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.