Microstructural evolution of Ti-6Al-4V alloy

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.Validerat; 20101217 (root