Geometrically necessary twins and their associated size effects

In metallic alloys or mineral crystals where twinning is a prominent deformation micromechanism, geometrically necessary twins (GNTs) have to be stored in order to accommodate plastic strain gradients, complementing or substituting the geometrically necessary dislocations (GNDs) that fulfil this function in materials deforming exclusively by crystallographic slip. The presence of GNTs will contribute to strain gradient hardening, just as GNDs contribute to it. Consequently, GNTs will be responsible for size effects.