Dislocation reactions

It may be energetically favorable for a dislocation, b, to spht into two dislocations if the product dislocation Burgers vectors and satisfy the condition b >b -hb. Dislocation reactions can even produce stable imperfect dislocations, if they result... 

The parent dislocations are in the same (I I I) plane. Thus, the product dislocation is also in this plane. Thus the dislocation reaction is ... 

Several models for the creation of interfaces by dislocation reactions have been surveyed in [5], The basic idea is shown in fig.2. In the interior of one crystal dislocations of opposite sign move in opposite directions. They... 

The micromechanisms of deformation and in particular the dislocation reactions have been analyzed in detail and have been discussed with respect to strength and ductility (Koss etal., 1990 Kim and Froes, 1990 Yamaguchi and Umakoshi, 1990 Froes et al., 1991 Umakoshi et al., 1993 a, b). In the DOjg structure five independent slip systems are possible (Kim and Froes,... 

In ionic crystals, the following dislocation reaction can occur. 

Suggest an energetically favorable dislocation reaction for the splitting of a unit dislocation into partial dislocations for basal slip in sapphire (a-alumina) single crystals. 

The following dislocation reaction has been proposed for dislocation motion in graphite ... 

Lomer WM (1951) A dislocation reaction in the face-centred cubic lattice. Phil Mag 42(334) 1327-1331... 

Fig. 3.60 A Thomson s tetrahedron helps in visualizing dislocation reactions in FCC structures on 111) planes. Decomposition into Shockley and Frank paitials is shown...

This LD was formed by the interaction of two nondissociated full dislocations with Burgers vectors a/2[l01] and a/2[110], respectively, moving under applied stress on two intersecting slip planes, (ill) and (111). The dislocation reaction may be written as ... 

Fig. 5.32 Dislocation substructure in 9.4 mol% Y203-stabilized Z1O2 sample deformed along (112) by 001 (110) slip to iil4 % strain at 1400 °C in air, shown as stereo parr with 6 = 17°, using b = 220 in [00 1] zone. Loops Ly and L are of vacancy and interstitial character, respectively. Dislocation reaction at node N gives rise to dislocation segments with different Burgers vectors (Si, S2 and S3) [6]. With kind permission of John Wiley and Sons...

T. Kawabata and O. Iztuni, Dislocation Reactions and Fracture Mechmiism in TiAl LL Type Intermetallic Compound, Scr. Metall., Vol 21,1987, p 435-440... 

Wadsleyite ((3-Mg2Si04) is a high-pressure polymorph of forsterite. In wadsleyite deformed at pressures of 14 GPa and 1450 °C, dislocations with [100] Burgers vectors were identified, many in tangles, although creep had caused numerous dislocations to form walls [428]. In wadsleyite synthesized from forsterite in a multianvil apparatus and deformed in compression in another multianvil apparatus at 15-19 GPa and temperatures ranging from room temperature to 1800-2000 °C, TEM and LACBED studies identify [100], <111 >, [010], <101>, and [001] dislocations [85], the lattermost resulting from dislocations reactions, not slip. Wadsleyite is elastically almost isotropic but slip occurs predominately on planes that do not break Si-O bonds (e.g., (010) and (001)) and dislocation dissociation is crucial [86]. 

There are several approaches for dislocation density reduction [35]. It was shown [36, 37, 38] that the number of dislocations decreases with layer thickness, which results from dislocation reactions. The experimental behavior of dislocation density versus layer thickness h was found to be proportional to 1/h", where n = 0.66 [39]. This means that to obtain dislocation density in GaN in the mid 10 " cm , the thickness of the layer should be in submillimeter range. However, cracking can appear in thick layers due to difference in thermal expansion coefficients between the substrate and the layer. Experimental [40] and theoretical calculations [41] show that screw and edge TDs in GaN are parallel to the [0001] direction and only mixed type... 

The next possible mechanism that we wish to consider is based on the following idea. It was noted above that in disordered alloys grain boundaries are usually strong and that an embrittling impurity is required to cause them to be weak fracture paths. Consequently, it has been thought that if the material near the grain boundaries became disordered in these compounds, an improvement in ductility would be realized. King and Yoo (1987) formally demonstrated that disorder would allow many additional dislocation reactions to occur at... 

Once plastic deformation has started in a specimen an increase in stress is needed to produce further deformation. In microscopic terms this means that as deformation proceeds, the movement of dislocations on their slip planes becomes progressively more difficult. This hardening of the material may arise from elastic interactions between dislocations, through their strain fields, it may arise from dislocation reactions that produce segments that cannot sUp and it may also arise from interactions with other defects such as vacancies, impurities and grain boundaries. 

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