Creating Dislocations

Tinplate and Solder. Metallurgical studies were performed to determine the effect of irradiation at low temperature on the corrosion resistance of tinplate and on the mechanical properties and microstructure of tinplate and side-seam solder of the tinplate container. The area of major interest was the effect of low-temperature irradiation on the possible conversion of the tin from the beta form to the alpha form. In the case of pure tin, the transition occurs at 18 °C. It was feared that low-temperature irradiation would create dislocations in the crystal lattice of tin and enhance the conversion of tin from the silvery form to a powdery form rendering the tin coating ineffective in protecting the base steel. Tin used for industrial consumption contains trace amounts of soluble impurities of lead and antimony to retard this conversion for several years. 

Suppose now that such a source is present in a crystal that is rapidly quenched from a temperature Tq to a temperature Ta to produce supersaturated vacancies. Find an expression for the critical value of the quenching temperature, Tq, which must be used to produce sufficient supersaturation to activate the source so that it will be able to create dislocations loops capable of destroying the supersaturated vacancies by climb. The vacancy formation energy is Ey and the segment length is L. 

In order to investigate the spatial distribution of B atoms in a B-doped HOD film, Graham et al. [415] deposited a 4-pm thick B-doped diamond layer on an undoped HOD film of 30-pm thickness. The B-doped layer was deposited using CH4, H2, and B2H2, where B/C = 44 ppm in the source gas. The specimen was thinned from the HOD film side so that TEM and CL measurements could be done for the same position of the specimen. In the CL spectrum related with dislocations, there were two bands at 2.87 eV (431 nm) and 2.32 eV (535 nm) due to bound excitons. A comparison between the TEM image and the monochromatic CL images for 2.87 and 2.32 eV indicated that the B dopants were distributed uniformly within the film on the submicron level. Furthermore, the incorporation of B dopants created dislocations in the film. 

The lack of a strong correlation between dislocation densities and reaction rates has also been demonstrated experimentally, as shown in Table II. In these studies the dislocations were mechanically induced, creating dislocation densities which varied by several orders of magnitude. The upper limit for defect densities exceeded the 10 /cm threshold density proposed by Blum and Lasaga (83). However, the large range in dislocation densities produced only factor of two increases in the reaction rates. 

Strained-layer superlattice An epitaxial thin film where the lattice spacing of the crystalline structure of the film material has been strained but not to the point of creating dislocations. 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

A hardness indentation causes both elastic and plastic deformations which activate certain strengthening mechanisms in metals. Dislocations created by the deformation result in strain hardening of metals. Thus the indentation hardness test, which is a measure of resistance to deformation, is affected by the rate of strain hardening. 

So far we have discussed the surface of a perfect crystal. But for an imperfect crystal there is another possibility to provide a step source. This is due to the screw dislocation. Assume that one cuts a crystal half-way from one side into the center, and slides the freshly created two faces against each other in... 

As an indenter creates an indentation it causes at least three types of finite deformation. It punches material downwards creating approximately circular prismatic dislocation loops. At the surface of the material it pushes material sideways. It causes shear on the planes of maximum shear stress under itself. Therefore, the overall pattern of deformation is very complex, and is reflected... 

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. 

Figure 5.9 Plan view of the (111) plane of the diamond structure. A—Normal structure with open circles in the plane of the paper, and crossed circles in the plane above. Each pair is connected by a covalent bond. B—Partial shear of the upper plane over the lower one on the right-hand side creating a screw dislocation line with a kink in it (dashed line). C—Upper plane sheared down-ward by the displacement, b.

Surface layers interfere with the motion of dislocations near surfaces. Among other effects, this causes local strain-hardening, creating a harder surface region which thickens with further deformation, and eventually affects an entire specimen. A specific way in which this happens is through curving... 

A screw dislocation creates a fault in a crystal that looks rather like a spiral staircase. The dislocation can be conceptually formed by cutting halfway through a crystal and sliding the regions on each side of the cut parallel to the cut, to create spiraling atom planes (Fig. 3.6). The dislocation line is the central axis of the spiral. 

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