Transmission electron microscopy investigation of Ti 2Al precipitation in titanium aluminides during high-strain torsion

Abstract

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 Ti 2Al phase with lattice parameters a = 0.29 nm and c = 1.39 nm coexists with Ti 3Al (α 2) phase at grain boundaries or within the grain interior of the TiAl (γ) matrix phase. The orientation relationship of Ti 2Al with α 2 and γ determined by electron diffraction is ( 0 0 0 1 ) < 1 1 2 ¯ 0 > T i 2 Al / / ( 0 0 0 1 ) < 1 1 2 ¯ 0 > α 2 / / { 1 1 1 } < 1 1 0 > γ . The mechanism of Ti 2Al phase formation is discussed.