Grain boundary hardening of poly-crystals
Compared to single crystals, poly-crystals tend to have higher yield stresses. This is because each grain in the poly-crystals has to undergo a complex shape change which is consistent with those of their neighbors, requiring multiple slips from the start. Therefore, unlike single-crystals, poly-crystals do not exhibit any kind of ‘easy glide’ when they are deformed.
Below is an explanation of how grain boundary hardening arises in a poly-crystal: