Displacement Of Grains Research Articles

A quantitative hot tearing criterion for aluminum alloys

In this study, a new hot tearing criterion was proposed to quantitatively predict open and segregated hot tears in aluminum alloys. In the suggested model, the displacement of grains at a grain boundary near the solidus temperature was considered as the main reason for crack formation. In this model, it is assumed that when the distance between two neighbor grains exceeds the distance between those grains at the coherency temperature, the enriched liquid can penetrate to the root of the dendrites and form segregated and open hot tears. The main parameters of the model are microstructure (grain size and number of grain boundaries), critical strain, applied strain, and width of generated cracks. The index was verified by the solidifying shell tensile (SST) test of an Al alloy containing 0.95 wt% Cu and 0.12 wt% Fe. Also, the critical strain for crack formation in the test was calculated by using accumulated strain theory and load-time curves. The results revealed that the proposed model can suitably predict the number of open and segregated hot tears generated in the SST test samples.

Numerical Investigation of Funicular Liquid Bridges between Three Spherical Grains in a Bidisperse Particulate System.

Appropriate capillary effects are beneficial for controlling the wet powder performance and agglomerate formation. As water content rises, the funicular regime supplants the pendular regime as the predominant state in wet granular media. The displacement of grains leading to the stretching of funicular liquid bridges until rupture is an interesting and common phenomenon. Utilizing Surface Evolver software (an energy minimization approach), this work develops an efficient and accurate numerical model to describe liquid interactions among three spherical grains. The effects of liquid volume, contact angle, grain size ratio, grain-pair gap, and separation distance on the capillary forces and rupture distances are investigated. Notably, we present a modified closed-form equation for predicting the rupture distance of funicular bridges between three grains, which reflects the coupled effects of the contact angle, grain size, and liquid volume on rupture distance. This present study provides insights for incorporating capillary effects into mechanical models relying on microassembly composed of several grains in bidisperse particulate systems. Additionally, the numerical findings confirm some findings regarding the splitting of funicular bridges.

A new OSL dose model to account for post-depositional mixing of sediments

In applications of optically stimulated luminescence (OSL) dating to unconsolidated sediments, the burial age of a sample of grains is estimated using statistical models of the distribution of the experimentally determined equivalent doses of the grains, together with estimates of the environmental dose rate. For grains that have been vertically mixed after deposition (e.g., due to bioturbation), existing dose models may fail to appropriately account for the complexity of the mixing process, thus producing inaccurate age estimates of the original time of deposition of the ‘native’ grains in any particular sample (usually the quantity of most interest). Here we introduce a new dose model, the asymmetric Laplacian mixture model (ALMM), developed for vertically mixed samples with single-grain dose distributions. The approach is based on a continuous statistical mixture that models the displacement of grains in both upward and downward directions. The central dose of the native grains in each sediment sample is estimated by the ALMM, as well as the parameters associated with overdispersion of single-grain dose distributions and the (modelled) mixing process. Using Bayesian methodology, we apply the model to two series of vertically contiguous samples collected at the site of Nawarla Gabarnmang in northern Australia. Independent age estimates obtained from radiocarbon dating of charcoal fragments support the OSL ages for the native grains estimated by the ALMM. Moreover, our study includes sensitivity analyses that show the model is robust to variation in the experimental error of the OSL data, as well as a simulation study that demonstrates the model’s good ability to recover the simulated central dose and its excellent coverage properties. The ALMM is introduced in the context of compound Gaussian distributions, a broadly encompassing statistical framework that includes many of the most commonly used dose models. This unifying and accessible perspective on the statistical modelling of dose distributions will support practitioners in selecting an appropriate model for samples affected by post-depositional mixing, and hopefully stimulate further theoretical developments. A new R package rstanosl is provided that fits the ALMM and other commonly used dose models using Hamiltonian Monte Carlo methods via the Stan programming language.

Robust experimental study of avalanche precursory events based on reproducible cycles of grain packing destabilizations

Quasi-periodic collective displacements of grains at the free surface of a tilted grain packing constitute precursors of granular avalanches. Laboratory experiments are commonly performed by slowly tilting the packing from 0° to the maximal stability angle θA. In these conditions, the number of precursors is too small to assess reproducible and robust statistical analyses of the precursor activity. To go beyond this limitation, we have developed a specific experimental protocol consisting of tilting the packing with successive oscillation cycles. We use a high-resolution optical camera and process the images of the packing free surface to identify precursory events during many consecutive cycles of a single packing. We observe the same behavior for all half-cycles, forth and back: appearance of the first precursors after the same variation of inclination, exponential evolution of the weak surface activity for the first precursors and linear growth of stronger surface activity for the following ones. The experimental protocol provides both reproducible precursor measurements based on large sample statistical inferences and a quasi-stationary state after one full-cycle. This approach is very promising for highlighting the effects of external parameters, including humidity and packing geometry.

