Twin-twin interactions in magnesium

When twin variants interact, TTBs form and consequently affect twinning and detwinning processes. In this paper, we study twin-twin interactions by combining experimental observations and theoretical analysis. Mg single crystals are cyclically loaded in [0 0 0 1] and [101¯0] directions, respectively. Experimental characterization reveals the character of the twin-twin boundary and three kinds of twin-twin structures: a quilted-looking twin structure consisting of twins arrested at other twin boundaries, an “apparent crossing” twin structure which links twins impinging independently on each side of twin lamella and a double twin structure that results from secondary twins being nucleated at twin-twin interfaces. According to their crystallography, twin-twin interactions are classified into Type I for two twin variants sharing the same 〈112¯0〉 zone axis and Type II for two twins with different zone axes. For Type I twin-twin interactions, one twin does not transmit across the twin boundary and into the other twin. For Type II twin-twin interactions, one twin can transmit into the other only under some special loading conditions. In most cases twin transmission does not occur but, instead, twin-twin boundaries form that contain boundary dislocations. For Type I twin-twin interactions, the twin-twin boundary is a low angle tilt boundary with the habit plane being either the basal or the prismatic plane. For Type II twin-twin interactions, the twin-twin boundary is a high index crystallographic plane according to geometry analysis. Twin-twin boundary dislocations can be inferred by reactions of twinning dislocations associated with the two twin variants. An “apparent crossing” twin structure is thus a consequence of twin-twin boundary formation. Under reversed loading, detwinning is hindered because of the energetically unfavorable dissociation of boundary dislocations. Most interestingly, secondary twinning is activated at Type II twin-twin boundaries under reversed loading.