Habit Planes of Twins in a Deformed Mg Alloy Determined from Three-Dimensional Microstructure Analysis

Serial sectioning and electron backscatter diffraction were used to image the three-dimensional microstructures of Mg alloy AZ31B deformed in tension and compression along sample directions that promote twinning. The habit planes of boundaries within the samples were determined from these measurements. For the sample deformed in tension, the boundary population is dominated by the so-called extension twin (lattice disorientation of 86° about [21 1 0] with symmetric {011 2} boundary planes) and boundaries formed by the intersection of these defects (lattice disorientation of 60° about [011 0] with {112 1} and {011 1} boundary planes). The sample deformed in compression, however, was different. In this case, the dominant boundary also had the lattice disorientation of 86° about [21 1 0], consistent with an extension twin, but the boundary plane has a broad range of asymmetric orientations in the [21 1 0] zone ranging from (01 11) to (011 3). It is proposed that the extension twin in the compressed sample initially formed to elongate the sample in the direction perpendicular to the compressive load. Later, during plastic deformation from the compressive load, dislocations intersect the twin boundaries, creating boundary segments with basal||prismatic orientations that change the orientation of the boundary plane in the [21 1 0] zone. Although such boundaries have been studied theoretically and detected by transmission electron microscopy, this work provides a statistical measurement of the distribution of twin boundary orientations within deformed magnesium alloys.