Transformation of BCC and B2 high temperature phases to HCP and orthorhombic structures in the Ti-Al-Nb system: Part II -- Experimental TEM study of microstructures

Journal Article · · Journal of Research of the National Institute of Standards and Technology; (United States)

DOI:https://doi.org/10.6028/jres.098.039· OSTI ID:5434694

Bendersky, L A; Boettinger, W J ^[1]

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1. National Inst. of Standards and Technology, Gaithersburg, MD (United States)

Possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted by Bendersky, Roytburd, and Boettinger for Ti-Al-Nb alloys cooled from the high temperature BCC/B2 phase field into close-packed orthorhombic or hexagonal phase fields. These predictions were based on structural and symmetry relations between the known phases. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti-25Al-12.5Nb (at%), the path includes the formation of intermediate hexagonal phases, A3 and DO[sub 19], and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti-25AL-25Nb (at%), the path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing at 700 C. The structure forms by congruent ordering of the DO[sub 19] phase to the O phase, and then reprecipitation of the DO[sub 19] phase possibly by a spinodal mechanism. The thermodynamics underlying the path selection and the two-phase formation are also discussed.

• OSTI ID: 5434694
• Journal Information: Journal of Research of the National Institute of Standards and Technology; (United States), Journal Name: Journal of Research of the National Institute of Standards and Technology; (United States) Vol. 98:5; ISSN 1044-677X; ISSN JRITEF
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360102* -- Metals & Alloys-- Structure & Phase Studies
ALLOYS
ALUMINIUM ALLOYS
BCC LATTICES
CRYSTAL LATTICES
CRYSTAL STRUCTURE
CUBIC LATTICES
ELECTRON MICROSCOPY
HCP LATTICES
HEXAGONAL LATTICES
MICROSCOPY
MICROSTRUCTURE
NIOBIUM ALLOYS
ORTHORHOMBIC LATTICES
TITANIUM ALLOYS
TITANIUM BASE ALLOYS
TRANSMISSION ELECTRON MICROSCOPY