L12-TYPE TERNARY TITANIUM ALUMINIDES AS ELECTRON-CONCENTRATION PHASES

The structural change from aluminium-rich titanium aluminides (TiAl2 or TiAl3) to L1(2) type ternary titanium aluminides is examined in terms of the two alloying variables; these are atomic radius ratio (R(A)/R(B)) and electron concentration (e/a). Similarity in the stoichiometry of the L1(2) alloys (generally Ti25X8Al66 where X = Cr, Mn, Fe, Co, Ni, Pd, Ag or Zn) and the differences in the atomic radii of ternary alloying elements makes it difficult to use R(A)/R(B) criterion as an alloy variable. It is also found that the structural change in these alloys cannot be explained by the classical definition of e/a because it gives an increase in this ratio rather than a decrease upon the addition of the ternary alloying element to TiAl2 or TiAl3. On the other hand, if a definition given by the Engel-Brewer theory is used, a decrease in e/a is found to occur upon the addition of ternary alloying element to either TiAl2 or TiAl3 with the consequence of achieving L1(2)-type ternary titanium aluminides at definite e/a values around 2.5.