DYNAMIC RECRYSTALLIZATION DURING HOT-WORKING OF ZR-2-CENTER-DOT-5NB -CHARACTERIZATION USING PROCESSING MAPS

The characteristics of the hot deformation of Zr-2.5Nb (wt-%) in the t emperature range 650-950 degrees C and in the strain rate range 0.001- 100 s(-1) have been studied using hot compression testing. Two differe nt preform microstructures: equiaxed (alpha + beta) and beta transform ed have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the d ynamic materials model. The efficiency of power dissipation given by [ 2m/(m + 1)], where m is the strain rate sensitivity, is plotted as a f unction of temperature and strain rate to obtain a processing map. A d omain of dynamic recrystallisation has been identified in the maps of equiaxed (alpha + beta) and beta transformed preforms. In the case of equiaxed (alpha + beta), the stress-strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak ef ficiency of 45% at 850 degrees C and 0.001 s(-1). On the other hand th e beta transformed preform exhibits stress-strain curves with continuo us flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of alpha platelets followed by globularisation with a peak efficiency of 54% at 750 degr ees C and 0.001 s(-1). The characteristics of dynamic recrystallisatio n are analysed on the basis of a simple model which considers the rate s of nucleation and growth of recrystallised gains. Calculations show that these two rates are nearly equal and that the nucleation of dynam ic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow insta bilities using a continuum criterion revealed that Zi-2.5Nb exhibits f low localisation at temperatures lower than 700 degrees C and strain r ates higher than 1 s(-1).