Final dislocation density

Champion and Rohde [42] investigate the effects of shock-wave amplitude and duration on the Rockwell C hardness [41] and microstructure of Hadfield steel over the pressure range of 0.4-48 GPa (pulse duration of 0.065 s, 0.230 ls, and 2.2 ps). The results are shown in Fig. 7.8. In addition to the very pronounced effeet of pulse duration on hardness shown in Fig. 7.8, postshoek electron microscope observations indicate that it is the final dislocation density and not the specific microstructure that is important in determining the hardness. 

A sheet of aluminium is rolled from an initial thickness of 10 mm to a final thickness of 5 mm. Using a transmission electron microscope, it was found that the dislocation density increases from = 10 m to q = 10 m . The prefactor in equation (6.20), needed to calculate the hardening contribution, is ky = 0.1. The length of the Burgers vector in a face-centred cubic lattice is h = y/2/2 a. The lattice constant of pure aluminium is a = 4.049 x 10 mm, the shear modulus is G = 26 200 MPa, and the Taylor factor is M = 3.1. Calculate the increase in strength due to this deformation ... 

Not all of the dislocations can contribute because of their orientation. Assume that the shear is in the x direction. If we consider screw dislocations, all dislocations with a line vector in the y direction can contribute (one third of all screw dislocations). Of the edge dislocations, only those with line vector in the y direction can contribute that have the additional half-plane in the 2 direction (one sixth of all edge dislocations). As we are interested in an estimate only, we can assume that about one fifth of all dislocations contribute to the deformation. This results in a final estimate for the dislocation density of about 3.5 X 10 °m l... 

Except for solidification of photovoltaic silicon, a cyhndrical crucible is normally applied with a seed channel at the bottom with a diameter of typically 5-lOmm. The seed channel is followed by a conical part with a certain cone angle. Within the conical part the diameter increases continuously from the value of the seed diameter up to the final crystal diameter in the upper cyhndrical part of the crucible. The main reason for this conical shape of the crucible is to use a seed with small diameter and eventually to reduce the dislocation density after seeding during the growth of the crystal in the seed chaimel [46]. The latter has been demonstrated... 

In this section, as examples of nonpolar GaN on lattice-mismatched substrates, the surface morphology and microstructure of a-plane GaN on an r-plane sapphire substrate and m-plane GaN on m-plane 4H-SiC are presented. Next, the SELO method of reducing threading-dislocation and stacking-fault densities is described in detail. This is followed by a description of the properties of the conductivity control of n-type andp-type nonpolar GaN, and the growth of the heterostructure/quantum well structure. Finally, the performances of the violet and green LEDs on nonpolar GaN are discussed with respect to the threading-dislocation density dependence of the output power. 

Finally, incoherent interfaces can be regarded as the limiting case of semicoherent interfaces for which the density of dislocations is so great that their cores overlap and that essentially all of the coherence characteristic of the reference structure has been destroyed. The cores of incoherent interfaces are therefore continuous slabs of bad material, and consequently the interfaces lack long-range order. 

---