Dynamic fragmentation and spheroidization of α phase grains during hot deformation of Ti-6Al-4V alloy

Development of mechanical properties of titanium alloys is usually achieved by the control of their microstructure (globular, lamellar, bimodal). Dynamic fragmentation and spheroidization of elongated alpha phase grains, formed from martensite laths, in the microstructure of Ti-6Al-4V alloy are considered in the paper. The effects of alpha phase grain spheroidization was evaluated in tensile tests over a temperature range of 800-900 degrees C and strain rate range of 10(-3) - 10(-2) s(-1). Completed spheroidization of elongated alpha phase grains and subsequent growth of equiaxed grains for all deformation modes were found. A low value of apparent activation energy for deformation was calculated - suggesting grain boundary sliding was the main deformation mechanism. The results showed good correlation with the kinetic rate equation based on the Zener-Hollomon parameter.