Fig. 1. (a) Tensile engineering stress-strain curves of the samples with strain levels of 0, 0.12 and 0.63; (b) Yield strength and tensile strength of the samples with strain levels of 0, 0.12 and 0.63.

Fig. 2. Macroscopic optical images of the samples with deformation strains of 0.12 (a) and 0.63 (b); (c,d) macroscopic composition distributions of samples with strain of 0.63

Fig. 3. EBSD and high-resolution emission transmission electron microscope (HRTEM) results of the samples with deformation strains of 0.12 and 0.63. (a-c) and (d-f) are the EBSD results of the cross-sectional samples with strain levels of 0.12 and 0.63, respectively. Most HCP structures (ε-martensite, indicted in blue in Fig. 3 (c) and (f)) formed near the coherent Σ3 60° deformation twin boundaries (marked by red lines). (g) and (h) are the HRTEM images of ε-martensite shaped with a five-layer zipper in the samples with strain levels of 0.12and 0.63, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4. (a),(b) TEM bright field image and schematic diagram of triangular intersecting twins in sample with a strain level of 0.63. (c),(f) [01$\bar 1$] and [10$\bar 1$] electron diffraction patterns of matrix and the selected three sides of the triangle in Fig. 1 (b). All sides of the triangle were proven to be twins.

Fig. 5. (a) Schematic diagram of intercepting tetrahedral twins by a certain crystallographic plane. (b,c) Comparison of the CBED results with the standard Kikuchi diffraction patterns provided by spherical Kikuchi maps. Good agreement between the calculation and experimental results proved the existence of the tetrahedral structures bounded by twins.

Fig. 6. (a) Schematic diagram of intersection structure patterns of folded lamella-twins. (b-d) Corresponding interception type of pattern 1, pattern 2, pattern 3 and pattern 4 in Fig. 6 (a).

Fig. 7. (a,b) Geometrically necessary dislocation density distribution maps of the sample with deformed strain of 0.63, circumferential direction (a) and drawing direction (b), respectively; (c) Enlarged image of the red box area in Fig. 7 (b). (d) Histogram of the geometrically necessary dislocation density changes on the triangular path (A-B-C-A) marked in Fig. 7 (c) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 8. Curve of stress concentration factor $K_2^*$ versus the polar angle φ at the boundary of a triangular hole.