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

In metastable beta-Ti alloys, nanoscale isothermal (alpha-phase (omega(iso)) precipitates are regarded as the nucleation sites for the alpha strengthening phase. Here we investigate the precipitation kinetics of the omega(iso) precipitates as a function of cooling rate (air cooling and water quenching) after beta-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 degrees C and 325 degrees C up to 8 h. The SANS modelling was consistent with ellipsoid shaped particles for the omega(iso) precipitates, for both air-cooled and water-quenched samples. The precipitates attained a maximum size (equatorial diameter) of similar to 21 nm and similar to 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 (similar to 11%) after ageing for 8 h. The average value of the activation energy for omega(iso) nucleation from the beta-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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

In metastable beta-Ti alloys, the precipitation kinetics of omega(iso) precipitates were investigated with different cooling rates, showing that water quenching led to a higher hardness value than air cooling.