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

doi:10.1016/S0022-3115(02)00923-6

Journal of Nuclear Materials

Volume 305, Issues 2–3, October 2002, Pages 175-186

https://doi.org/10.1016/S0022-3115(02)00923-6 Get rights and content

Abstract

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.

Introduction

Zirconium alloys, which are widely used in the nuclear industry especially as cladding tubes and guide tubes for pressurized water reactors, can present a variability of their thermomechanical behaviour (e.g. thermal creep) as a function not only of the chemical composition but also of the microstructure. For a better control of the mechanical behaviour of the actual alloys, but also to take into account the evolution of the industrial elaboration processes (chemical composition, thermal treatments, etc.), different studies have been engaged, for instance [1], [2], to build a predictive modelling aiming at a precise description of the relationship between microstructure and effective mechanical behaviour. Following recent studies [3] on the optimization of the M5™ cladding tubes made of ternary alloy (Zr–Nb–O), this paper deals with the understanding of the influence of the microstructure on the thermal creep behaviour of Zr–Nb1%–O alloys at 400 °C. We have worked on fully annealed cladding tubes issued from the same ingot (i.e. same chemical composition) but elaborated using two different thermomechanical treatments which mainly differ by the time spent above the monotectoı̈d transformation (620 °C). These two thermomechanical treatments give rise to two different types of microstructure which apparently mainly differ by the spatial repartition of precipitates. The two alloys are found to exhibit significantly different viscosities under creep loading. Based on an experimental analysis coupled with an homogenization approach, this study contributes (i) to analyze and understand the observed differences on the mechanical behaviour and (ii) to establish a predictive modelling of the thermal creep, based on a physically relevant averaged constitutive relation at the grain scale and an accurate scale-transition scheme.

Section snippets

Elaboration

The products used for this study are Zr–Nb1%–O cladding tubes. All tubes are issued from the same ingot (i.e. same chemical composition, Table 1) and were obtained with identical cold-rolling operations (i.e. identical number of passes and reduction ratio). They only differ by the intermediate heat treatments performed during the cold-rolling process. According to the Zr–Nb phase diagram [4], a monotectoı̈d transformation β-Zr↔α-Zr+β-Nb takes place at about 620 °C, where β-Zr and β-Nb are body

Influence of the microstructure on thermal creep behaviour

The creep response of the Zr–Nb1%–O alloy was investigated at 400 °C in the low-stress domain (i.e. below the yield strength). Indeed, these stress-temperature conditions correspond to the one used industrially to characterize the out-of-pile creep behaviour. In this context, the response to an internal pressure loading with a hoop stress σ_θθ=130 MPa was first studied. As reported in Fig. 4, the evolution of the effective hoop strain during creep clearly indicates a strong dependence on the

Elastoviscoplastic micromechanical modelling

The homogenization techniques aim at deriving the effective behaviour of heterogeneous materials from the behaviour of their elementary constituents using appropriate average operators. Such an approach is thus well adapted to deal with the relation between microstructure and effective properties. For a comprehensive discussion of this approach in the framework of polycrystals, refer to [11], [12].

In the sequel, the Zr–Nb1%–O polycrystals will be considered as composite materials. Assuming that

Conclusion

The creep responses of two Zr–Nb1%–O alloys have been investigated in details. The two alloys exhibit identical crystallographic textures, grain size, grain shape, and chemical composition. They only differ by the thermal cycles during the elaboration process, alloy A being elaborated at low temperature whereas for alloy B thermal treatments alternate below and above the monotectoı̈d temperature. It has been shown that alloy B deforms by creep about 4 times slower than alloy A. Mechanical tests

Acknowledgements

This work was partially supported by EdF and Framatome ANP Nuclear Fuel, and the cladding tubes were provided by Framatome ANP Zircotube. The authors wish to thank J.P. Mardon (Framatome ANP Nuclear Fuel) for fruitful discussions and express their gratitude to F. Gregori (LPMTM, CNRS) for her generous contribution to the TEM observations.

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

Abstract

Introduction

Section snippets

Elaboration

Influence of the microstructure on thermal creep behaviour

Elastoviscoplastic micromechanical modelling

Conclusion

Acknowledgements

J. Nucl. Mater.

C.R. Acad. Sci. Paris sér. IIb

J. Nucl. Mater.

Int. J. Plast.

J. Nucl. Mater.

Int. J. Plast.

J. Mech. Phys. Solids

Binary Alloy Phase Diagrams

Met. Rev.

J. Phys. IV

Dislocations

J. Appl. Phys.

Texture and Anisotropy