Dynamic behavior of humid granular avalanches: Optical measurements to characterize the precursor activity.

Laboratory study of slope stability of granular media remains a challenge for modeling, understanding, and predicting natural hazards, such as avalanches and landslides, precursory signs of which are controlled by numerous physical parameters. The present work focuses on the impact of the humidity, in the range of 40-90%, on the stability of monodisperse dense packings of spherical beads. The beads are in a transparent box that is slowly and continuously tilted and allows simultaneous top and lateral optical measurements of global displacements of grains at the surface, defined as precursors. Humidity increases the cohesion between the grains. By performing successive avalanches that destabilize deeper granular layers, we assess the role of the exposure time to the high humidity rates in the diffusion process to reach the hygroscopic equilibrium inside the packing. We highlight an increase of the stability and first precursor angles, associated to a constant angle increment between two consecutive precursors, with a dependency with both the diameter (0.2,0.5, and 0.75mm) and the material (glass and polystyrene) of the grains.

Hysteresis of the drag force of an intruder moving into a granular medium.

We numerically investigate the force-displacement relation of a moving intruder initially at rest into a granular medium. Our model granular medium is composed of one layer of coplanar polydisperse spheres subjected to a gravity field. The interactions between the grains are modelled by Hertzian contacts to which a viscous damping is applied. Moving it horizontally and with alternating positive and negative velocity, we recover a hysteresis of the force-displacement curve. Considering that the flow is plastic as the yield strength has been reached, we describe the transient part of the flow around the intruder. We show that the drag stress increases as its distance to an ultimate drag stress [Formula: see text] with a typical deformation [Formula: see text]: the drag stress-strains curve appears to exponentially decay as it saturates to this ultimate drag stress. This protocol of deformation highlights that the deformation of the grains is negligible compared to the deformation of the packing, i.e. related to the irreversible displacements of grains allowing the intruder to pass through. Simultaneously, the lift force is constant on average during the displacement of the intruder. We then give the different scaling laws of the yield strength, this ultimate drag stress, the characteristic deformation of the packing and the lift stress. Finally, we recover the complete hysteresis cycle of the drag force around the intruder.

Acoustic emission avalanches during compression of granular manganites

We have studied acoustic emission (AE) during compression of La0.8Ca0.28Sr0.08MnO2.7 manganites with different microstructures obtained by selected synthesis techniques. In ceramic samples with large grains obtained by a solid-state method, avalanche criticality is confirmed when grain fracture is the AE dominant mechanism. In samples synthesized by means of micro-wave and sol-gel techniques, grains are much smaller and the AE is mainly originated from friction effects associated with relative displacement of grains during deformation. In this case, significant deviations from avalanche criticality have been detected.

THE CHARACTERISTICS OF ORE MINERALIZATION IN THE WEISSWASSER COPPER DISTRICT, GERMANY

The research deals with drill core samples collected during exploration of copper in the Weisswasser area in Saxony. The area is the north-western part of the North-Sudetic Basin, the lithostratigraphic section of which is similar to the typical one known from the Lubin district. As a result of mineralogical observations using reflected light microscopy and scanning electron microscopy, the following minerals were identified: chalcopyrite, chalcocite, bornite, digenite, galena, sphalerite, native silver, Ag-amalgams, mckinstyite, anilite and betechtinite. Detailed characteristics of the occurrence of sulphide mineralization is presented, describing its form, mutual inclusions and paragenesis, relationship to microtectonic conditions, as well as implications of morphological changes on the bottom of the Zechstein. Three forms of mineralization were found: (1) horizontal, mid-layered copper sulphide lenses formed by hydraulic peeling along the sludge lamination surfaces caused by mineralized high-pressure solutions, (2) horizontal copper sulphide lenses replacing previous framboidal pyrite lenses, and (3) replacement of bioclasts. Copper sulphides were concentrated at the expense of sulphur deposited during sedimentation and early diagenesis as bacterial pyrite. Ore mineralization occurs in the thin horizon with an extensive microtectonic net. Two tectonic episodes can be distinguished: Mesozoic tectonic events that enabled seismic pumping of hot fluids, and microfractures that developed during ore minerals precipitation. Futhermore, single sharp-edged grains of chalcopyrite disturbing lamination of copper shale are observed. It indicates the displacement of grains from crushed conglomerate and resedimentation together with copper shale.

