It seems to me all the answers for your questions could be found in a new book by Prof. Rostislav A. Andrievskii published in 2016 in Springer. Or maybe in his previous works...

Very interesting question. I personally feel that the concurrent changes in the substructure will have significant effect on grain boundary dynamics and therefore the former has to be well understood. You may find information on substructure in nanocrystals in the article:http://www.sciencedirect.com/science/article/pii/037596019190435B

It is interesting that there is alternative opinion on nano-sized grain deformation which involves dislocation mechanisms. It is the fact that no dislocations are found inside the nanograins after deformation in the bulk of a sample but still we don't know what happens to grains during the deformation. It is inconceivable that nanosize grains are deformed by slipping but at the end of the deformation cycle all there dislocations sink to the grain boundaries. Extra contribution to this efffect may be from the foil preparation procedure including the effect of thinning.

I think there is a conceptual error in the chain of arguments. If you reduce the diameter of the wire the number of grain boundaries in the cross-section will be reduced but not the grain boundary density (SV), i.e. the grain boundary area per unit volume. There could be some sort of transition from polycrystalline to oligocrystalline behavior at some point though. Still, that doesn’t necessarily mean that the deformations mechanisms change unless the behavior of grains with a free surface is significantly different to those without.

However, you could be right with your assumption that the deformation mechanisms will change due to the fact that the structure will coarsen if you reduce the cross-section by wire drawing. This would change SV and consequently the deformation mechanisms. I’d recommend the following paper on deformation mechanisms in nanocrystalline metals with respect to their grain size:

http://www.sciencedirect.com/science/article/pii/S1359645413006289

Argument that dislocations sink to grain boundaries in nanograined material is very old. But how these dislocations appear?  Just calculate the critical stress necessary for dislocation nucleation. I will be much higher than   flow stress measured in experiments.

I agree with Dr. Yu. Ivanisenko completely.

Thank you Prof Igor L Shabalin, Prof Malur Srinivasan and Dr. Enrico for the useful insight and references. I am still processing the details in these articles to make sense in regard to the question. Thank you also to Dr. Yu Ivanisenko and Dr. Sergei Tarasov for the insight.

It is a challenging problem indeed since the dislocation plasticity is hard to accept here: ////// 1) As Dr. Ivaniseko said, even if dislocations crossed grain volumes, enormous stress would be needful to activate dislocation sources with the so short "ledge";/////////// 2) To keep plastic strain COMPATIBLE, difference in slip system orientations of neighboring grains should be compensated by internal stresses of the order of 0.1S (S about 0.01G is external stress, G is shear modulus), generated by induced interfacial defects; i.e. (b/d) should be about 0.001 (b-Burgers vector of lattice dislocation, d-grain size). Evidently, it becomes impossible for d

Article A Short Review on Mechanical Behavior of Nanocrystalline Materials

The grain size dependent deformation mechanism