Microstructural evolution of Ti-6Al-4V alloy

Abstract: Ti-6Al-4V alloy undergoes flow softening during hot working at temperatures
around 500-800ºC. Different research studies suggest that the flow
softening is caused by different mechanisms such as dislocation effects and
microstructure changes i.e. recovery and recrystallization. Different
studies of this behavior have been carried out: however, there is a lack of
the information regarding the static processes. Different specimens were
deformed up to 10% strain at 0.001s-1 and then annealed at different
temperatures. The specimens were characterized using different techniques
in order to evaluate the microstructural evolution. Optical analyses show
that the microstructure of the alloy starts changing at 500ºC forming an
equiaxed microstructure: at higher temperatures, the microstructure
develops elongated grains which later form a lamellar structure due to the
phase transformation. At high temperatures (800ºC), a restoration process
can be observed in the hardness measurements. EBSD maps show the formation
of subgrains at 600ºC, which tend to disappear at higher temperatures
(800ºC): however, quantification of the low angle grain boundaries
fraction shows that there is not a decrease of these boundaries, (formed by
dislocations), when the annealing temperature is increased. Orientation
maps of single grains at higher magnification were acquired.
Misorientations along single grains were observed at 600ºC, which reveals
the presence of dislocations and hence internal stresses. At 800ºC, the
orientation of the grains become uniform along the grains, this confirms
qualitatively the annihilation of dislocations, which causes the drop of the
hardness.