Influence of cooling rate on the precipitation kinetics of nanoscale isothermal ω-phase in metastable β-Ti alloy, Ti–5Al–5Mo–5V–3Cr

Highlights: • ω nucleates slowly in the air-cooled sample than the water-quenched sample. • Heat treatment of 300 °C/8 h produces ∼11% of ω volume fraction in both samples. • The average activation energy for ω formation is ∼122 kJ mol⁻¹. • ω grows only in equatorial dimension at later stages of transformation. • Ageing of water-quenched samples produces higher hardness than air-cooled samples. -- Abstract: In metastable β-Ti alloys, nanoscale isothermal ω-phase (ω_iso) precipitates are regarded as the nucleation sites for the α strengthening phase. Here we investigate the precipitation kinetics of the ω_iso precipitates as a function of cooling rate (air cooling and water quenching) after β-solutionising. A combined in situ small-angle neutron scattering (SANS) and electrical resistivity measurement approach was used during ageing of Ti–5Al–5Mo–5V–3Cr wt% (Ti-5553) alloy at 300 °C and 325 °C up to 8 h. The SANS modelling was consistent with ellipsoid shaped particles for the ω_iso precipitates, for both air-cooled and water-quenched samples. The precipitates attained a maximum size (equatorial diameter) of ∼21 nm and ∼17 nm after 2 h and 4 h of ageing the water-quenched and air-cooled samples respectively. Although the air-cooled samples showed delayed nucleation in comparison to water-quenched sample, the volume fraction became approximately the same (∼11%) after ageing for 8 h. The average value of the activation energy for ω_iso nucleation from the β-phase matrix was determined as 122 kJ mol⁻¹ from electrical resistivity data using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The hardness increased with ageing time, with water quenching leading to a higher final value of hardness than air cooling.