Dislocation model

From the early work of Taylor [63T01] connecting dislocation behavior to observed viscoplastic shock-compression response, numerous studies have attempted to relate conventional dislocation dynamics models to experimental observations. Theory and observations consistently require unusually large numbers of mobile dislocations. Although qualitatively descriptive, progress to date on dislocation models has not proven to provide quantitative descriptions to the observations in metals. 

F. Minari and B. Pichaud, Dislocations and Free Surfaces in the Micro-plastic Deformation of F.C.C. Metals , in Dislocation Modelling of Physical Systems", Edrs. M. F. Ashby, R. Bullough, C. S. Hartley, and J. P. Hirth, p. 551, Pergamon Press, New York, USA (1980). 

The Anderson—Hyde dislocation model differs from the earher model based on the cooperative diffusion mechanism described by Andersson and Wadsley (1966), in which CS planes, e.g. in rutile, were diought to be produced by cation migration during reduction.The reduced oxygen potential at die surface means an enhanced Ti-potential and dierefore die Ti ions diffuse cooperatively into the crystal down diis Ti-potential gradient. However, diere is no experimental evidence to support this hypodiesis. This mechanism is also less hkely since diis would involve a large number of cations. 

We describe the development of in situ (dynamic) ETEM for direct imaging of CS defects in dynamic catalytic oxides in chapter 3. These studies have recently led to better insights into the formation of CS planes (leading to further developments in the dislocation model) and their role in oxidation catalysis. By directly probing the formation of CS planes and their growth by in situ ETEM... 

Figure 3-1. a) Edge dislocation model b) Burgers vector h with Burgers circuit and glide plane indicated. Dislocation motion during plastic deformation under the action of force F. Jog and kink. 

W.T. Read and W. Shockley. Dislocations models of grain boundaries. In Imperfections in Nearly Perfect Crystals. John Wiley Sons, New York, 1952. 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

To complete the overview, we can mention also the dislocation model of micropore formation, but this model does not give any results principally different from these of the thermodynamic model [6]. 

A screw dislocation model that predicts preferred growth along a defined dislocation. Here,... 

Crystal growth by the layer growth mechanism describes the formation of steps (i.e., layers) by two different mechanisms—2-D nucleation and screw dislocation. The model for 2-D nucleation was developed by Volmer[ ° l and Stranski. The screw dislocation model was first described by Burton, Cabrera, and Frank (BCF). The details of the derivations for these models have been summarized in a number of other references. ° ° ... 

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