Ti has a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

Precipitate evolution in Ti-5Al-5Mo-5V-3Cr-0.3Fe wt.% (Ti-5553) has been studied in-situ by small angle neutron scattering (SANS) during a two step ageing heat treatment of 300• C/8 h + 500• C/2 h. The first heat treatment step precipitates ω, with a corresponding increase in hardness of ∼ 15% compared to quenched material. The second heat treatment step precipitates fine scale α from the ω phase, with a ∼ 90% increase in hardness compared to quenched material.The SANS measurements are complemented by atom probe tomography (APT) to give compositional information, ex-situ transmission electron microscopy (TEM) to confirm phase identification and size distribution locally, and X-ray diffraction (XRD) for additional confirmation of phase identification.The ω phase is depleted in all the solute additions following 300• C/8 h ageing heat treatment. The volume fraction of the ω phase from APT is estimated to be ∼ 7%. SANS modelling is consistent with disc shaped particles for the ω phase. The mean particle diameter increases from ∼ 7.5 nm to 9.5 nm diameter between 1 h and 8 h heat treatment at 300• C, while the thickness increases from ∼ 4 nm to ∼ 5 nm. The SANS model estimates the volume fraction to be ∼ 10% for the 8 h heat treatment, using the phase compositions from APT.

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Ti-Al alloys can suffer from a chemical decomposition on ageing around 500 C or on air cooling. At long ageing times this results in the formation of a 2 (Ti 3 Al) precipitates. At reduced times or elevated temperatures, diffuse electron or neutron diffraction peaks can be observed, which has been termed 'short range ordering' (SRO). Here, we present correlative transmission electron microscopy (TEM) and atom probe tomography (APT) results showing that the reaction proceeds through the formation of ordered Al-rich precipitate clusters. Notably, Al-Al clustering could be observed well before the appearance of distinct precipitates in the TEM. In addition, the Vcontaining a phase of Ti-6Al-4V formed ordered clusters much faster than in binary Ti-7Al. This implies that the ternary addition of b stabilisers exacerbates the problem of a 2 precipitate formation in commercial dual phase titanium alloys.