Dislocation concentration

Meir and Clifton [12] study shocked <100) LiF (high purity) with peak longitudinal stress amplitudes 0.5 GPa. A series of experiments is reported in which surface damage is gradually eliminated. They find that, while at low-impact velocities the dislocations in subgrain boundaries are immobile and do not affect the dislocation concentration in their vicinity, at high-impact velocities ( 0.1 km/s) dislocations emitted from subgrain boundaries appear to account for most of the mobile dislocations. 

For the case of LiF crystals, both the dislocation concentration and the incremental stress caused by plastic deformation are proportional to the amount of deformation. This indicates that the hardening is caused by impediments created by dislocations and dipoles to the motion of subsequent dislocations. 

On heating, constituents of the crystal within the strained zone of a line dislocation can be displaced relatively readily and the dislocations can thus migrate to surfaces where they are eliminated, or those with opposing Burgers vectors may cancel each other. Such reduction in dislocation concentration is termed annealing. Cold working (distortion or strain imposition) of a crystal increases its dislocation density. Impurities may retard the movement of dislocations through the crystal (e.g. carbon in steel). 

The dislocation structure of the RPV changes (Figure 4.12) with the number of anneals. In the final state, the average dislocation concentration is 2 x 10 m for the BM (15Kh2MFA steel) [41]. 

As the grains grow under maximum temperature gradients, their microstructural character is heterogeneous and non-isotropic. Two types of carbidic phase were observed in the volume M3C and M7C3. The dislocation concentration is 1.2-2.0 X lO m-2. 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Two point defects may aggregate to give a defect pair (such as when the two vacanc that constitute a Schottky defect come from neighbouring sites). Ousters of defects ( also form. These defect clusters may ultimately give rise to a new periodic structure oi an extended defect such as a dislocation. Increasing disorder may alternatively give j to a random, amorphous solid. As the properties of a material may be dramatically alte by the presence of defects it is obviously of great interest to be able to imderstand th relationships and ultimately predict them. However, we will restrict our discussion small concentrations of defects. 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

Dislocations are known to be responsible for die short-term plastic (nonelastic) properties of substances, which represents departure from die elastic behaviour described by Hooke s law. Their concentration determines, in part, not only dris immediate transport of planes of atoms drrough die solid at moderate temperatures, but also plays a decisive role in die behaviour of metals under long-term stress. In processes which occur slowly over a long period of time such as secondaiy creep, die dislocation distribution cannot be considered geometrically fixed widrin a solid because of die applied suess. 

Introduction of the surface-nucleation mechanism in numerical computation of elastic-plastic wave evolution leads to enhanced precursor attenuation in thin specimens, but not in thicker ones. Inclusion of dislocation nucleation at subgrain boundaries indicates that a relatively low concentration of subgrain boundaries ( 2/mm) and nucleation density (10"-10 m ) is sufficient to obtain predicted precursor decay rates which are comparable to those obtained from the experiments. These experiments are only slightly above the threshold necessary to produce enhanced elastic-precursor decay. 

The dislocation cannot glide upwards by the shearing of atom planes - the atomic geometry is wrong - but the dislocation can move upwards if atoms at the bottom of the half-plane are able to diffuse away (Fig. 19.2). We have come across Fick s Law in which diffusion is driven by differences in concentration. A mechanical force can do exactly the same thing, and this is what leads to the diffusion of atoms away from the... 

Most ceramics are intrinsically hard ionic or covalent bonds present an enormous lattice resistance to the motion of a dislocation. Take the covalent bond first. The covalent bond is localised the electrons which form the bond are concentrated in the region between the bonded atoms they behave like little elastic struts joining the atoms (Eig. 17.1b). When a dislocation moves through the structure it must break and reform... 

Figure 2 CL micrographs of Te-doped GaAs dark-dot dislocation contrast (a) in GaAs doped with a Te concentration of 10 cm and dot-and-halo dislocation contrast (b) in GaAs doped with a Te concentration of 10 cm. ...

Just recently (Wilde et al. 2000), half a century after the indirect demonstration, it has at last become possible to see carbon atmospheres around dislocations in steel directly, by means of atom-probe imaging (see Section 6.2.4). The maximum carbon concentration in such atmospheres was estimated at 8 2 at.% of carbon. 

However, it is not yet clear why the ener es of the SISF and the twin boundary increase with increasing A1 concentration. To find a clue to the problem, it would be needed to make out the effects of the short-range ordering of A1 atoms in excess of the stoichiometric composition of the HAl phase on the energies of planar faults and the stmcture of dislocation cores in the Al-rich HAl phase. 

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