Effect of annealing treatment on the creep behaviour of Zr-2.5Nb alloy

In this study, the effect of cold work and annealing treatment on the creep behaviour of Zr-2.5Nb material is investigated. The creep properties of Zr-2.5 wt.% Nb pressure tube material was evaluated under two material conditions: i) cold-worked and stress-relieved (CWSR) and ii) annealed. The annealing treatment was carried out in a vacuum furnace at 800 degrees C, corresponding to the two-phase (alpha+beta) region of the Zr-Nb phase diagram, with a 30-minute soaking time and then furnace-cooled. Constant load creep tests were performed as per ASTM 139-2011 using flat creep specimens with their axis along the axial direction of the pressure tube at 450 degrees C under three normalized stress levels corresponding to 0.7, 0.8 and 0.9 times the yield strength of the respective material condition. The minimum creep rate in the annealed material was found to be two orders lower with a wide range of steady-state creep regimes as compared to the CWSR material condition. The change in texture after annealing and creep exposure was found to be marginal. The two-order difference in creep rate could be attributed to the elongated grain structure and to the presence of high density of dislocations in CWSR material.