This is a good question. Not an easy one to answer. Dislocation climb down is one possible mechanism. Another possibility is two opposite edge dislocation on parallel but adjacent slip planes coming together. If they are on the same slip plane they will annihilate. But if they are are on adjacent slip planes they may generate a row of interstices or a row of vacancies depending on whether the half-planes overlap or not. However, the increase in concentration of vacancies due to deformation may not be very high.
No. During recovery, there is no SIGNIFICANT change in dislocation density. A slight decrease can happen due to a combination of dislocations of opposite signs.
During recovery, we are only heating the workpiece. How is then the dislocations are moving? Previously we studied movement of dislocation under the action of external forces. But here, there is no application of force. Is it because of diffusion of atoms?
This is indeed a very intresting question. 'Interesting' means not an easy one two answer. Let me try. There is attractive force between two dislocations of opposite sign on the same slip plane. But this may not be sufficient to move them as a minimum critical force is required for movement. I guess that the thermal energy helps the dislocation in overcoming the barrier. For edge dislocations of the same sign on parallel planes there is a climb force as well. This is what creates low-angle tilt boundary in a process called polygonization. But climb requires vacancy movement which is again assisted by higher tempearture. These are my current guess.I think I have to give a more careful thought to sort this out. Thanks for asking.
In Annealing we are heating the specimen so number of vacancies should increase according to relation, n = N*exp(-H/RT). But in this lecture you are saying that point defects decrease. Sir, kindly justify it.
The comparison is being made between the vacancy concentration in deformed and annealed samples, both at room temperature. Let NER and NEA be the vacancy concentration in equilibrium at room temperature and the annealing temperature respectively. Obviously NAE>>NTE. But the deformed sample at room temperature has a higher vacancy concentration NDR=NER+DN. Upon annealing the excess vacancy concentration DN disappears. On cooling back to room temperature one again gets a equilibrium concentration NER. Thus the decrease in vacancy concentration during annealing is NDR->NER or NER+DN->NER. The sequence of evolution of vacancy concentrations is as follows: NER (undeformed, room temperature)-> NER+DN (Deformed room temperature)->NAR+DN (At annealing temperature, beginning of annealing)->NAR (at annealing temperature, end of annealing)->NER(at room temperature, cooled after annealing).
