ON THE THERMAL STABILITY OF Ti ALLOYS II. The Behaviour of Transition Elements in Ti_3X-Phase Formation

Four ternary alloy systems, including Ti-Al-Sc, Ti-Al-V, Ti-Al-Nb and Ti-AlMo, other than previous work, were selected to investigate the behaviour of transition elements in the formation of Ti_3X-phase. Results show that the formation ofTi_3X-phase in the Ti-Al solid solution containing the transition elements Sc, V, Nb or Mo obeys the electron concentration rule as well, and the valence electron number of the transition elements depends on the structures of their d energy bands, i.e., N(Sc)=2(S~2) for the elements, e.g. Sc, on the left of Ti in the periodic table; N_v=3(d~3), N_(Nb)=4(d~4) and N_(Mo)=5(d~5) for the elements V, Nb and Mo on the right of Ti, respectively. Furthermore, it seems that the formation of Ti_3X-phase could not be'restrained by an addition of any transition element to the alloy.