{1 1 ¯ 01} tension twins and {1 1 ¯ 01}-{2 2 ¯ 01}/{2 2 ¯ 01}-{1 1 ¯ 01} double twins in the D019 ordered hexagonal α2-Ti3Al phase

Compared to the disordered HCP metals, the α ₂-Ti ₃Al phase is reluctant to twin due to its D0 ₁₉ ordered structure. Recently, abundant {20 2 ¯ 1} compression twins were observed in this phase. However, other type of twin has not been reported besides the {20 2 ¯ 1} ones. In this study, we observed the {1 1 ¯ 01} tension twins in the ordered α ₂-Ti ₃Al phase for the first time in the α ₂+γ two phase TiAl alloy. In addition, the {1 1 ¯ 01}< 1 ¯ 102> tensile mode occurs together with {20 2 ¯ 1}< 1 ¯ 014> compression twins already known for the α ₂ phase, and these two modes are often twinned internally. Two types of double twins ({1 1 ¯ 01}-{2 2 ¯ 01} and {2 2 ¯ 01}-{1 1 ¯ 01}) were observed. The structure of the various interfaces was examined with high-resolution electron microscopy. The factors governing the evolution of the observed twin architecture are discussed with particular emphasis on deformation induced internal stresses and the interchange shuffles necessary to restore the D0 ₁₉ ordering of the α ₂ phase in its twinned counterpart. The transformation matrix was calculated for the observed two types of double twin structures, which explains the reorientation of the grains and the shear continuity, further confirming the double twin mechanism. The variant selection mechanism of the secondary twin is also discussed. These results indicate that abundant twinning activity can be initiated in the ordered α ₂ phase under severe deformation.