Dislocation formation

The dislocation formation mechanism described in the previous section generates dislocation loops. A dislocation loop can also form by the aggregation of vacancies on a plane in a crystal. Vacancy populations are relatively large at high temperatures, and, if a metal, for example, is held at a temperature near to its melting point, considerable... 

Based on the discussion in earlier sections of this chapter, one may expect atomically flat incommensurate surfaces to be superlubric. Indeed the first suggestion that ultra-low friction may be possible was based on simulations of copper surfaces.6,7 Furthermore, the simulations of Ni(100)/(100) interfaces discussed in the previous section showed very low friction when the surfaces were atomically flat and misoriented (see the data for the atomically flat system between 30° and 60° in Figure 21). In general, however, it is reasonable to assume that bare metals are not good candidates for superlubric materials because they are vulnerable to a variety of energy dissipation mechanisms such as dislocation formation, plastic deformation, and wear. 

Creep and fracture in crystals are important mechanical processes which often determine the limits of materials application. Consequently, they have been widely studied and analyzed in physical metallurgy [J. Weertmann, J.R. Weertmann (1983) R.M. Thomson (1983)]. In solid state chemistry and outside the field of metallurgy, much less is known about these mechanical processes [F. Ernst (1995)]. This is true although the atomic mechanisms of creep and fracture are basically independent of the crystal type. Dislocation formation, annihilation, and motion play decisive roles in this context. We cannot give an exhaustive account of creep and fracture in this chapter. Rather, we intend to point out those aspects which strongly influence chemical reactivity and reaction kinetics. Illustrations are mainly from the field of metals and metal alloys. 

If a crystal is exposed to stress in such a way that the strain is kept constant, the stress will decrease with time as shown in Figure 14-4. One concludes that stress relaxation has occurred. Conversely, strain does not remain constant under constant load. Time dependent (i.e., plastic) strain in stressed crystals is called creep. It was already mentioned that elastic strain due to the applied stress is usually less than 1%. Plastic strain definitely dominates beyond the elastic limit which, to a large extent, is due to dislocation formation and motion. Since the crystal lattice is conserved during this... 

L.T. Chadderton, Fission Fragment Damage to Crystal Lattices Dislocation Formation , Froc-RoySoc A269, 143-64 (1962) 98) M.J. 

The local deformation, cracking and the dislocation formation in the composites1 under the oxygenation treatment, obviously, occur at the expense of the energies of the internal reactions17. 

Effect of Oxygen Content and Sinter Conditions on Dislocation Formation... 

The criterion for interfacial dislocations is that aacks start to move along the intaface, but the aack opening is constrained to cause healing. Thus the condition for dislocation formation is the same as that for interface cracking in the lap joint (Chapter 15). Dislocations will form at a stress... 

Fig. 3.33 Dislocation formation by cutting a slot in the body and gluing the faces formed together a ABCD is the slip plane and EF is the dislocation line at the start of deformation b edge dislocation AB, formed by shear displacement in the slip plane ABCD following a cut in the plane and the gluing of the faces formed by the cut [14]...

The primary one-dimensional or line defects seen in sohds are dislocations. In general, dislocation formation and motion require more energy in ceramics than in metals. This is due to the complex atomic structures of ceramics, which often have large unit cells, and to the typically strong and localized bonding between atoms in... 

It was considered that an important role was played by dislocation formation during diffusion annealing. 

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