ON THE THERMAL STABILITY OF Ti ALLOYS I. The Electron Concentration Rule for Formation of Ti_3X-Phase

Based on the Hume-Rothery rules and the connections between phase stability and electron structure, it was noticed that both the atomic radii and electronegativities of Ti with Ti_3X forming elements Al, Ga, In, Sn and Zr are favourable factors for alloying. Under the examination of atomic properties of these elements, it seems to be drawn that the electron concentration is the chief controlling factor for the a-Ti_3X-phase boundary. The investigation was made with four selected ternary alloy systems: Ti-Al-Ga, Ti-Al-Sn, Ti-Al-Zr and Ti-Al-O. The experimental resuits show that the formation of Ti_3X-phase obeys the electron concentration rule, and the valence electron number of the alloying elements depends on their own electronic structures, i.e., N_(Ti)=N_(Zr)=2 for the transition elements Ti and Zr; N_(Al)=N_(Ga)=3(S~2P~1), N_(Sn)=4(S~2P~2) and N_o=6(S~2P~4) for non-transition elements Al, Ga, Sn and O. The characteristic electron concentration for the formation of Ti_3X-phase may be expressed as: