Extension twin variant selection during uniaxial compression of a magnesium alloy

► Activation of extension twin variants during compression of AZ31 investigated. ► Significant number of variants cannot be predicted using the Schmid factor (SF). ► At low strains twinning is delayed in the smallest grains. ► Wide scatter in relationship between twin fraction and SF in individual grains. ► Envelope for the maximum twinned fraction as function of SF can, however, be defined.