Precipitation processes in the beta-titanium alloy Ti-5Al-5Mo-5V-3Cr

Abstract 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/8h+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.