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

The effect of heating rate to aging temperature, and beta grain size on the aging behavior of three metastable beta 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 a. at slow heating rates to the aging temperature. This behavior was determined to be due to the precipitation of isothermal omega at low temperatures, which serves as nucleation sites for alpha. The slow heating rate yielded the best balance of strength and ductility, particularly in alloys with a fine (similar to 10 mu m) beta grain size. At high heating rates, the formation of isothermal omega was avoided, leading to coarse, plate-like alpha microstructures with less desirable properties. Ti-15-3, on the other hand, exhibited P phase separation during isothermal aging rather than isothermal to formation. Much slower cooling rates were required to form fine a 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 P titanium alloys was thus established. (c) 2005 Elsevier B.V. All rights reserved.