Abstract: The precipitation characteristics of the a2 ordered phase in Ti-Al-Sn-Zr-Mo-Si-Nd alloys with various content of Al, under different aging conditions, were investigated. The distribution and size of the a2 ordered phase changed with temperature and Al content. The dislocations were the only places where the a2 ordered phase could precipitate at higher temperature near the critical transformation temperature for each alloy experimented. With the addition of Al content, the critical transformation temperature of a2 ordered phase increased. When the aging temperature was relatively low (650ºC), the precedent precipitation of a2 ordered phase took place in primary a phase at the early stage of aging, in the duplex microstructure (the primary a with the transformed b) of the alloys with lower Al content. But after certain aging time (50h), the size of a2 particles was almost equal in both the primary a and the transformed b. And no obvious growth of a2 particles could be observed after 50 h.

Key words: Ti-Al-Sn-Zr-Mo-Si-Nd alloys, a2 ordered phase, Precipitation