On the partial atomic volume of aluminium in the titanium-rich phases of the binary system Ti-Al

The dependence of the unit cell parameters upon the mole fraction of alumin ium as well as that upon temperature and also the macroscopic density were investigated in the homogeneity range (0.25 less than or equal to x(Al) les s than or equal to 0.33) of the intermetallic com pound Ti3Al (hP8, Ni3Sn t ype). Both hexagonal unit cell parameters, a and c, decrease with increasin g mole fraction of aluminium. For the compositions x(Al) > 0.25, titanium i s substituted by aluminium without structural-vacancy formation. The axial ratio c/a changes neither with temperature nor with mole fraction. The aver age atomic volume deviates strongly from the additivity of the pure element s titanium and aluminium. The partial atomic volume of aluminium in the tit anium-rich phases belonging to the magnesium family shows a significant sma ll value, (V) over bar(Al/Ti) = 14.75 Angstrom(3), and indicates (i) an eff ective charge transfer and (ii) the strong bonding between aluminium and ti tanium atoms in this advanced engineering intermetallic.