Effect of prior cold rolling and test temperature on stress-strain rate behaviour of a Zr-2.5Nb alloy

Zr-2.5 wt % Nb alloy sheet, obtained by unfolding and straightening a press ure tube, was further cold rolled upto 39% reduction in thickness to invest igate the effect of cold working on the stress (sigma)-strain rate (epsilon )over dot behaviour over a strain rate range of similar to 2 x 10(-5) to 5 x 10(-3) s(-1) and a temperature range of 625 to 700 degrees C. Irrespectiv e of the amount of rolling, the log a vs log (epsilon)over dot plots exhibi t superplastic behaviour with strain rate sensitivity index, m, as high as 0.8, which decreases to 0.2 at higher strain rates. On the other hand, the activation energy for deformation, Q, increases from 171.1 kJ/mol for super plastic deformation to 249 kJ/mol in Region ill. The tendency for improved superplasticity (m) is seen upon cold working by 22% or more at the test te mperatures 675 and 700 degrees C. (C) 1999 Kluwer Academic Publishers.