Fig. 1. (a) TEM micrograph of a {10$\bar{1}$1} twin. (b) Schematic illustration of {10$\bar{1}$1} twin in a HCP unit cell. (c) Enlarged micrograph of the twin tip marked in (a). (d) SAED pattern corresponding to the {10$\bar{1}$1} twin along [$\bar{1}$2$\bar{1}$0] zone axis, yellow arrows indicate the SFs on (0001)T plane and (e) enlarged micrograph of the front part of the twin marked in (a). The green, red, and yellow lines represent the CTBs, steps and SFs, respectively.

Fig. 2. (a, b) HRTEM images of {10$\bar{1}$1} twin tip. (c, d) Enlarged HRTEM images of Step A and Step B, respectively. The dashed yellow lines represent the boundaries and the white arrow shows the SF in the twin. The insets of atomic planes schematically show the stacking sequence of the close-packed planes, in which the yellow, blue, and green balls represent the layer-A, layer-B, and layer-C, respectively. The red vector circuits are drawn surrounding Step A and Step B, and the direction vector of each circuit segment along the [1$\bar{2}$10] projection is accordingly listed the built-in tables, in which Λ= $ (2/3)^ \frac{1}{2}$c/a, P is the transformation matrix from the coordinate frame of the twin (marked by a subscript T) to that of the matrix (marked by a subscript M). (e, f) εyy strain maps of Step A and Step B. (g) Strain distribution curves of the lines 1-3 marked in (e) and (f).

Fig. 3. (a, b) The atomic marked HRTEM image and corresponding simulation diagram of Step A. The dashed circles and dashed red lines represent the ideal position of the atoms and the (0001)M planes, respectively. The inset shows the difference of the atomic arrangement and lattice spacing in the (0001)M and ({10$\bar{11}$})T planes. (c, d) The atomic marked HRTEM image and corresponding simulation diagram of Step B. The yellow, blue, and green balls represent the atoms on layer A, B, and C, respectively. The inset shows the two-layer FCC stacking sequence.

Fig. 4. (a, b) Schematic illustration of the high density SFs expanding with the {10$\bar{1}$1} twin thickening. (c, d) HRTEM images of the PSFs terminated at the bottom TBs of the {10$\bar{1}$1} twins.