Modulated two-domain structure of the O phase formed in the Ti-25Al-12. 5Nb (at %) alloy
- NIST, Gaithersburg, MD (United States). Metallurgy Div.
Transformation of the high temperature Ti-Al-Nb cubic [beta]-phase (bcc or ordered B2) to low temperature ordered close-packed (2H-type) phases, the hexagonal DO[sub 19] (Ti[sub 3]Al-type) and the orthorhombic O (Cmcm with Ti[sub 2]AlNb stoichiometry), has been a subject of numerous papers by different research groups. For alloys from the Ti[sub 3]Al-Nb[sub 3]Al section with Nb < 30 at%, the following partitionless (no long-range diffusion) reactions were observed after continuously cooling: (a) bcc [r arrow] hcp [r arrow] DO[sub 19] for the Ti rich alloys; (b) B2 [r arrow] O phase for the Nb rich alloys; (c) bcc [r arrow] hcp [r arrow] DO[sub 19] [r arrow] O for the intermediate composition alloys. In the (c) case the O phase is metastable and transforms to an equilibrium mixture of O+DO[sub 19] phases after prolonged annealing. In this paper the authors will show evidence that short annealing (at 600--700 C) of quenched specimens results in a different transformation path, namely B2 [r arrow] O. The paper will also discuss preliminary results on a microstructure resulting in large prior [beta]-grains after this transformation. The microstructure has very regular modulations of only two rotational variants of the O phase and, to the knowledge, was identified here for the first time. The two-variant microstructure is very different from the microstructures observed in the continuously cooled alloys where all six 2H variants are present in a comparable volume (of either bcc [r arrow] DO[sub 19] or B2 [r arrow] O transformations in the Ti-25Al-12.5Nb (at%) and Ti-25Al-25Nb (at%) alloys, respectively).
- OSTI ID:
- 6008282
- Journal Information:
- Scripta Metallurgica et Materialia; (United States), Journal Name: Scripta Metallurgica et Materialia; (United States) Vol. 29:12; ISSN SCRMEX; ISSN 0956-716X
- Country of Publication:
- United States
- Language:
- English
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