Nucleation of dislocation

Dick et al. [29] present additional data on the <100) shock compression of LiF which further establishes a threshold shear stress of between 0.24 GPa and 0.30 GPa for nucleation of dislocations in the shock front. 

The next example that will be set forth as an example of a problem in which multiple defect types can be present simultaneously is that of fracture. Our discussion will begin with an examination of the way in which cracks can be built up as superpositions of dislocations. This leads naturally to questions of the interaction of dislocations and cracks and culminates in the analysis of the intriguing problem of crack tip nucleation of dislocations. 

If the same level of spatial resolution is attained near a crack tip, we will see the nucleation of dislocations in its vicinity. An example of crack tip dislocation nucleation as evidenced in electron microscopy is given in fig. 13.3. Once the material fails, we can also subject it to post-mortem chemical and structural analysis. Using techniques such as Auger spectroscopy, the chemical profile of the failed material may be queried. Again, our main point is to note the diversity of the various geometric signatures of mechanical response. 

Xu G., Argon A. S. and Ortiz M., Nucleation of Dislocations from Crack Tips under Mixes Moded of Loading Implications for Brittle vs Ductile Behavior of Crystals, Phil. Mag. 72, 415 (1995). Xu G., Argon A. S. and Ortiz M., Critical Configurations for Dislocation Nucleation, Phil. Mag. 75, 341 (1997). 

The critical stress needed for homogenous nucleation of dislocations is considerable larger than that for dislocation propagation or multiplication. In order to tolerate fairly high stress levels during processing, the heterogenous nucleation of dislocations has to be avoided /13/ (fig.8). 

Adsorption-Enhanced Plasticity Models According to fractographic studies the cleavage fracture is not an anatomically brittle process, but occurs by alternate slip at the crack tip in conjunction with the formation of very small voids ahead of the crack. It is also thought that the chemisorption of environmental species facilitates the nucleation of dislocations at the crack tip, promoting the shear process responsible for brittle-like fracture (4). 

Nanoindentation load-displacement curves of Ge are not as characteristic as in Si. The only feature discussed in the literature is the formation of numerous small discontinuities ( pop-ins ) in the upper portion of the loading curve (Fig. 30a). As Ge is known to be veiy prone to radial cracking, it has been argued that the pop-ins occur as a result of the discontinuous propagation of radial cracks [13S]. Another explanation of loading discontinuities associated the pop-ins with the nucleation of dislocation slips [121]. 

Xu, G. and Argon, A. S. (2000) Homogeneous nucleation of dislocation loops under stress in perfect crystals, Phil. Mag. Lett., 80, 605 11. 

The activation volumes, normalized by b, predicted by the models using eqs. (9.30) are compared in Fig. 9.23 with some of those measured by Kazmierczak et al. (2005) in strain-rate-jump experiments. In the model calculation for the nucleation of the monolithic-screw-dislocation emission A = 20 nm X/b = 78.5) was used. The data for 13 up-jump strain-rate-change experiments are also plotted in Fig. 9.23. The details of these experiments are given by Kazmierczak et al. (2005). The measured Av /Z> for the jump experiments from s = 5.5 X 10 to 5.5 X 10 fall quite close to the models for nucleation of dislocation half loops. The data for jumps at considerably lower strain rates and smaller flow stresses fall a bit closer to the model of nucleation of monolithic screw dislocations but are in much less satisfactory agreement. In all cases it was assumed that the externally applied stresses directly apply locally, which is the assumption in the Sachs model. However, considering the generally confused morphology of... 

The proponents of this model reasoned that similar fracture processes occur in liquid-metal embrittlement, hydrogen embrittlement, and S.C.C., with chemisorption facilitating the nucleation of dislocations at the crack tip and promoting the shear processes that result in brittle, cleavage-like fracture. It was found that cleavage fracture occurs by alternate slip at the crack tip and formation of voids ahead of the crack tip [35-37]. 

Xu G and Argon A S (2001) Energetics of homogeneous nucleation of dislocation loops under a simple shear stress in perfect crystals. Mater Sci Eng A319-321 144-147. 

Xu G, Argon A S and Ortiz M (1995) Nucleation of dislocations from crack tips under mixed modes of loading Implications for brittle against ductile behavior of crystals, Philos Mag A72 415-451. 

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