Appearance Of Slip Research Articles

Low temperature in-situ micro-compression testing of iron pillars

An in-situ nanomechanical cooling system has been developed to study the temperature dependency of local mechanical properties and slip behavior of bcc α-Fe. Uniaxial compression tests with Focused Ion Beam (FIB) fabricated pillars with a diameter of 1µm, were performed in the single slip orientations 2¯35 and 1¯49 at room temperature and 198K. The testing was conducted inside a Scanning Electron Microscope (SEM) equipped with a nanoindenter. Slip trace analyses revealed occurrence of slip in the {112}<111> family of slip systems for 2¯35 pillars at both room temperature and 198K while the predominantly slip systems governing the deformation on 1¯49 pillars were {110}<111> for both test temperatures. The stress-strain response showed an increased strength with decreasing temperature for the 2¯35 pillars, in contrast to 1¯49 pillars, where only a weak temperature dependence is observed. Furthermore, for 2¯35 pillars, the appearance of slip is less prominent at 198K, indicating that the temperature strongly influences the relative motion of screw and edge dislocations. Molecular Dynamics (MD) simulations performed at 15K and 300K, was used to study dislocation mechanisms for the two orientations. 1¯49 pillars exhibit a change in deformation mechanisms at low temperature and the evolution of dislocation density during deformation, display distinct differences for the two loading orientations.

Yielding the yield-stress analysis: a study focused on the effects of elasticity on the settling of a single spherical particle in simple yield-stress fluids.

The sedimentation of a single particle in materials that exhibit simultaneously elastic, viscous and plastic behavior is examined in an effort to explain phenomena that contradict the nature of purely yield-stress materials. Such phenomena include the loss of the fore-and-aft symmetry with respect to an isolated settling particle under creeping flow conditions and the appearance of the "negative wake" behind it. Despite the fact that similar observations have been reported in studies involving viscoelastic fluids, researchers conjectured that thixotropy is responsible for these phenomena, as the aging of yield-stress materials is another common feature. By means of transient calculations, we study the effect of elasticity on both the fluidized and the solid phase. The latter is considered to behave as an ideal Hookean solid. The material properties of the model are determined under the isotropic kinematic hardening framework via Large Amplitude Oscillatory Shear (LAOS) measurements. In this way, we are able to predict accurately the unusual phenomena observed in experiments with simple yield-stress materials, irrespective of the appearance of slip on the particle surface. Viscoelasticity favors the formation of intense shear and extensional stresses downstream of the particle, significantly changing the entrapment mechanism in comparison to that observed in viscoplastic fluids. Therefore, the critical conditions under which the entrapment of the particle occurs deviate from the well-known criterion established theoretically by Beris et al. (1985) and verified experimentally by Tabuteau et al. (2007) for similar materials under conditions that elastic effects are negligible. Our predictions are in quantitative agreement with published experimental results by Holenberg et al. (2012) on the loss of the fore-aft symmetry and the formation of the negative wake in Carbopol with well-characterized rheology. Additionally, we propose simple expressions for the Stokes drag coefficient, as a function of the gravity number, Yg (related to the Bingham number), for different levels of elasticity and for its critical value, under which entrapment of particles occurs. These criteria are in agreement with the results found in the recent work by Ahonguio et al. (2014). Finally, we propose a method to quantify experimentally the elastic effects in viscoplastic particulate systems.

Microfluidics and rheology of carbon black suspensions for In-Mold Coating applications: some insights into the slip flow phenomena

Microfluidics and rheology are critical to modeling material processes such as In-Mold Coating (IMC). In the IMC process, a carbon black suspension is injected onto the surface of the molded part while the part is still in the mold. Due to the microscopic length scale of the IMC flow (10–25 μm), a study of the slip flow and rheological properties at high shear rates of the coating liquid becomes significant. A customized microslit rheometer was developed and used to measure the viscosity of a coating material being considered for IMC in the channel gaps between 25 and 100 μm (C. Aramphongphun Ph.D. Dissertation, The Ohio State University, 2006). The results were interpreted assuming true slip at the wall. In this paper, we present further insights into the slip flow phenomena. It is assumed that there is a fluid layer of lower viscosity near the wall, which accounts for the appearance of slip. The previously obtained data is interpreted using this assumption.

Stokes drag and lubrication flows: A molecular dynamics study.

We have studied the translational and rotational motion of a sphere in a viscous Lennard-Jones liquid using molecular dynamics simulations. The drag and torque on a sphere in an effectively unbounded fluid are found to agree with continuum hydrodynamics results even when the size of the sphere is comparable to that of the fluid molecules. The diffusivity of a spherical tracer particle is in accord with the Stokes-Einstein relation, and the corresponding Brownian motion is determined by its interaction with the layers formed by fluid molecules around it. When a sphere moves near a solid wall, we find that the drag and torque agree with lubrication theory down to molecular scales, but the predicted divergence is regularized at very short distances due to depletion of fluid molecules near the wall and the appearance of slip at high shear stress.

Defect Structure in Te‐doped GaAs Single Crystals after Plastic Deformation (II). Dislocation Slip and Cell Formation

AbstractAt temperatures above the brittle‐to‐ductile transition (490 °C) in Te‐doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈550 °C) and (iii) dislocation cells (580 … 590 °C). In Part II quantitative details of the appearance of slip and cell formation are given. Leading segments of gliding half loops are mainly of 60° type. Cell walls were formed by multiple slip of perfect dislocations.

Serrated yielding in Al-Zn alloys

Serrated yielding was observed in quenched Al single crystals and quenched dilute Al-Zn single crystals, which had been deformed by tension at room temperature.The characteristics of serrated yielding of these crystals were investigated by its dependence on quenching temperature, tempering temperature, and strain rate, and appearance of slip lines.The principal results obtained were as follows:(1) The serrated yielding was observed only in single crystals containing Frank-sessile dislocation loops as secondary lattice defects.(2) The dependence of the serrated yielding on quenching and tempering temperatures would by attributed to sizes and densities of the Frank-sessile dislocation loops.(3) The dependence of the yielding on strain rate would be attributed to the interaction between moving dislocations and Frank-sessile dislocation loops.(4) It would be concluded from these experiments that the serrated yielding may be controlled by superjogs formed by the interaction between the moving dislocations and the Frank-sessile dislocation loops.

First Stage of Work Hardening in Aluminium Single Crystals

PREVIOUS work1 has indicated that the only metallographic features appearing during stage I of the deformation of single crystals of face-centred cubic metals (the ‘easy glide’ region) is slip on the (111) plane of maximum resolved shear stress (laminar flow2). The transition to stage II has been related to the appearance of slip on secondary (111) planes (turbulent flow2). However, it is known1 that the rate of hardening during stage I is dependent on the orientation of the axis of the specimen, particularly with respect to the [100]–[111] boundary of the basic stereographic triangle. This has led to the theoretical conclusion3 that some secondary slip must occur during stage I. This communication provides experimental confirmation.