Triple and double twin interfaces in magnesium—the role of disconnections and facets

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.