Insight into threshold creep behaviour in Zr-2.5Nb alloy

In this study, the threshold creep behaviour of cold-worked and stress-relieved (CWSR) Zr-2.5Nb material is investigated at 350 degrees C, 400 degrees C and 450 degrees C. Constant-load uniaxial creep tests were carried out using flat creep specimens in accordance with ASTM 139-20. The specimens were tested at three different normalized stress levels, which corresponded to 0.7, 0.8 and 0.9 times the yield strength at the respective temperatures. The stress exponents and activation energy were determined in the temperature range of 350 degrees C-450 degrees C. For the determination of threshold stress, Li and Langdon proposed a simple extrapolation method to predict the threshold stress, defined as the limiting stress conforming to a strain rate of 10-10 s -1 . To evaluate the threshold creep behavior, stress increment creep tests were conducted at the estimated threshold stress as the initial stress level, along with two subsequent stress increments. The minimum creep rates, obtained at the estimated threshold values, were found to be at least one order higher than the proposed threshold creep rate value of 10 -10 s -1 . The discrepancy in the obtained minimum creep rates at the predicted threshold stress value could be attributed to change in creep mechanism at much lower stress levels. Towards microstructural investigation, Transmission Electron Microscopy (TEM) analysis of as-received (CWSR) and creep-deformed specimens was carried out to study the dislocation substructures.