L12-type ternary titanium aluminides as electron concentration phases

Abstract

The structural change from aluminium-rich titanium aluminides (TiAl2 or TiAl3) to L12 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 L12 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 useR 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 ofe/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 ine/a is found to occur upon the addition of ternary alloying element to either TiAl2 or TiAl3 with the consequence of achieving L12-type ternary titanium aluminides at definitee/a values around 2.5.