PPT Slide
This condition of strain compatibility leads a polycrystalline sample to have multiple slip in the vicinity of grain boundaries.
The smaller the grain size, the larger will be the total boundary surface area per unit volume.
In other words, for a given deformation in the beginning of the stress-strain curve, the total volume occupied by the work-hardened material increases with the decreasing grain size.
This implies a greater hardening due to dislocation interactions induced by multiple slip.