Fig. 1. (a) Scanning electron microscopy (SEM) image showing the lamellar structure of α2-Ti3Al and γ-TiAl phases in as-grown polysynthetic twinned (PST) TiAl samples. Inset presents a SEM image with higher magnification showing detailed α2-Ti3Al lamellae. (b) Transmission electron microscopy (TEM) image of primary twin lamellae in the (111ˉ) plane of γ-TiAl in the as-grown samples. (c, d) TEM images showing primary twin lamellae in the (111ˉ) plane (c) and secondary twin lamellae in the (11ˉ1ˉ) plane inside a primary twin lamella (d) in the impact deformed samples.

Fig. 2. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image showing a microtwin tip viewed along [1ˉ10] zone axis. The strong and weak bright spots stand for Ti and Al atom columns, respectively. The Burgers circuits of the dislocation b1, the superposition of dislocations b1 and b2, the superposition of dislocations b1, b2 and b3 are outlined by the yellow, white solid lines, and red dotted lines.

Fig. 3. (a) HAADF STEM image showing a step dislocation with the Burgers vector of 1/2?<?110> at the coherent twin boundary (TB) viewed along [101] zone axis. (b, c) Schematic illustrations showing the formation processes of the 1/2[11ˉ 0] dislocation (b) and 1/2[01ˉ1ˉ] dislocation (c), respectively.

Fig. 4. (a) HAADF STEM image showing two step dislocations with the Burgers vector of 1/3[11ˉ1ˉ] at the coherent TB viewed along [101] zone axis. (b) Schematic illustration showing the formation process of the step dislocations in (a).

Fig. 5. (a) HAADF STEM image showing a step dislocation with the Burgers vector of the 1/6?<?211> at the coherent TB viewed along [101] zone axis. (b) Schematic illustration showing the formation process of the 1/6[211] dislocation.

Fig. 6. HAADF STEM image showing the antiphase domain boundary (APDB) indicated by yellow dotted line viewed along <100] zone axis. (001) planes are marked by red solid (Ti) and dotted (Al) lines.

Fig. 7. (a) HAADF STEM image of an abnormal stacking fault between atom layers L1 and L2 viewed along [1ˉ 10] zone axis. The corresponding Burgers circuits show that the Burgers vectors of dislocations b1-b5 are 1/12[112], 1/12[1ˉ1ˉ2ˉ], 1/3[111ˉ], 1/2[1ˉ01] and 1/6[1ˉ1ˉ2ˉ]. (b) Geometric phase analysis (GPA) result of (a) showing strain along [111ˉ] direction. The yellow rectangular box denotes the abnormal stacking region. Magnified image (c) and structural model (d) of the abnormal stacking fault. (e) The simplified atom stacking illustration showing the slipping result of the abnormal stacking dislocation b1. The A′, B′ and C′ indicate that the change of atom occupation is induced by the shear vector of 1/12[112] (1/4[1ˉ10] + 1/6[21ˉ1]) viewed along [1ˉ10] zone axis in γ-TiAl.

Fig. 8. Schematic of the shearing process for the abnormal stacking fault’s formation viewed along [111ˉ] direction.

Fig. 9. (a, b) TEM images of a twin intersection with slightly different tilting angles away from the [1ˉ10] zone axis. (c) HAADF STEM image showing grain boundaries in the intersection area. (c) was obtained at the region R1 in (b).

Fig. 10. (a) TEM image of an ordinary twin intersection with constant number of incident twin viewed along [1ˉ10] zone axis. (b) Schematic illustration showing the formation process of the twin intersection in (a). IT and OT stand for incident and obstacle twin.