Thermal Stability of Severe Plastically Deformed VT-6 (Ti-6Al-4V)

The thermal stability of equal channel angular extruded VT-6(Ti-6Al-4V) has been examined using micro-hardness, nano-hardness of the individual αand β phases backscattered scanning (BSEI) and transmission electron microscopy (TEM). After straining to an equivalent total equivalent of 6.5 samples were annealed for 1 h at temperatures between 175 and 800 o C followed by water quenching. Micro and nano-hardness measurements showed an initial hardness increase, the former rising to a maximum at 175°C, while the latter exhibited a maximum at 500°C. BSEI and TEM analysis showed that these observations can be understood by considering the microstructure changes occurring at different length scales. Annealing in the temperature range of 175 to 500°C did not significantly alter the α and β particle size, while TEM showed that recovery and continuous recrystallization occurred in the α phase, higher temperatures being required to activate the recovery and recrystallization processes within the β phase. Finally at temperatures above 600°C spheroidization and growth of the β phase occurred with the volume fraction of this phase increasing from 15 pct at lower temperature to 25 pct at 800°C, an equi-axed α+ β microstructure being observed at this temperature.