Flow stress and substructural change during transient dynamic recrystallization of nickel

The transient behaviour of dynamically recrystallizing nickel following a change in strain rate has been studied using hot tensile testing, with microstructural observations being carried out by optical and transmission electron microscopy. After a decrease in strain rate, the flow curve displayed multiple peaks accompanied by grain coarsening, whereas after an increase in strain rate, the flow curve displayed a single peak associated with grain refinement. Grain coarsening after a strain rate decrease occurred in three distinct stages, in contrast to the monotonous grain refinement taking place after a strain rate increase. In the first stage, rapid grain boundary migration caused a sharp drop in flow stress over a strain of 0·002. In the second, the new grains grew, causing coarsening and impingement, with rapid work hardening in the softer regions. Finally, in the third stage, the grain size increased further, approaching the steady state value, while multiple peaks in flow stress were observed.

MST/372