In spite of being sheared along the so-called pseudo-twinning 2-bar 11 direction, y-TiAl undergoes true twinning under zero pressure or hydrostatic tension by means of a specific combination of 1/62-bar 11, 1/61-bar 2 1-bar and 1-bar 1-bar 2 shears in two consecutive (111) matrix planes allowing the adjacent twin to thicken over one (111) atomic layer. The corresponding total shear strain of 2 2~(1/2) is four times as large as that generated by conventional deformation twinning or during the Ll_0 to L_0 transformation by 1/611 2-bar or 1/62-bar 11 shears, respectively. This shear is substantially more effective in accommodating stress concentration and high strain rate than conventional deformation twinning. The conditions under which twinning by 2/32-bar 11 dislocations operates are interpreted based on a modified gamma-surface and discussed in terms of zonal partial dislocations.
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