Dislocations interaction with impurities

For the detection of dislocations by electroetching as well as by chemical etching, it is frequently necessary to "decorate " the dislocations by means of one or more impurities in the base metal. The impurity atoms interact with the dislocations. This idea was first put forward by Wyon and Laeombe (11) in the case of the Al. The same approach has also been extended to Fe (12), Si-Fe alloys (13), Zn (14), Cu (15), and others. 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

Even the cleanest of all the substrates shows areas where the periodic surface potential is perturbed. These sites can be generally called defects. Defects are generally classified into two main subclasses point defects, like corners, kinks, impurities or missing atoms and extended defects, like dislocations and steps. The type, concentration and characteristics of defects depend on several factors but the nature of the oxide and the history of the sample are no doubt the most important ones. In this section, two of the most commonly found MgO defects21,126 — low coordinated anion sites (steps and corners) and oxygen vacancies — will be considered with special emphasis on their interaction with metal atoms. 

Dislocations in ceramics can be pinned by solute atoms just as they can in metals as shown in Figure 17.14. The dislocations are impeded because of their interaction with the stress field around the impurity. This effect has long been used to strengthen metals. 

Sokaski (1975) investigated the deformation rate-controlling mechanisms of yttrium and concluded that below 300 K deformation was controlled by impurities with a barrier energy of about 0.3 eV the impurities were not oxygen. It was suggested that a dislocation interaction mechanism was rate-controlling at temperatures above 300 K. 

K. Sumino, 1. Yonenaga, 2002, Interactions of impurities with dislocations mechanical effects . Solid State Phenom. 85-86, 145-176. 

Through the associated strain field, a dislocation is able to interact with other dislocations and also with point defects such as vacancies and impurities. The c.r.s.s. can be affected by the presence of these defects so that the c.r.s.s. is, to some extent. 

Once plastic deformation has started in a specimen an increase in stress is needed to produce further deformation. In microscopic terms this means that as deformation proceeds, the movement of dislocations on their slip planes becomes progressively more difficult. This hardening of the material may arise from elastic interactions between dislocations, through their strain fields, it may arise from dislocation reactions that produce segments that cannot sUp and it may also arise from interactions with other defects such as vacancies, impurities and grain boundaries. 

Describe and explain solid-solution strengthen ing for substitutional impurity atoms in terms of lattice strain interactions with dislocations. 

Because there are a large number of sites along the dislocation line, a large number of impurity atoms can be accommodated in the dislocation core. The interaction energy of an impurity located at position r and angle 0 with respect to a pure edge dislocation, where the impurity has a size AR different from the matrix atom radius R can be shown to be ... 

For the deformation of NiAl in a soft orientation our calculations give by far the lowest Peierls barriers for the (100) 011 glide system. This glide system is also found in many experimental observations and generally accepted as the primary slip system in NiAl [18], Compared to previous atomistic modelling [6], we obtain Peierls stresses which are markedly lower. The calculated Peierls stresses (see table 1) are in the range of 40-150 MPa which is clearly at the lower end of the experimental low temperature deformation data [18]. This may either be attributed to an insufficiency of the interaction model used here or one may speculate that the low temperature deformation of NiAl is not limited by the Peierls stresses but by the interaction of the dislocations with other obstacles (possibly point defects and impurities). 

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