Deformation mechanisms dislocation motion

The development of nanocrystaUine structures under the severe plastic deformation introduced by milling is explained by the mechanisms of the generation of a large number of dislocations, that on further deformation forms grain boundaries [ 14]. It has been suggested [14,77] that a small grain size in itself prevents further plastic deformation via dislocation motion and therefore, further grain... 

In the absence of cavitation, creep in vitreous-bonded materials would occur by S-P, wherein material dissolves from one side of the grain and deposits on another [48, 49]. No definitive studies have been made to date that support the dislocation creep models in which the grains of silicon nitride deform by dislocation motion. Studies of deformed silicon nitride grains have provided no evidence of the types of dislocation pileup that should be present in order for this type of mechanism to be active [50]. 

In the Sect. 8.2, one of the dislocation mechanisms was discussed-the pile-up concept-as the origin of brittle fracture due to the stress concentration occmring at the leading dislocation in the pile-up. However, in materials that is entirely brittle, as are most ceramics at RT and low temperatures, plastic deformation by dislocation motion does not occur or occurs to such a limited extent that cracks are sharp up to the atomic level. In order to understand the fracture behavior of ceramic materials, it is first necessary to understand the fracture mechanisms of materials that are entirely brittle. In such materials, the mechanism of fracture is associated with various flaws inherent or intentionally added to the ceramics. A fist of most of the flaws that may induce brittle failure by crack formation is given below, as indicated in Rg. 8.10 ... 

Movement of dislocations is a primary mechanism for plastic deformation. A dislocation s motion is impeded when they encounter obstacles, causing the stress required to continue the deformation process to increase. Grain boundaries are one of the obstacles that can impede dislocation glide, so the number of grain boundaries along a slip direction can be expected to influence the strength of a material. In the early 1950s, two researchers, Hall (1951) and Petch (1953),... 

In addition to the importance that attaches to rigid body motions, shearing deformations occupy a central position in the mechanics of solids. In particular, permanent deformation by either dislocation motion or twinning can be thought of as a shearing motion that can be captured kinematically in terms of a shear in a direction s on a plane with normal n. 

For coarse-grained metals, dislocation movement and twinning are well known primary deformation mechanisms. Ultrafine, equiaxed grains with high-angle grain boundaries impede the motion of dislocations and... 

In this chapter we are concerned with the deformation of ceramics leading to a permanent shape change. This is known as plastic deformation and is both nonrecoverable and irreversible. There are several mechanisms that are responsible for plastic deformation in crystalline materials dislocation motion, vacancy motion, twinning, and phase transformation. In metals at room temperature dislocation motion is the most important of these mechanisms. In Chapter 12 we already noted that dislocations do not move easily in ceramics and this is the reason for their inherent brittleness. Nevertheless, dislocation motion is observed in ceramics under specific loading conditions. In general, plastic deformation of ceramics requires high temperatures and this is important because... 

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