Discrete element simulation on vortex structure in granular materials subject to biaxial compression

There is the fluctuation of the residual displacement field, which is the affine displacement subtracted from the displacement of grains. In this study, the role of inclusions in granular materials in the fluctuation is investigated under biaxial compression by the discrete element method. Local particle groups with lower/higher stiffness are used to represent the inclusions. The distributions of the fluctuation and strain are presented under loading stage. Numerical results show that the fluctuation of the displacement exhibits different structure according to different inclusion or assembly with different packing. In addition, by comparing the distribution of displacement and strains, it is indicated that whether the fluctuation will present vortex structure depends on not only the non-uniform of strain distribution but also the heterogeneity of granular assembly itself.

Simulation of competitive grain growth upon the directional solidification of a Ni-base superalloy

Using the numerical simulation in the CA-FE module of the ProCAST simulation program, a systematic investigation of competitive growth of grains in a thin plate has been performed over a wide range of values of the temperature gradient and solidification rate. It has been established that the result of the simulation in the case of converging grains depends on the only parameter, i.e., the ratio of the value of the overgrowth of the grain with the preferred orientation to the size of the cell of the computational grid. Thus, the size of the cell is an important adjusting parameter of the model and must be coordinated with the parameters of the dendritic structure under given growth conditions. The grain with the preferred orientation always displaces neighboring diverging grains. The converging grains are eliminated if their deviation from the vector of the temperature gradient exceeds 20°. At the smaller angles of deviation, the result of the competitive growth depends on the size of the computational cell and varies from their joint growth (at the cell size of 5 μm) to the displacement of grains with the preferred orientation (at the cell size of 20 μm). However, all of the results of the simulation agree with the experimental data available in the literature. For the efficient selection of grains with the preferred orientation, regimes with a low temperature gradient and high growth rate are favorable.

Dynamics of grain boundaries under applied mechanical stress

Recent results of experimental research into stress-induced grain boundary migration in aluminum bicrystals are reported. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for planar symmetrical 〈100〉 tilt boundaries. This coupling proved to be the typical migration mode of any 〈100〉 tilt boundary, no matter whether low- or high-angle, low Σ CSL coincidence or non-coincidence boundary. The measured ratios of normal boundary motion to the tangential displacement of grains are in an excellent agreement with theoretical predictions. The migration-shear coupling is also observed for asymmetrical 〈100〉 boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress-induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Stress-driven migration of symmetrical 〈1 0 0〉 tilt grain boundaries in Al bicrystals

Stress-induced migration of planar grain boundaries in aluminum bicrystals was measured for both low- and high-angle symmetrical 〈1 0 0〉 tilt grain boundaries across the entire misorientation range (0–90°). Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains. Boundaries with misorientations smaller than 31° and larger than 36° moved in opposite directions under the same applied external stress. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with theoretical predictions. The coupled boundary motion was measured in the temperature range between 280 and 400 °C, and the corresponding activation parameters were determined. The results revealed that for mechanically induced grain-boundary motion there is a misorientation dependence of migration activation parameters. The obtained results are discussed with respect of the mechanism of grain-boundary motion.

Mechanically Driven Migration of <100> Tilt Grain Boundaries in Al-Bicrystals

The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical <100> tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.

Diffusion model of internal friction in nanocrystalline materials

The stress normal and tangenital components arise in grain-boundary segments differently oriented with respect to an external periodic load, causing the fluxes of vacancies and impurity atoms between neighboring segments. By solving the diffusion problem, one can find the velocity of mutual displacement of grains, the stress distribution in the segments with allowance for stress adjustment, and the amount of internal friction. The frequency dependence of the internal friction shows peaks associated with the redistribution of impurity atoms over the segments, grain sliding, and a high-temperature background.

