Aspects of Dynamic Recrystallization in Cobalt at High Temperatures

Deformation behavior of cobalt in the fcc phase was studied over a wide range of strain rates (0.001 to 1 s(-1)) and temperatures (600 A degrees C to 950 A degrees C). At the lower temperatures and higher strain rates, a single peak in flow stress followed by softening to steady state was observed, whereas at higher temperatures and lower strain rates, an oscillatory flow stress behavior finally converging to a steady state was observed. Both these represent typical flow stress responses during dynamic recrystallization (DRX). The critical condition for the initiation of DRX was determined by the inflection point in the work-hardening vs stress plot. This critical condition in terms of the work done was found to increase with temperature-compensated strain rate. Electron backscattered diffraction (EBSD) revealed that, on quenching after deformation, a certain fraction of the fcc phase was always retained, the extent of retention being higher for smaller grain sizes. Misorientation measurements across boundaries revealed a significant amount of twins in the retained fcc phase. The grain average misorientation (GAM), signifying the misorientation within grains, was found to be inversely related to the twin boundary fractions in the fcc phase