Microstructure formation and mechanical behaviour of titanium aluminides during torsion

Two-phase Ti-47Al-4(Cr, Nb, Mn, Si and B) alloys (at.%) with fine-grained globular and coarse-grained lamellar structures were deformed in torsion at 1000 degrees C under 400MPa hydrostatic pressure at a maximum shear strain rate of 2x10(-4)s(-1) in a Paterson-type rock deformation machine. The evolution of microstructural characteristics during thermomechanical processing was investigated by transmission electron microscopy (TEM). It is observed that the -TiAl + (2)-Ti3Al lamellar structure breaks down, and a fine-grained duplex globular structure with a grain size on the order of 5 m develops. TEM observation indicates that the phase transformation of (2) to occurring during torsion shows diffusional character. Ordinary dislocations 1/2[110] and superdislocations 1/2[11] are observed. The microstructural and mechanical characteristics indicate that superplastic flow occurs during torsion of titanium aluminides.