Question

Dislocations can work as sinks and sources of vacancies. Explain.

Answer 1

Answer:

As we know that the lines and the burger vector for any edge are perpendicular to each other. There is always a plane which is available and on that plan ethe dislocation can slip.There are also some other mechanism available by which dislocations can be moved to the different plane and that is known as climb.So the climb is the phenomenon by which an edge dislocation is moved from one slip plane to the another slip plane by the incorporation of vacancies or by the atoms.

As we defined the climb above, climb can be positive or negative.In case of the positive climb, the dislocation which is available acts as a vacancy sink by which it absorbs a vacancy to shift itself in the upward direction with relation to its initial position. While in case of negative climb, Dislocations acts as a vacancy source, in which the vacancy which is available at the bottom from the half plane which is extra, is replaced by generally by an atom, it caused the dislocation to shift in the downward direction.

At some arbitrary temperatures, slobal thermal equilibrium can only be acheived if there is some mechanism for the total annihilation of the point defects. Indirectly it says that there must be source and sinks for the vacancies and also some interstitial sites which generally work with the very low activation energy. Other wise to reach the global equillibrium, it will take to much time.

In the case of real life scenerio, we need some defects as grain boundaries, crystal melt surfaces, dislocations and precipitates which acts as source and sinks for the point defects present in the crystal structure.Grain boundaries and the dislocations acts as the sources and the sink for the vacancies so hat dislocation is not accumulated in the grain interior structure. The grain boundary of dislocations are absorbed by the grain boundaries.

The above metioned statement is generally applicable perfectly to the clinb model. High concentration of jogs are available with and along the dislocation lines this can also be said like that dislocation acts as source and sink for the vacancies. Also the vacancies are absorbed or emitted bt the climbing dislocation. These climb properties are acheived at high temperatures and high energy states.

Physical behaviour of materials are significantly affected by the atomic vacancies which acts as the point defects in the crystal lattice.