Thermal creep of Zr–Nb1%–O alloys: experimental analysis and micromechanical modelling

Zirconium alloys present a large variability of their mechanical behaviour with respect not only to their chemical composition but also to their microstructure. We analyze here the creep behaviour at 400 °C of two Zr–Nb1%–O alloys presenting identical chemical composition, crystallographic texture, grain size and grain shape. Both alloys only differ by the thermal cycles imposed during the fabrication process, either below (alloy A) or alternatively above and below (alloy B) the monotectoı̈d transition. This sole difference gives rise to creep rates varying by a factor of about 4 between the two alloys. From a microstructural point of view, alloys A and B differ by the precipitates distribution and the thermodynamical state (alloy B is in a metastable equilibrium state). Our experimental analysis based on mechanical tests, transmission electron microscopy (TEM) observations and phase analysis by X-ray diffraction strongly suggests an hardening effect of Nb in solid solution to explain the differences between alloys A and B. This result is confirmed by TEM X-ray spectrometry which gives a weight content of Nb in solid solution differing by about 0.1% between the two alloys. A predictive micromechanical model, based on the self-consistent affine scheme, is then applied. This model well captures the anisotropy of the specimens, and describes accurately both transient and secondary creep regimes. As a result of the identification procedure, identical hardening laws are obtained for the two alloys at the grain scale, and the saturating reference stress for prismatic slip is found to be higher for alloy B by about 30 MPa with respect to alloy A.