Effect of rolling-assisted deformation on the formation of an ultrafine-grained structure in a two-phase titanium alloy subjected to severe plastic deformation

The effect of rolling in the temperature range 450-650 degrees C on the fragmentation of the primary phase in a hot-rolled VT6 alloy rod preliminarily subjected to severe plastic deformation by equal-channel angular pressing at 700 degrees C (scheme B (c), the angle between the channels is 135 degrees, 12 passes) is studied. Rolling at 450 degrees C without preliminary ECAP is shown not to cause alpha-phase fragmentation and to favor intense cold working of the alloy due to multiple slip. ECAP provides partial fragmentation of the initial structure of the alpha phase and changes the morphology of the retained beta phase: it transforms from a continuous matrix phase into separated precipitates located between alpha particles. This transformation activates the fragmentation of the alpha phase during rolling at 550 degrees C owing to the development of twinning and polygonization processes apart from multiple slip. Both a decrease (to 450 degrees C) and an increase (to 625-650 degrees C) in the rolling temperature as compared to 550 degrees C lead to the formation of a less homogeneous and fragmented structure because of weakly developed recovery and intense cold working in the former case and because of the beginning of recrystallization and the suppression of twinning in the latter case. A relation between the structure that forms upon SPD followed by rolling and the set of its properties is found. A general scheme is proposed for the structural transformations that occur during ECAP followed by rolling at various temperatures.