This might help. From my 2nd year lecture course.  Steve Roberts

This might help. From my 2nd year lecture course.  Steve Roberts

When solute and solvent atoms differ in size, local stress fields are created. Depending on their relative locations, solute atoms will either attract or repel dislocations in their vicinity. This is known as the size effect. This allows the solute atoms to relieve either tensile or compressive strain in the lattice, which in turn puts the dislocation in a lower energy state. In substitutional solid solutions, these stress fields are spherically symmetric, meaning they have no shear stress component. As such, substitutional solute atoms do not interact with the shear stress fields characteristic of screw dislocations. Conversely, in interstitial solid solutions, solute atoms cause a tetragonal distortion, generating a shear field that can interact with both edge, screw, and mixed dislocations. The attraction or repulsion of the dislocation centers to the solute particles increase the stress it takes to propagate the dislocation in any other direction. Increasing the applied stress to move the dislocation increases the yield strength of the material.

Excellent reply by Anshuman. I can only add that one should also take into account diffusion and thermal activation for dislocation movements. For some alloys at some point at certain strain rate / temperature you'll have dynamic strain aging condition.

Hähner models describe it in detail for instance

ftp://dinamico2.unibg.it/erizzi/CV/HaehnerRizziTRDI0602.pdf

Hm, strange, just last week I could access it. But google schoolar should provide plenty of different articles on the subject

This should in free access it's my PhD in the summary part I have discussed some basics of DSA as well

http://lib.tkk.fi/Diss/2010/isbn9789526034454/

This video may help you!!!

https://www.youtube.com/watch?v=RUuLusenhfA

@ MR. Steve Roberts could u please mention the references also , since the pdf which you have attached is of great use and i would like to go a bit deep into the subject. thanking you

I like the argument that a shear component in the strain field of the interstitial solute may make dislocation motion more difficult. This could maybe also related to the formation of clustering and ordering of substitutional solutes in alloys of higher concentration. I personally observed it in binary Ti-Al alloys under formation of Ti3Al, and GP-zones in Al-Cu alloys may act similarly.

Mr. Arnas Fitzner , due to the clustering  and ordering of solutes , do they form any stretcher stains?

since , at the dislocations if solute atoms forms clusters and when stress is applied , the velocity at which the dislocation moves should be equal to the velocity at which the solute cluster should also move , but it doesn't happen. as the velocity of solute atoms are less compared to the dislocation. the result of this effect is stretcher strain.

have u observed any, if so could u give me a more clear idea.

Kranthi, I hear stretcher strain the first time but it seems to be equivalent to slip bands or so called Luders bands. In that case I have observed them and they seem to become more intense with increasing Al addition. A journal publication is in preparation to discuss and visualise this detailed. For now you may look on page 19 of the attached link to see one published image. 

Is that what you would image under stretcher strains??

http://www.latest2.manchester.ac.uk/documents/LATEST2%20Mid%20Term%20Report%202010%20-%202013.pdf

I add on to the answer given by Prof. Steve Roberts (univ. of Oxford). Please see the attached notes (draft) on dislocation - solute interaction, serrated flow and dynamic strain ageing.

I forgot to mention the classic papers on interaction of screw dislocation with solute.

1) A. W. COCHARDT, G. SCHOEK, and H. WIEDERSICH, Acta Metall., Vol. 3, 1955, p. 533.

2) R. L. FLEISCHER, Acta Metall., Vol. 11, 1963, p. 203.

Best wishes.