Mechanical twinning in a 45° (321) oriented polysynthetically twinned TiAl crystal at a high strain rate and a high temperature

Mechanical twinning behaviour in a 45° 〈321〉 oriented (the lamellar interface was tilted 45° from the loading axis about the 〈321〉 direction in the lamellar interface) polysynthetically twinned crystal deformed in compression at a strain rate of 3000 s^-1and a temperature of 800° C was characterized. (111) [112] parallel twinning was observed to be the dominant deformation mode in domains [III] and IV] where the transmission electron microscopy foil normals were close to [101] and [110] respectively. Cross-twinning of the (111) [112], (111) [112] and (111) [112] variants in the remaining domains was found to be a complementary deformation mode to accommodate deformation. The mechanical twin propagation stress was found to be less than 150 MPa at 3000 s^-1and 800° C in TiAl. Mechanical twin propagation was found to respond to the global sample stress state and thus the crystal yield and flow stresses. When mechanical twinning was the dominant deformation mode in a domain, it was observed to obey Schmid’s law.