Twin boundaries showing very large deviations from the twinning plane

In deformation twinning, twin boundaries (TBs) should coincide with the twinning plane. Here we show that the TBs of the most common twinning mode in hexagonal close-packed metals, {101¯2}〈101¯1¯〉, may not lie on the {101¯2} twinning plane. Examinations using transmission electron microscopy (TEM) reveal that the TBs in Co and Mg deviate significantly from the {101¯2} plane. High-resolution TEM confirms that the incoherent TBs entirely depart from the twinning plane with a magnitude greater than 45°.