Slip planes interaction

When crystals yield, dislocations move through them. Most crystals have several slip planes the f.c.c. structure, which slips on 111) planes (Chapter 5), has four, for example. Dislocations on these intersecting planes interact, and obstruct each other, and accumulate in the material. 

The qualitatively new feature is that f(app) does not increase continually but attains a plateau, which is lower for higher values of c /. The physical reason is that, with increasing y, the electric field in the double layer also rises and hence the part of the double layer that is immobilized. If translated in terms of the familiar slip picture, one could say that the slip plane moves outward if y Increases. In fig. 4.12 the nature of the surface (hydrophilic or hydrophobic) does not arise, apart from possible differences with respect to. However, this is perhaps a small effect because the packing of liquids near a surface is more determined by liquid-liquid than by liquid-surface interactions. 

Both the above simulations considered identical tips and substrates. Failure moved away from the interface for geometric reasons, and the orientation of the interface relative to easy slip planes was important. In the more general case of two different materials, the interfacial interactions may be stronger than those within one of the materials. If the tip is the weaker material, it will be likely to yield internally regardless of the crystallographic orientation. This behavior has been observed in experiments between clean metal surfaces where a thin tip is scraped across a flat substrate [31]. When the thin tip is softer than the substrate, failure is localized in the tip, and it leaves material behind as it advances. However, the simulations considered in this section treated the artificial case of a commensurate interface. It is not obvious that the shear strength of an interface between two incommensurate surfaces should be sufficient to cause such yield, nor is it obvious how the dislocation model of Hurtado and Kim applies to such surfaces. 

Fig. 8.22. Force of interaction between two dislocations on adjacent slip planes.

Although there are a number of interesting features of this analysis, it also leaves us with serious concerns about the formulation of an elastic theory of the obstacle forces that impede dislocation motion. In particular, this analysis suggests that for an obstacle on the slip plane itself, there is no interaction with the dislocation. Despite this elastic perspective, it seems certain that core effects will amend this conclusion. 

In FCC materials, there are 12 different slip systems, which can contribute to the deformation process. Dislocation density histories at a peak stress of 4.5 GPa for [001], [111] and [Oil] orientations and isotropic case with [001] orientation are calculated and plotted as shown in Fig. 13. It is clear that the dislocation density is very sensitive to crystal orientation with the highest density exhibited by [111] orientation followed by the isotropic media, [011] and [001] orientations respectively. This may be attributed to the number of slip systems activated and to the way in which these systems interact. The [001] orientation has the highest symmetry among all orientations with four possible slip planes 111 that have identical Schmid factor of 0.4082, which leads to immediate work hardening. The [011] orientation is also exhibits symmetry with 2 possible slip planes that have Schmid factor of 0.4082. 

The particular merit of transmission electron microscopy is that the three-dimensional arrangements of dislocations can often be observed and that the identity, i.e., the slip plane and Burgers vector, can be determined. Dislocation interaction and movements may also be observed directly. However, one serious limitation devolves upon the fact that thin sections have to be used, which means that the arrangement of dislocations and their mutual interaction may be influenced by the sample size. In addition, rather small and unrepresentative samples have to be used and, most inconvenient of all for those interested in... 

Nevertheless, at some distance from the interface forces of intermolecular and electrostatic interaction weaken and become comparable with hydrodynamical forces. The surface, beyond which ground water is subjected to the effect of gravity forces and participates in the flow, is called slip plane ot plane of shear. Actually, it serves the outside distribution border of ion-salt complex in the rock. The layer of immobile water behind the slip plane is often called the Nemst layer. Substance migration with the gravity water due to the flow is called mass transport. 

The rock-salt structure is shown in Fig. 6.13. In crystals having this structure, the smallest spacing between ions of the same type is along <110> and the most widely spaced planes with these closely packed directions are the 100 planes. Experimental observations confirm the slip direction as <110> but the slip planes are usually found to be the 110 planes. The systems for which slip is easiest are termed the primary slip systems and, thus, for rock-salt structures they are usually 110 < li0>. Slip may occur with greater difficulty on other systems and these are termed secondary slip systems. Slip does not occur on 100 planes because of the electrostatic interaction that occurs between the ions in this process. This is depicted in Fig. 6.14, in which the initial (a, c) and mid-shear (b, d) positions of the ions are shown. For slip on 100 planes (a to b), the distance between like ions is increased and between opposite ions, it is decreased. For slip... 

Stage II Interaction of dislocations on intersecting slip planes resulting in work hardening... 

The (110) dislocations are dissociated into four 1/4(110) collinear partials. The SFE of the middle fault is surprisingly low ( 80 mj m ), and it is suggested that this is because only the fourth plane-plane interactions are changed by the fault [178]. For Er203 single crystals [35, 179], temperatures in excess of 1600 °C and stresses of 100 MPa were required for plastic deformation at a strain rate of 10 s . Only (111) 110 slip was observed. The 1/2(111) dislocations were found to be dissociated according to Eq. (21), and the authors pointed out that the partial Burgers vectors connect cation positions in the structure. 

This LD was formed by the interaction of two nondissociated full dislocations with Burgers vectors a/2[l01] and a/2[110], respectively, moving under applied stress on two intersecting slip planes, (ill) and (111). The dislocation reaction may be written as ... 

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