Dear Bharat Reddy,
such an elements as H, O, N, ... usually creates atmospheres under dislocation line, called Cottrell, Maxwell or Suzuki atmosphere. These atmospheres make dislocations more difficult to move. Only Suzuki atmosphere, created between partials dislocations may cause ambivalent reaction, and make dislocations easier or difficult to move.
In addition, these soluted atoms can form precipitates, which acts as obstacles for dislocation moving. Both effects is determined by diffusion moving of soluted atoms H, O, ...
The presence of interstitial atoms such as Hydrogen and Oxygen can affect dislocation mobility significantly. This occurs in many FCC metals as well as other metals like Plutonium and Uranium. The interstitial elements have also been shown in our studies to increase the stacking fault energies resulting in greater yield strength. Like the previous author mentioned, the hydrogen in Pu and U can form hydrides, which in turn can change mechanical behaviors through Orowan strengthening mechanisms.
N, C, O and H solutes are interstitial-impurities and locate at tetrahedral and octa-hedral vacant sites in crystal units. this atoms introduce an extra stress on atomic structure and increase total energy of system. in this condition, these atoms trend to diffuse to sites with higher atomic distance. below the dislocations line, due to local fault in atomic arrangement, distance between atoms is higher than other locations in system. in other word, dislocation line act as a sink for solutes. as a result, there in a absorption between solutes and dislocation. then higher shear stress would be required to separate dislocation kine from absorbed atoms and activate dislocation movement. it means that these solutes reduce mobility of dislocation. such condition result into strain hardening in some steels.
I want to correct one item in my answer (final line):
such condition result into strain aging in some steels. (strain hardening is wrong)
the answer of Xu Wang can not be correct. Bharat Reddy mentioned H and O as solute in this question. in the W and Mo metals, addition of Re may be resulted into facilitate dislocation motion, but H and O solutes almost led to inhibition of its motion.
Here is apossible michanism for H/dislocation interaction:
Softening and hardening of yield stress by hydrogen–solute interactionshttp://www.tandfonline.com/doi/abs/10.1080/14786435.2016.1263402
Anyways, there have been some solid solution weaking theories claim solution weakening (aids dislocation motion) would occur at specific interstitial concentrations.
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