PPT Slide
A single crystal subjected to a shear stress can deform extensively with slip on only a single slip system. However, this is not the case for polycrystals.
Since each grain in a polycrystal has a different crystallographic orientation, each will respond differently (slip in a different direction) when subjected to a shear stress.
Thus, unless the region around grain boundaries can be undergo arbitrary shape changes, voids at grain boundaries will be opened up and the material will fracture at low strains.