5. Surface morphology Etching treatment with an etchant removes slip steps leaving no trace, but reveals twins by delineating the interface between the twin and the matrix 6. Conditions ① Structure: twinning is easy to take place in crystals of low symmetry ② Temperature dy ③ Strain rate dt 7. Twinning plays an essential role in BCC and HCP lattices5. Surface morphology: Etching treatment with an etchant removes slip steps leaving no trace, but reveals twins by delineating the interface between the twin and the matrix. 6. Conditions: ① Structure: twinning is easy to take place in crystals of low symmetry. ② Temperature ③ Strain rate dt d 7. Twinning plays an essential role in BCC and HCP lattices