of such favored nucleation sites is also largely due to a heterogeneous dislocation density distribution. To properly account for processes such as these in microstructure modeling, dislocation density gradients and grain boundary influence need to be considered. In the present contribution, the evolution of dislocation density is viewed as a reaction–diffusion system, involving mobile and immobile dislocations. Gradient effects are introduced by making the immobilization of mobile dislocations sensitive to the distance to grain boundaries. Through simulations in both single grains and polycrystals, it is shown that the present model provides a macroscopic yield stress behavior of Hall–Petch type, without explicitly incorporating a yield stress dependence on the grain size. In addition, the model is employed in a cellular automaton algorithm, allowing a polycrystalline microstructure to evolve due to dynamic recrystallization. It is shown that the introduced gradients provide important additions to recrystallization modeling.

Application of ultrasonic methods to determine elastic anisotropy of polycrystalline copper processed by equal-channel angular pressing

Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and four passes of equal-channel angular pressing (ECAP) is investigated by means of ultrasonic methods. For each material, Young’s and shear moduli in the principal processing directions are evaluated and the symmetry and orientation of the anisotropy are identified. The relation between the determined symmetry and the processing mechanisms is discussed. It is shown that the material after one and two passes of ECAP exhibits a measurable anisotropy, while the material after the fourth pass behaves isotropically. Within the discussion, it is shown that the origin of the observed anisotropy may be attributed to the spatial arrangement of grain boundaries rather than to the crystallographic texture. In the light of this conclusion, the obtained results correspond well with optical and transmission electron microscopy observations of the microstructures of ECAPed materials documented in the literature.

Characterization of the quality of ZnO thin films using reflective second harmonic generation

A polar mirror symmetrical contribution originated from the arrangement of grain boundaries existing in the ZnO film is detected by reflective second harmonic generation pattern. The ordering of ZnO grain boundary is dependent on the kinetic energy of deposited atoms and affects the quality of ZnO films. The net direction of the grain boundary in ZnO film trends toward the [1¯10] direction of Si(111) to reach the minimum grain energy for better quality ZnO film. The polar structure of the mirrorlike boundaries under the optically macroscopic viewpoint presents a correlation with film quality.

Statistics of Grain Boundaries in Polysilicon

A nanometer-scale variation of grain boundary locations in gate polysilicon is investigated in detail based on the assumption that the arrangement of grain boundaries obeys Poisson distribution. The statistics of grain boundaries described here reveals a relation between nanoscopic location and the arrangement of grain boundaries, which implies fluctuation in transistor characteristics of 45-nm and beyond MOSFETs

Critical currents and microstructures of LPE grown YBCO bicrystal films with large single facet grain boundaries

The microstructures and electric transport properties across grain boundaries (GBs) are studied for YBaCuO bicrystal films grown by liquid phase epitaxy (LPE) method. Large single facet GBs with typical size of 10–100 μm are obtained in the films on MgO bicrystal substrates. Microstructures and atomic arrangements of GBs with different misorientation angles ϑ are investigated by TEM. The critical current densities across [001] tilt GBs as a function of ϑ show that J c values are almost constant up to ∼12° and then decrease exponentially, as usually observed for bicrystal films grown by physical vapor deposition methods. The presence of a plateau region up to 12° demonstrates a feature of the clean and meandering free GB structure in the LPE bicrystal films. The temperature and field dependencies of I c for 24° GB are presented and discussed based on the microstructures.

Evolution of microstructure during high temperature low cycle fatigue of high purity aluminiumo ligocrystals

For a variety of metallic materials, conspicuous microstructural changes were observed during elevated temperature cyclic deformation owing to grain growth, grain boundary motion, and reorientation of the grain boundaries under 45° with respect to the stress axis. These phenomena were investigated in high purity aluminium. The evolution of microstructure and microtexture during high temperature low cycle fatigue can be observed very well using oligocrystals (i.e. specimens consisting of less than 15 grains in the deformed area). The orientation of individual grains and the spatial arrangement of grain boundaries can be characterised completely owing to the extremely large grain size. After several deformation steps the movement of the grain boundaries was measured using optical microscopy without additional preparation, therefore, each migration step of the grain boundary and each microtextural change of grains can be analysed.

Evolution of microstructure during high temperature low cycle fatigue of high purity aluminiumo ligocrystals

For a variety of metallic materials, conspicuous microstructural changes were observed during elevated temperature cyclic deformation owing to grain growth, grain boundary motion, and reorientation of the grain boundaries under 45° with respect to the stress axis. These phenomena were investigated in high purity aluminium. The evolution of microstructure and microtexture during high temperature low cycle fatigue can be observed very well using oligocrystals (i.e. specimens consisting of less than 15 grains in the deformed area). The orientation of individual grains and the spatial arrangement of grain boundaries can be characterised completely owing to the extremely large grain size. After several deformation steps the movement of the grain boundaries was measured using optical microscopy without additional preparation, therefore, each migration step of the grain boundary and each microtextural change of grains can be analysed.

Modelling of intergranular damage propagation

The ultimate goal of grain boundary engineering with regard to the intergranular damage propagation is to find a relationship between the resistance to intergranular degradation of a polycrystal and the characteristics of the interface distribution. In this study, a new probabilistic approach for the intergranular damage propagation is proposed, which combines the analysis based on the percolation theory and the consideration of the crack path as a Markov chain. The study was performed by means of computer simulation. The following qualitative conclusions concerning the influence of the microstructural parameters of the grain boundary network on the likelihood of failure of a polycrystalline specimen have been drawn from this study: (1) The analysis of a microstructure's susceptibility to intergranular damage should be based on the determination of the maximum crack length possible in the microstructure rather than on the average crack length. (2) Under certain conditions, the behaviour of cracks can be analyzed within the framework of the percolation model. (3) The resistance to intergranular degradation depends both on the parameters of the grain boundary network and the conditions of crack nucleation. (4) The fraction of crack-resistant boundaries is not the only factor controlling the intergranular damage resistance. The spatial arrangement of grain boundaries, in particular the triple junction distribution, is important as well. (5) Crystallographic texture and the development of annealing twinning may improve resistance to intergranular damage in materials prone to annealing twinning.

Grain boundary ensembles in polycrystals

Topological characteristics of interface aggregates are considered and experimental results on the connectivity of grain boundary networks in materials prone to annealing twinning are presented. The impact of spatial arrangement of grain boundaries on material resistance to intergranular degradation is discussed. This consideration shows the importance of studying not only the grain boundary character distribution, but also the grain boundary network arrangement for the development of the grain boundary engineering approach.