MSE 57: Dislocation cannot end abruptly in a crystal: Dislocation loop

MSE 57: Dislocation cannot end abruptly in a crystal: Dislocation loop

[@vishnu6398]

What an amazing lecture, Prof Rajesh is Leonardo Da Vinci, good at science and art as well, from Prismatic dislocation loop onwards, it is crystal clear. But before that it is hard to imagine. At 3:46 tangent vector is moving along the core of the dislocation (like in an edge dislocation, if we imagine the tangent vector parallel to the core/line of the dislocation) then burger vector is perpendicular to the tangent vector along that core of the dislocation. The relation between tangent vector and burger vector is same everywhere along that ellipse/circle.

[@user-xv9yi9ti7v]

THank you sooooo much!!

[@hitesh7469]

Sir, In the first figure, how did you assign signs such as _ve and +ve to edge dislocation and RH and LH to the screw dislocation ? Are these arbitrarily chosen or there is any convention ?

[@rajeshprasadlectures]

-Ve and +Ve edge dislocations are in fact arbitrary. There is no physical difference between them. If I turn a model containing +ve edge dislocation, with extra half plane above the slip plane, upside down then the dislocation becomes a -Ve edge. But RH and LH screw dislocations are physically distinct just as RH and LH mechanical screws are physically distinct.

[@hitesh7469]

@@rajeshprasadlectures Thank you so much Sir

[@sumandutta876]

@@rajeshprasadlectures sir why have you taken the Bottom point as negative and above one as positive edge isn't clear even after this comment .

[@vishalpannu9504]

Sir, You're videos have been very helpful and this one is also not an exception. I just couldn't understand why the burger vector was directed upwards in the end? Is it because the upper and lower planes will be moving towards it i.e. perpendicular to it's plane?

[@rajeshprasad101]

If you see carefully, in the last figure half plane is horizontal. The dislocation line, perpendicular to this half-plane is vertical. The Burgers vector thus also is vertical. It can point up or down based on the sense of line vector chosen.

[@vishalpannu9504]

Ohkay Sir. Thank you

[@pailasaisravan5423]

@@rajeshprasad101 sir could you please eloborate it i couldnt understand it why burgers vector direction has taken in the vertical direction . the dislocation line is also in horizontal plane according to the picture .

[@sarveshswamy9708]

Very nice lecture, thank you sir

[@introductiontomaterialsscience]

Best wishes Sarvesh.

[@g.prasanthswamy1716]

Sir why burgers vector is constant when dislocation line is in closed loop even tangent vector changes......???

[@rajeshprasadlectures]

Tangent vectors change as the orientation of the line changes. But the line is still a boundary between slipped and unslipped regions. The magnitude and direction of the slip are not changing. Thus the Burgers vector is costant.

[@Vickysahu0092]

Sir, why RH screw disclocation shown by CCW direction? Is it a convention ? Same for LH screw dislocation.

[@rajeshprasadlectures]

You can imagine the stretched thumb of the right hand to be coming out of the paper. Then the fingers of the right hand will curl CCW. Therefore RH screw is shown by CCW curved arrow. Similarly for LH screw.

[@Vickysahu0092]

Thank you very much sir.

[@mahdiziaei2201]

Or is it possible to have a ring of dislocation where all points have a screw dislocation?

[@introductiontomaterialsscience]

A dislocation ring cannot be purely screw. The Burgers vector remains the same everywhere whereas the line vector is changes direction. Thus they cannot be parallel or antiparallel at all points.

[@mahdiziaei2201]

Why does the dislocation in the crystal not end?

[@blaz988]

watch the first video on this theme (with this headline)

[@ashutoshsingh-pk4tv]

Respected Sir, How to decide whether the screw dislocation is LH or RH.

[@introductiontomaterialsscience]

For the convention I am using, b parallel to t is LH, and b antiparallel to t is RH.

[@ashutoshsingh-pk4tv]

@@introductiontomaterialsscience thank you very much sir

[@gunasai9039]

Sir, when a material is quenched from high temperature to room temperature, the equilibrium concentration of vacancies is reduced. And, you said that to reduce this number of vacancies, there is a formation of vacancy loop. But, how this number is reduced? Do we need to consider an entire loop as one vacancy?

[@rajeshprasadlectures]

There is no vacancy inside the prismatic dislocation loop created by condensation of several vacancies as a disc on an atomic plane. What happens that after such condensation the atomic planes above and below the disc find themselves at a distance 2d instead of d, the spacing between the atomic planes. Thus to reduce energy, the planes collapse towards the disc decreasing their distance to d and eliminating all the vacancy. However, in this process, the boundary of the disk becomes an edge dislocation loop with the Burgers vector perpendicular to the plane of the loop. The net result is that vacancies are eliminated and an edge dislocation loop is created. This will happen if there is an overall decrease in the free energy of the system.