Stability of the ordered orthorhombic phase in Ti-24A1-11Nb

The existence of an ordered orthorhombic (O) phase in the Ti-Al-Nb ordered alloy system was first reported by Banerjee et. al. and has subsequently been confirmed by several investigators. The O phase has been found in Ti-Al-Nb alloys either slowly cooled from the temperature regime above 1200 degrees C β stable regime) or after quenching from the temperature regime followed by isothermal aging. Recently, the authors have also reported the formation of the O phase through a sequential transition (β → B2→ T→ O) in an aged Ti-24Al-11Nb alloy. This study showed that the O phase was metastable; an O→ α2 transition took place when the O phase was further aged. Detailed studies of the B2 → T and T→ O transitions have been reported elsewhere. In this paper the structural relationships between the O and α2 are investigated in order to elucidate the mechanism of the O→ α2 transition