Plastic deformation dislocation movement

Beside dislocation density, dislocation orientation is the primary factor in determining the critical shear stress required for plastic deformation. Dislocations do not move with the same degree of ease in all crystallographic directions or in all crystallographic planes. There is usually a preferred direction for slip dislocation movement. The combination of slip direction and slip plane is called the slip system, and it depends on the crystal structure of the metal. The slip plane is usually that plane having the most dense atomic packing (cf. Section 1.1.1.2). In face-centered cubic structures, this plane is the (111) plane, and the slip direction is the [110] direction. Each slip plane may contain more than one possible slip direction, so several slip systems may exist for a particular crystal structure. Eor FCC, there are a total of 12 possible slip systems four different (111) planes and three independent [110] directions for each plane. The... 

The densification of the parts by HIP implies primarily three phenomena i) fragmentation of the particles and rearrangement, ii) deformation of the interparticle areas of contact and iii) elimination of the pores. The first process is transitory and hardly contributes to the overall densification, at least if the initial forming (for example, by CIC) has been correcdy carried out. The second process brings into play effects of plastic deformation by movement of dislocations and diffusion phenomena that are similar to those indicated in the case of uniaxial pressure sintering. Lastly, by considering the final reduction of porosity, we can write phenomenologically ... 

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

This complex form of precipitation in the Al-Cu system is of great practical importance. The finely dispersed precipitates act as effective barriers to the glide movement of dislocations during plastic deformation and harden and strengthen the material. This has led to the development of a number of widely used precipitation-hardened Al-Cu alloys [1]. 

This is the process by which a crystal undergoes plastic deformation, as a result of which one atomic plane moves over another. Slip is believed to occur through the movement of dislocations. The total deformation of a given crystal is the sum of many small lateral displacements in parallel crystallographic planes of a given family. Moreover, each slip plane becomes more resistanl to further deformation than the remaining potential slip planes. 

This is slip that occurs simultaneously on several slip planes having Ihe same slip direction. See Fig. 14. This type of plastic deformation is normally associated with the movement of screw dislocations. Screw dislocations can move on any slip plane that passes through the dislocation. This is a result of the fact that the slip plane of a dislocation is that plane which contains both the dislocation and its Burgers veclor, and the fact that the Burgers vector of a screw dislocation lies parallel io the dislocation itself,... 

It is well established that the plastic deformation of crystalline solids occurs by the movement of lattice dislocations and/or diffusional creep. The rate of diffusion is expressed as... 

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),... 

Normally, dislocation-based plastic deformation is irreversible, that is, it is not possible to return the material to its original microstructural state. Remarkably, fully reversible dislocation-based compressive deformation was recently observed at room temperature in the layered ternary carbide Ti3SiC2 (Barsoum and El-Raghy, 1996). This compound has a hexagonal stmcture with a large cja ratio and it is believed that the dominant deformation mechanism involves dislocation movement in the basal plane. 

It has been shown that several polymers exhibit instabilities in their plastic deformation process. It should finally be mentioned that instabilities may also occur during the plastic deformation of metals This phenomenon which is called the Portevin-Le Chatelier effect, is generally interpreted in terms of different modes of dislocation movement depending on whether or not dislocations move by dragging along their atmosphere of impurities behind them. 

The plastic deformation in crystalline objects is related to the appearance and movement of specific linear structure defects, referred to as the dislocations (see Chapter IV, 4) [37]. Within the slip plane, dislocation separates the portion of a crystal in which the position of atoms was shifted by one interatomic distance from the portion of crystal in which such displacement has not yet occurred (Fig. IX-35). The movement of dislocation... 

According to Shchukin [9], the mechanism of adsorption plasticizing is based on facilitation of dislocation movement. It was experimentally established that upon deformation of crystals (e.g., of naphthalene or sodium... 

Glen, J. W. (1968). The effect of hydrogen disorder on dislocation movement and plastic deformation of ice. Phys. kondens. Materie... 

In Si wafer processing, there are primarily two causes for stress apart from oxygen precipitation. The first one are temperature gradients. The second causes are surface-near layers with structural properties different from that of the Si-substrate, e.g. oxides, nitrides, poly-Si, or doped layers. If these stresses in a wafer grow beyond a critical, temperature-dependent level, plastic deformation will occur by movement of dislocations. Dislocations will lead mostly to catastrophic device failure /11,12/. 

In many crystals, plastic deformation occurs by movement of partial dislocations. What defect arises from this phenomenon ... 

There are many mechanisms that can lead to plastic deformation in single crystals, but the most important is slip. The two things that we need to consider are the inherent resistance to the movement of dislocations provided by the periodicity of the lattice and the orientation of the crystal with respect to the applied stress. 

The mechanical strength of hard materials is critical for load-bearing, structural applications. These brittle materials only deform plastically at high temperatures, or under severe hydrostatic constraint, since the Peierls stress for dislocation movement is high. Failure is usually by unstable crack propagation under a tensile stress that exceeds the tensile strength of the material. In terms of fracture mechanics, brittle failure occurs when the Mode I stress intensity factor Kj reaches the fracture toughness of the material, Kic (see below). 

The so called nanoindentation , which is frequently used to measure < 1 pm thin films, is subject to a number of possible errors [27,28]. For example, when measuring soft materials, such as aluminum or pure iron with a small load and indentation depth, the dislocations are pinned in the surface contaminant layer (oxides, carbides) and, consequently, unrealistically high values of hardness are found. The plastic deformation may also need a certain time to reach equilibrium under the given load because of a finite velocity of dislocation movement. This can be seen as creep (increase of the indentation depth) when a constant load is applied for 10-30 s. 

In the case of borides, the production of pure dense materials is more difficult because TiB2 does not deform plastically even at very high temperatures due to its intrinsically high concentration of Peierls barriers to dislocation movement. Recent investigations on borides have been totally devoted to the synthesis of composites either metal-ceramic or ceramic-ceramic. Woodger et al. [94,95]... 

When accommodated by some of the mechanisms involving dislocation movement or the diffusion of point defects, GBS forms the basis of the structural superplastic behavior of these materials (see Section 15.2). By taking advantage of the processes involved in superplasticity, it is possible to join ceramics super-plastically. For example, when two pieces of the same ceramics in contact are deformed within a superplastic regime (i.e., as soon as GBS is activated), the grains of one part interpenetrate those of the other part. This produces a rapid and perfect junction of the two, in such a way that a shorter time and a lower temperature can be used than are commonly required in other conventional process for ceramics joining [90]. 

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