Transmission electron microscopy investigation of Ti2Al precipitation in titanium aluminides during high-strain torsion

Ti-45Al-4(Cr, Nb, Ta, B) was deformed in torsion at 900 °C under hydrostatic pressure of 400 MPa in a Paterson type rock deformation machine at a maximum constant shear strain rate of 2 × 10−4 s−1. The microstructure was investigated by transmission electron microscopy (TEM). TEM observation shows that a hexagonal Ti2Al phase with lattice parameters a = 0.29 nm and c = 1.39 nm coexists with Ti3Al (α2) phase at grain boundaries or within the grain interior of the TiAl (γ) matrix phase. The orientation relationship of Ti2Al with α2 and γ determined by electron diffraction is (0 0 0 1)<1 1 2¯ 0>Ti2Al//(0 0 0 1)<1 1 2¯ 0>α2//{1 1 1}<1 1 0>γ. The mechanism of Ti2Al phase formation is discussed.