Generation of dislocations

Figure 6.5. X-ray topographs demonstrating the differences in inheritance and generation of dislocations when (a) an etched crystal and (b) an as-grown crystal are used as seed crystals (ref [11], Chapter 7). The crystal here is BalNOj), grown from aqueous solution, g = Burgers vector S = seed crystal.

Crack tips are line defects, but may resemble disclinations more than dislocations if the sides of the crack are inclined to one another rather than parallel. The differences are significant deformation at the crack tip may be very large leading to the generation of dislocations in the plastically deformed region even in brittle ceramics. If this plastic deformation is large, it actually blunts the crack tip and toughens the ceramic. 

The movement of mobile dislocations plays a significant role in the fatigue failure preparation. If a specimen is subjected to alternating stresses the generation of dislocations is known to occur. There is a threshold stress tg at which this process begins [81]... 

If the capillary pressure at the neck region between two particles can cause a plastic deformation of the region, the capillary pressure must be larger than the yield strength. What condition (an equation) should be provided for the generation of dislocations by the capillary pressure Discuss the possibility of dislocation generation by capillary pressure in real systems. 

The overall process can now be viewed to involve the following steps (Fig. 5) Asperities of the material approach each other and collide. This is seen not just as a Hertzian pressing but as an impact event. The result is more then the generation of dislocations, because it shatters the structure of the material to disassemble the original carbide grains into smaller grains... 

Another such example of cross-slip in oxide ceramics may be observed during the generation of dislocations in plastic deformation, as illustrated in Fig. 3.76. 

A mechanism of the double-cross slip mechanism is suggested for the generation of dislocations during deformation which increases the density in large amounts in addition to the known Frank-Read mechanism. 

The intial densification kinetics in the sintering of powder compacts have been examined for five oxides ZrOz-CaO, MgO, CaO, CeO, and SiOz. Study of the initial sintering, the densification which occurs as the temperature of the specimen is increased, permits the observation of densification kinetics which may not be diffusion controlled. The densification kinetics demonstrate several features which are not compatible with a diffusion-controlled process. For example, there is a disproportionately large increase in the densification rate when the temperature is increased compared to that expected from a volume diffusion process. Other tests indicate that isostatic pressing of partially sintered powder compacts causes densification on subsequent heat treatment. An explanation of these effects by a diffusion sintering process, either volume or grain boundary diffusion, is not evident however, a qualitative explanation based on dislocation transport of material can be proposed. This explanation rests on the premise that as the temperature is increased, the stress required for motion and generation of dislocations decreases. 

These conclusions are compatible with Radcliff s ( ) observation of what is thought to be a similar phenomenon with respect to isostatic pressure generation of dislocations which move on subsequent activation. He found that if iron-carbon alloys were isostatically pressed, discontinuous yielding was eliminated. Bullen et al. ( °) observed a similar effect in iron specimens containing carbon and were also able to obtain up to 60% plastic deformation... 

A quantitative comparison between calculated and measured dislocation densities can only be performed when more sophisticated models are used. A popular model for the description of the generation of dislocations by plastic deformation is the Alexander-Haasen model [39]. Volkl and Muller [40] used the Alexander-Haasen... 

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