Observations of the Atomic Structure of Tensile and Compressive Twin Boundaries and Twin–Twin Interactions in Zirconium

The boundary structures of twins in the hexagonal close-packed metal zirconium were studied. High-resolution transmission electron microscopy was used to characterize the boundary structure of { 10 1 ¯ 2 } (T1), { 1 ¯ 1 ¯ 21 } (T2), and { 1 ¯ 1 ¯ 22 } (C1) twins on the atomic level. Basal–prismatic (B–P) plane faceting is observed along the T1 twin boundaries, matching previous observations of T1 twins in magnesium. C1 twins are observed to form basal–pyramidal (B–Py) facets along otherwise perfect twin planes. T2 twins exhibit faceting that aligns prism planes with second-order pyramidal planes across the boundary (P–Py facets). As a function of the crystallography, T2 twins appear less likely to accommodate large deviations from perfect twin planes by P–Py faceting alone, and may rely on small dislocation-accommodated facets to achieve arbitrary boundary planes. The structure of these boundaries, specifically the modes by which faceting is permitted, has a direct impact on boundary mobility. In addition, the boundary structure of two C1 twins during a twin–twin interaction event is observed, and is compared to previous observations of tensile twin–twin interactions in magnesium.