Kinetics screw dislocation mechanisms

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. 

In contrast to the growth kinetics of real crystals where dislocations and defects play a dominant role (cf. Section 5.3), the growth mechanism of crystallographic faces free of defects and particularly free of screw dislocations is completely different. The... 

The kinetics of metal deposition on crystal faces free from screw dislocations is determined by the rate of nucleation /, nuclei cm sec of new lattice nets and their rate of propagation v, cm sec over the face. Almost simultaneously Nielsen/ Chernov/ and Hillig " showed that, depending on the values of these two parameters, two different mechanisms can be distinguished, whereby the extension of the surface plays a significant role (i) a layer by layer growth and (ii) a multinuclear multilayer growth. 

A series of considerations including a correct value of /x modulus and its dependence on temperature, the kinetic crystallographic shear strain rate expression encountered in crystal plasticity, and a Coulomb law where the shear resistance r is dependent on the normal stress glide plane, lead to detailed expressions for yield stresses for all three cases of monolithic, half loop screw and half loop edge dislocations engaged in plastic deformation. The comparison of compressive yield stress of all three mechanisms with experimental data of Kazmierczak et al [149] is illustrated in Figure 1.12. 

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