Experimental evidence of concurrent compositional and structural instabilities leading to ω precipitation in titanium–molybdenum alloys

The ω phase is commonly observed in many commercial β or near-β titanium alloys on rapidly cooling from the single β phase field and also during subsequent isothermal annealing. However, the crystallographic formation mechanism for the ω particles is hitherto unclear/under discussion. The present st...

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Bibliographic Details
Published in:Acta materialia 2012, Vol.60 (2), p.596-609
Main Authors: Devaraj, A., Nag, S., Srinivasan, R., Williams, R.E.A., Banerjee, S., Banerjee, R., Fraser, H.L.
Format: Article
Language:English
Subjects:
Alloys
Applied sciences
Atom probe tomography
Body centered cubic lattice
Collapse
Collapse of atomic planes
Cooling
Embryos
Exact sciences and technology
Instability
Metals. Metallurgy
Omega phase
Phase separation
Precipitation
Titanium alloy
Titanium base alloys
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Summary:The ω phase is commonly observed in many commercial β or near-β titanium alloys on rapidly cooling from the single β phase field and also during subsequent isothermal annealing. However, the crystallographic formation mechanism for the ω particles is hitherto unclear/under discussion. The present study primarily focuses on ω precipitation within the β (body-centered cubic (bcc)) matrix of simple model binary titanium–molybdenum (Ti–Mo) alloys. It provides direct experimental evidence of the formation of ω-like embryos from competing compositional and structural instabilities arising in the bcc lattice of Ti–Mo alloys during rapid cooling from the high-temperature single β phase field. The displacive partial collapse of the {1 1 1} planes of the parent bcc structure within compositionally phase-separated regions containing several at.% less of Mo, forming ω-like embryos, has been conclusively shown by coupling aberration-corrected high-resolution scanning transmission electron microscopy with atom probe tomography observations. Growth and coarsening of these ω-like embryos take place during subsequent isothermal annealing, accompanied with both a completion of the collapse of the {1 1 1} β planes leading to a fully developed ω structure as well as rejection of Mo from these precipitates, resulting in near-equilibrium compositions.
ISSN:1359-6454
1873-2453
DOI:10.1016/j.actamat.2011.10.008