Characterization of hot deformation behaviour of Zr-2.5Nb-0.5Cu using processing maps

Abstract

The characteristics of hot deformation of β-quenched Zr---2.5Nb---0.5Cu in the temperature range 650-1050°C and in the strain rate range 0.001-100 s^-1 have been studied using hot compression testing. For this study, the approach of processing maps has been adopted and their interpretation done using the Dynamic Materials Model. The efficiency of power dissipation given by [2m/(m+1)], where m is strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. The processing map for Zr-2.5Nb-0.5Cu within (α + β) phase field showed a domain of dynamic recrystallization, occurring by shearing of α-platelets followed by spheroidization, with a peak efficiency of 48% at 750° C and 0.001 s^-1. The stress-strain curves in this domain had features of continuous flow softening and all these are similar to that in Zr-2.5Nb alloy. In the β-phase field, a second domain with a peak efficiency of 47% occurred at 1050°C and 0.001 s^-1 and this domain is correlated with the superplasticity of α-phase. The β -deformation characteristics of this alloy are similar to that observed in pure β-zirconium with large grain size. Analysis of flow instabilities using a continuum criterion revealed that the Zr-2.5Nb-0.5Cu exhibits flow localization at temperatures higher than 800° C and strain rates higher than about 30 s^-1 and that the addition of copper to Zr-2.5Nb reduces its susceptibility to flow instability, particularly in the (α + β) phase field.