Thermal creep fracture of a Zr1%Nb cladding alloy in the α and (α+β) phase regions

Highlights: • Creep fracture processes in a sponge- based Zr1%Nb claddig alloy were investigated. • Relationship between creep strain and fracture in α-Zr and (α+β)-Zr regions was found. • Ductile dimple fracture mode and creep cavitation were observed in α-Zr region. • Strain-induced matrix instability leading to necking was relevant to (α+β)-Zr region. The objective of the present study was to provide new relevant information on creep behaviour and fracture processes in a sponge-based modified Zr1%Nb cladding alloy (modified E110 alloy) in the α-Zr and (α+β)-Zr phase regions. To this end, constant load creep tests were carried out in argon at testing temperature intervals from 350°C to 950°C, and applied tensile stresses ranging from 5 MPa to 210 MPa, corresponding to the power-law breakdown creep regime and/or high testing temperatures, and thus to simulate disaster conditions. Creep tests were followed by metallographic and fractographic analyses of the specimens to explain the observed creep behaviour. It was found that in the power-law region (at 350°C) the values of the stress exponent n of the minimum creep rate $\dot{\epsilon }$_m (n = ∂ln$\dot{\epsilon }$_m /∂lnσ)_T, and the stress exponent m of the time to fracture t_f (m = - ∂lnt_f /∂lnσ)_T, were very high and near to each other, indicating a close relationship between creep deformation and fracture. Further support for the idea that creep deformation and fracture are interconnected can be observed by the validity of the empirical Monkman-Grant relationship. Creep tests in the α-Zr phase region revealed creep cavitation near to, and at, the fracture surface. The final fracture is a ductile dimple mode with the synergistic effect of creep cavitation. By contrast, the final fracture in the (α+β)-Zr phase region is caused by a local strain-induced instability of the matrix leading to a loss of an external section of specimen (necking). There is a marked difference between the values of the strain to fracture ε_f. In the α-Zr region, typical values of ε_f ~ 0.3-0.5 were found, whereas for the (α+β)-Zr region, the value ε_f increases with testing temperature up to 850°C. Following drop of ε_f at temperatures ≥ 900°C was explain by intensive oxidation of the alloy.