Plastic deformation, dislocation related

In textbooks, plastic deformation is often described as a two-dimensional process. However, it is intrinsically three-dimensional, and cannot be adequately described in terms of two-dimensions. Hardness indentation is a case in point. For many years this process was described in terms of two-dimensional slip-line fields (Tabor, 1951). This approach, developed by Hill (1950) and others, indicated that the hardness number should be about three times the yield stress. Various shortcomings of this theory were discussed by Shaw (1973). He showed that the experimental flow pattern under a spherical indenter bears little resemblance to the prediction of slip-line theory. He attributes this discrepancy to the neglect of elastic strains in slip-line theory. However, the cause of the discrepancy has a different source as will be discussed here. Slip-lines arise from deformation-softening which is related to the principal mechanism of dislocation multiplication a three-dimensional process. The plastic zone determined by Shaw, and his colleagues is determined by strain-hardening. This is a good example of the confusion that results from inadequate understanding of the physics of a process such as plasticity. 

When normal sites in a crystal structure are replaced by impurity atoms, or vacancies, or interstitial atoms, the local electronic structure is disturbed and local electronic states are introduced. Now when a dislocation kink moves into such a site, its energy changes, not by a minute amount but by some significant amount. The resistance to further motion is best described as an increase in the local viscosity coefficient, remembering that plastic deformation is time dependent. A viscosity coefficient, q relates a rate d8/dt with a stress, x ... 

It is noted, however, that both of the above CaF2 and Ti02 nanoceramics had some amount of porosity. This may account for an apparent soft behavior related to the superplastic deformation at low temperature, which does not yet reveal the plastic deformation characteristics in nanoceramics. Localized superplastic deformation under cyclic tensile fatigue tests was observed by Yan et al. on 3Y-TZP nanoceramics at room temperature [25], The micromechanism behind this phenomenon is argued to be essentially governed by grain-boundary diffusion. The contribution of dislocation slip might be in operation as a parallel mechanism to develop slip band-like microfeatures. 

The history of the development of the theory of low-temperature plasticity of solids resembles very much the development of tunneling notions in cryochemistry. This resemblance is not casual it is related to the similarity of the elementary act pictures this was noted by Eyring, who successfully applied the theory of absolute rates to a description of fracture kinetics [202]. Plastic deformation at constant stress (creep) is stipulated by dislocation slip... 

After initial deformation, examination of the crystal surface, again by optical means, reveals a novel feature already discussed in fig. 8.3, namely, slip traces. These slip traces are the debris left in the wake of dislocations that have made their way to the crystal surface, leaving behind a jump across the relevant slip plane at its point of intersection with the crystal s surface. These slip traces bear a precise geometrical relation to the underlying crystalline geometry and thereby provide a central clue in ferreting out the microscopic origins of plastic deformation. 

Pulse duration is related to the time required for the dislocations to reorganize in certain patterns. During the shock time rise, dislocations are generated leading to permanent plastic deformation. However, some of the dislocations can possibly retrace their path during wave release reversibly [38], Pulse duration may influence the amount dislocation reversibility and as a result the saturated dislocation density. Fig. 12 shows that the saturation density of... 

Numerous Br SSNMR studies have been carried out on AgBr, many of which focused on the addition of dopants and their effects on the bromine SSNMR spectra (see below).The temperature and time dependence of the Br NMR signal of plastically deformed AgBr (both pure and NaBr doped) have also been discussed and related to the density of lattice dislocations. The reader is referred to Table 9 for additional information on AgBr. 

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... 

For the low-temperature region, several dislocation-related hypotheses were proposed to explain the mechanism of plastic deformation. Trefilov and Mil-man [76] suggested that during indentation the theoretical shear strength was... 

The yield strength (Oy) is expressed in terms of the yield stress, Oo (Oo is related to the intrinsic stress, Oi, resisting dislocation motion) and the grain size, d. When this relationship was deduced, it applied to a situation in which the grains were deformed by plastic deformation and the GBs acted as barriers to dislocation motion. This model is unlikely to be valid in general for ceramics since deformation by dislocation glide is not common. However, the relationship between d and Oy does hold as we saw for polycrystalline MgO in Figure 1.2. 

The lapping process is a very complex three-body abrasion. During friction and wear processes, many AE signals could be generated because of the interactions, impact, dislocation, deformation, and removal of materials. In the area of tribology, AE was used to characterize wear mechanisms, discern plastic deformation and fracture, and to monitor active friction and wear processes. AE signals were related to friction coefficient, abrasive grit size, and wear rate. 

The subgrains formation is related to high plastic deformations ratio and high density of dislocation lines (visible in the lower part of the Figure 5) formed in the process. 

Furthermore, if the quartz is dry - that is, if it contains little or no dissolved H2O or other water-related spedes (H, OH , etc.) - then any deformation is difficult even under hydrostatic-confining pressures. For example, under a confining pressure of 1.5 GPa, Griggs and Black 83,84] found the CRSS for basal slip of dry quartz to be 2.2 GPa at 300 °C and 1.6 GPa at 700 °C, which is about 5% of the shear modulus of 40 G Pa. This intrinsic resistance to plastic deformation of quartz remains high to at least 1300 °C ( 0.8 T ), where the CRSS is still in excess of 0.5 GPa [12,85). Where deformation has been induced under hydrostaticslip planes - (0001), 1010, 1120, lOll, Oil - and dislocations with various Burgers vectors-1/3 (1120), [0001], and 1/3 (1123)-have often been found. These Burgers vectors correspond to the three shortest lattice translations in quartz, namely 0.491, 0.541, and 0.730 nm, respectively. 

Although the observed mechanical properties in FG ceramics have been related to the GBS mechanism, the actual deformation mechanism is stiU under debate and may be material-dependent. A concept of plastic deformation has been suggested as a mechanism involving the non-local, homogeneous nucleation of nanoscale loops of partial dislocations also unusual, nonlinear stress and grain-size dependence is assumed to facilitate nanocrystalline plasticity. However, the dominant mechanism is stiU GBS. The stress level required to nucleate a dislocation is much higher than usually encountered in experimental data. Therefore, dislocation gliding itself is not expected to contribute to total strain. 

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