The positron lifetime spectra of TiAl, Ti 50 Al 48 Zr 2 and Ti 50 Al 48 Nb 2 alloys were measured. The densities of free electrons of the bulk and microdefects in these alloys were calculated by using the positron lifetime parameters. The density of free electrons is lower in the bulk of TiAl alloy than that of Ti or Al metal which implicates that some valence electrons of Ti and Al atoms are localized when they aggregate to form TiAl alloy. The bonding characteristic of TiAl is a mixture of metallic and covalent. The large open defects occur on the grain boundary in TiAl alloy and the bonding strength of grain boundary is weak due to the low density of free electrons there, and the brittle fracture along grain boundary is prone to occur. As TiAl alloy is alloyed with ternary element of Zr or Nb, the densities of free electrons in the bulk and the grain boundary will simultaneously increase. The brittle fracture along grain bourdary and cleavage fracture of alloys are restrained. <