Spheroidization of the lamellar microstructure in Ti-6Al-4V alloy during warm deformation and annealing

The spheroidization behavior of an alpha colony microstructure in Ti-6Al-4V alloy during warm working and subsequent annealing at 600 and 800 degrees C was established. The principal features of microstructure evolution were found to be temperature dependent. At 800 degrees C transformation of the lamellar microstrucuture into a globular one was associated primarily with the classical boundary splitting mechanism followed by further spheroidization of alpha particles by means of termination migration. For thick alpha lamellae, however, new grains were formed due to continuous dynamic recrystallization during deformation, but spheroidization per se was limited. A decrease in temperature to 600 degrees C resulted in increased shear strains, lower diffusivity, and a decrease in the volume fraction of the beta phase. Consequently, the thin beta interlayers transformed relatively quickly into separate particles while alpha became the matrix phase. Evolution of the alpha phase during deformation/annealing at 600 degrees C was associated with continuous dynamic recrystallization with only limited dynamic or static spheroidization. Static spheroidization kinetics during annealing following warm working were explained in the context of approximate models of boundary splitting and termination migration. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.