Flow behavior and globularization kinetics during hot working of Ti-6Al-4Vwith a colony alpha microstructure

The effect of process variables on how response and microstructure evolutio n during hot working of Ti-6Al-4V with a colony alpha preform microstructur e was established using isothermal hot compression tests. Testing was condu cted on material with prior-beta grain sizes of 100 mu m or 400 mu m at str ain rates of 0.001-10 s(-1) test temperatures between 815 and 955 degrees C , and height reductions of 40-80%. All of the flow curves exhibited a peak stress followed by moderate flow softening. The absence of a grain/colony s ize dependence of flow behavior, coupled with relatively low values of the strain rate sensitivity of the flow stress ( similar to 0.05-0.30). led to the conclusion that deformation was controlled by dislocation glide/climb p rocesses. Flow softening was interpreted in terms of deformation heating an d substructure/texture evolution. The dependence on strain rate and tempera ture of the kinetics of dynamic globularization of the colony microstructur e was complex and appeared to be of second-order importance compared to the effects of strain per se, thus suggesting the dominance of dislocation-typ e processes for the control of globularization as well. (C) 1999 Elsevier S cience S.A. All rights reserved.