Dynamic recrystallization during hot working of Zr-2·5Nb: characterizing using processing maps

The characteristics of the hot deformation of Zr-2·5Nb (wt-%) in the temperature range 650-950°C and in the strain rate range 0·001-100 s-1 have been studied using hot compression testing. Two different preform microstructures: equiaxed (α +β ) and β transformed, have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the dynamic materials model. The efficiency of power dissipation given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified in the maps of equiaxed (α +β ) and β transformed preforms. In the case of equiaxed (α +β ), the stress-strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak efficiency of 45% at 850°C and 0·001 s-1. On the other hand, the β transformed preform exhibits stress-strain curves with continuous flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of a platelets followed by globularisation with a peak efficiency of 54% at 750°C and 0.001 s-1. The characteristics of dynamic recrystallisation are analysed on the basis of a simple model which considers the rates of nucleation and growth of recrystallised grains. Calculations show that these two rates are nearly equal and that the nucleation of dynamic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow instabilities using a continuum criterion revealed that Zr-2·5Nb exhibits flow localisation at temperatures lower than 700°C and strain rates higher than 1 s-1