First report on observation of abnormal creep in a Zr-2.5wt.%Nb alloy at low stresses

Low-stress creep behaviour of a two-phase Zr-2.5%Nb alloy, differently heat treated, has been investigated using helical test specimens. The phase diagram of the α (hcp) + β (bcc) alloy is characterized by the monotectoid reaction at 893 ± 10 K: β1 → (α + β2) where β1 (Zr-20Nb) and β2 (Zr-80Nb) have widely differing compositions. At the creep testing temperature, 818 K, which is close to but below the monotectoid temperature, the creep rate for samples with the equilibrium α + β2 structure has been found to be considerably higher, over an order of magnitude, than that in samples with the metastable α + β1 structure. Microstructural changes accompanying the markedly enhanced creep rate for the α + β2 structure at stresses as low as 1-4 MPa have been explained in terms of the relative stability of the β1 and the β2 phases during the creep process. An attempt has been made to elicit the likely mechanism underlying the observed enhancement of creep rate and the changes in morphology, composition and volume fraction of the β phase