Positron lifetime measurements have been performed in binary TiAl alloys with Al contents from 47at.% to 53at.%. The experiment results indicate that the open volume of defect on grain boundary of all tested TiAl alloys is larger than that of a monovacancy of pure Al metal. In Al-rich TiAl alloys

with the increase of Al content

the open volume of defect on grain boundary increases while the density of free electron decreases. In contrast

in Ti-rich TiAl alloys

with the increase of Ti content

the open volume of defect on grain boundary decreases

and the density of free electron of the grain boundary increases

thus the cohesive strength between the grains increases. The duplex structure

α 2(Ti 3Al)+γ(TiAl)

was formed in Ti-rich TiAl alloys

and this give rise to the increase of the phase boundaries and the traps of positron. The effect of the microstructure of the alloy on the mechanical property has been discussed.