Effective temperature and jamming transition in dense, gently sheared granular assemblies

We present extensive computational results for the effective temperature, defined by the fluctuation-dissipation relation between the mean square displacement and the average displacement of grains, under the action of a weak, external perturbation, of a sheared, bi-disperse granular packing of compressible spheres. We study the dependence of this parameter on the shear rate and volume fractions, the type of particle and the observable in the fluctuation-dissipation relation. We find the same temperature for different tracer particles in the system. The temperature becomes independent on the shear rate for slow enough shear suggesting that it is the effective temperature of the jammed packing. However, we also show that the agreement of the effective temperature for different observables is only approximate, for very long times, suggesting that this definition may not capture the full thermodynamics of the system. On the other hand, we find good agreement between the dynamical effective temperature and a compactivity calculated assuming that all jammed states are equiprobable. Therefore, this definition of temperature may capture an instance of the ergodic hypothesis for granular materials as proposed by theoretical formalisms for jamming. Finally, our simulations indicate that the average shear stress and apparent shear viscosity follow the usual relation with the shear rate for complex fluids. Our results show that the application of shear induces jamming in packings whose particles interact by tangential forces.

Measurements of grain motion in a dense, three-dimensional granular fluid.

We have used an NMR technique to measure the short-time, three-dimensional displacement of grains in a system of mustard seeds vibrated vertically at 15 g. The technique averages over a time interval in which the grains move ballistically, giving a direct measurement of the granular temperature profile. The dense, lower portion of the sample is well described by a recent hydrodynamic theory for inelastic hard spheres. Near the free upper surface the mean free path is longer than the particle diameter and the hydrodynamic description fails.

Modification of thin gold films with a scanning force microscope

Thin gold films grown on mica were investigated with the scanning force microscope. By rising the set-point force of the microscope working in the constant-force mode holes and grooves were created in the films. Two basic mechanisms of thin film modification were observed: cutting of gold grains, and displacement of grains on the mica substrate as a whole. Small grains were moved on the mica substrate with forces of only a few nN. Bigger grains could be crushed or cut by the tip when forces in the range of μN were applied. Using this technique lines could be written in the films.

Critical currents of composite superconductors: Model calculations.

We calculate the I-V characteristics of composite superconductors modeled as random three-dimensional arrays of resistively shunted Josephson junctions at zero temperature, in zero and finite applied magnetic fields. Two types of disorder are considered: site dilution of a simple cubic lattice to a concentration p and a random displacement of grains in an ordered lattice. In the site-diluted lattice, we find that there exists a critical current density ${\mathit{J}}_{\mathit{c}}$(p), below which the voltage becomes very small, and which varies as a power law in (p-${\mathit{p}}_{\mathit{c}}$), ${\mathit{J}}_{\mathit{c}}$(p)\ensuremath{\propto}(p-${\mathit{p}}_{\mathit{c}}$${)}^{\mathit{v}}$ with v\ensuremath{\approxeq}1.7, in agreement with previous static estimates and in qualitative agreement with experimental studies on Ag/${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ composites. Near the percolation threshold ${\mathit{p}}_{\mathit{c}}$, we find a scaling relation between the current density J and the electric field E, analogous to that proposed by Fisher et al. near the vortex-glass transition in superconductors with microscopic disorder. In the presence of an applied magnetic field, we find numerically that the critical current initially decreases with field, then reaches a plateau. We present qualitative arguments to estimate both the crossover field ${\mathit{H}}_{\mathit{X}}$ at which the plateau is approached, and the value of the critical current density on the plateau. Possible connections between these results and the large plateau fields and critical currents observed by Sato et al. [IEEE Trans. Magn. MAG-27, 1231 (1991)] are discussed.

A technique for the measurement of diffusion creep from marker line displacements

A procedure is presented which allows the determination of the relative displacement of grains across a common grain boundary during diffusional creep deformation. This relative displacement comprises the strain produced by accretion of material at the grain boundary and by grain-boundary sliding. The only measurements necessary are of marker line displacements across the grain boundary.

Grain boundary sliding and diffusion creep

The theory of diffusion creep is reviewed. It is shown that if a polycrystalline material is to deform in a coherent manner without the development of voids, grain boundary sliding is necessary as well as relative motion normal to the boundaries by diffusion. Coherent deformation requires that the ratio of the sliding velocity and the normal velocity due to diffusion of a pair of adjacent grains is fixed by the geometry of the grains and the stress system. Current theories of the atomic mechanisms of sliding and of the normal displacement of grains by diffusion involve the climb and glide of grain boundary dislocations. It is shown that in their simple form these theories cannot satisfy the coherency requirements. Modification of these theories is discuss the motion of sets of dislocations in the boundary with at least three different non-coplanar Burgers vectors is required to maintain coherency.