Pinning dislocation

Where is the ratio of the irradiated to unirradiated elastic modulus. The dislocation pinning contribution to the modulus change is due to relatively mobile small defects and is thermally annealable at 2000°C. Figure 13 shows the irradiation-induced elastic modulus changes for GraphNOL N3M. The low dose change due to dislocation piiming (dashed line) saturates at a dose <1 dpa. 

Figure 3.13 Dislocation pinned at each end (a) can respond to stress by bowing out (b, c, d, and e) to form a dislocation loop and reform the pinned dislocation (/). The growth of the dislocation represented in (a)-(c) requires increasing local stress, whereas the steps in (d)-(f) are spontaneous once the point in (c) is passed.

Figure 3-3. Representation of dislocation movement in a Frank-Read dislocation source under stress a. Multiplication of dislocation pinned at a distance l.

Figure 3.23 A dislocation pinned at each end (part a) can respond to stress by bowing out (parts Iv l) to form a dislocation loop and reform the pirmed dislocation (part e)...

Figure 4.51 indicates that the yielding in sapphire undergoing basal slip is a consequence of dislocation multiplication and is not due to the unpinning of a Cottrell-type atmosphere, where dislocation pinning results from impurities. The study of two types of sapphires, with different initial dislocation densities, was meant to point out the difference in their surface dislocation densities and the consequent differences in their yield phenomena. 

Dislocation pinning Stiffening mechanism (typically experienced in metallic systems) in which defects hinder the motion of dislocation planes. For a more in-depth discussion, see a metallurgy text. 

Another possibility to increase the dislocation density is a spiral dislocation, pinned at the centre. Similar to a dislocation loop, the spiral dislocation extends when shear stresses are acting on it. As its centre is pinned, the length of the spiral grows (figure 6.20). This process does not increase the number of dislocations, but their density. 

Let there be several obstacles in our material with a distance of 2A between them (figure 6.23). Consider a dislocation pinned on these obstacles. When the external stress r acts on the dislocation, it tries to move on and bows out. Its shape is a segment of a circle because this covers the greatest area with the least-most energy to create new length of dislocation line. 

In this case, the strain rate is determined by the rate of emission or absorption of vacancies. Figure 11.5 shows the example of two edge dislocations pinned at two obstacles. Dislocation 1 has to absorb vacancies to climb dislocation 2 needs to emit them. Thus, vacancies can be transported from one dislocation to the other, with one dislocation acting as vacancy source, the other as vacancy sink. The vacancy current density, j, determines the rate of deformation. This quantity can be estimated. 

B.T. Kelly, A. Foreman, Theory of irradiation creep in reactor graphite — dislocation pinning-unpinning model, Carhon N.Y. 11 (1973) 694. 

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