The superplastic and strain-rate dependent plastic flow of zirconium-2.5 wt. % niobium in the 873 to 1373 K temperature range

The tensile properties of zirconium-2.5 wt.% niobium, cut longitudinally from a CANDU pressure tube, were determined in the 873 to 1373 K temperature range. These properties are strongly temperature and strain-rate dependent. The flow stress decreases with increasing temperature and, at constant temperature, increases with increasing strain-rate. Two distinct types of stress -strain curves, which change their appearance in the 1148 to 1173 K range, are observed. The activation energy for the deformation in the 873 to 1100 K range is 275 +- 20 kJ/mol. Superplasticity, i.e., elongation > 200%, is observed in the upper temperature range of the duplex α+β phase region. Superplasticity, which is attributed to the deformation of the softer β-Zr phase, results in an equiaxed, rather than elongated, microstructure. As well, a strain-rate sensitivity exponent of m > 0.3 is obtained when zirconium-2.5 wt.% niobium is superplastic. (auth)