It was recalled that, in metallic alloys or minerals where twinning was a prominent deformation micro-mechanism, geometrically necessary twins had to be stored in order to accommodate plastic strain gradients; complementing, or replacing, the geometrically necessary dislocations that fulfilled this function in materials which deformed exclusively by crystallographic slip. The presence of geometrically necessary twins then contributed to strain gradient hardening, just as geometrically necessary dislocations contributed to it. Thus, geometrically necessary twins were expected to be responsible for size effects.

Geometrically Necessary Twins and their Associated Size Effects. J.G.Sevillano: Scripta Materialia, 2008, 59[2], 135-8