Steady state creep of ZrNb alloys in a temperature interval 350 to 550°C

Abstract

Creep of three Zr-Nb alloys (0.5, 2.5 and 4.5 wt% Nb) was investigated in a temperature interval 350 to 550°C using the isothermal creep test and transmission electron microscopy techniques. Relations between steady-state creep rate and applied stress were determined; the parameter of applied stress sensitivity of steady-state creep rate increases with the applied stress, reaching values sometimes greater than 30 at 350°C. The apparent activation energy of creep was determined and the mean effective stress in steady-state creep measured. Possible creep-rate controlling mechanisms were discussed, with the conclusion that the creep is most probably recovery-controlled. Relations between steady state flow stress and temperature suggest a significant contribution of an athermal deformation mechanism to the measured steady-state strain rate at low temperatures and high applied stresses. The creep-strengthening effect of niobium increases linearly with niobium concentration at high steady-state creep rates, while at low steady-state creep rates the optimum niobium concentration does not exceed 2.5%. This, together with the temperature sensitivity of the strengthening effect of niobium, was explained by structural stability decreasing as niobium concentration increases from 2.5 to 4.5%. Creep-controlling mechanisms for very low creep rates are discussed.