Aging response of coarse- and fine-grained β titanium alloys

The effect of heating rate to aging temperature and β grain size on the aging behavior of three metastable β titanium alloys, TIMETAL-LCB, VT22 and Ti-15-3-3-3 ('Ti-15-3'), was established using in situ resistivity measurements, X-ray diffraction, optical microscopy, SEM, TEM and STEM characterization. The results revealed the alloys could be divided into two classes based on their aging behavior. TIMETAL-LCB and VT-22 formed fine plate-like α at slow heating rates to the aging temperature. This behavior was determined to be due to the precipitation of isothermal ω at low temperatures, which serves as nucleation sites for α. The slow heating rate yielded the best balance of strength and ductility, particularly in alloys with a fine (∼10 μm) β grain size. At high heating rates, the formation of isothermal ω was avoided, leading to coarse, plate-like α microstructures with less desirable properties. Ti-15-3, on the other hand, exhibited β phase separation during isothermal aging rather than isothermal ω formation. Much slower cooling rates were required to form fine α laths in Ti-15-3 compared to the other two alloys. The importance of specifying heating rate and aging temperature for the industrial heat treatment of β titanium alloys was thus established.