Creep behavior of niobium-modified zirconium alloys

Le comportement en fluage isotherme de l'alliage M5® a ete etudie a haute temperature dans le domaine biphase (α+β). Une premiere approche consiste en l'identification des lois de fluage des phases α et β dans leur domaine monophase respectif puis en l'extrapolation de ces lois dans le domaine biphase. Cette approche ne permet malheureusement pas de reproduire le comportement experimental. Une amelioration de ce modele est developpee dans cette etude en prenant en compte deux effets microstructuraux : (i) la taille de grains : des tailles de grains specifiques controlees ont ete obtenues en appliquant des traitements thermo-mecaniques au materiau. Des essais de fluage dans les domaines quasi-α et quasi-β ont ainsi mis en evidence un fort effet de la taille de grains, en particulier dans le regime de fluage diffusionnel. (ii) le contraste micro-chimique entre les phases α et β dans le domaine biphase : d'apres des calculs thermodynamiques et des analyses microstructurales, la phase β est enrichie en Nb et appauvrie en O (inversement pour la phase α). Des essais de fluage ont alors ete mis en œuvre sur des alliages Zr-Nb-O dont les teneurs en Nb et O sont representatives de chaque phase dans le domaine biphase. Cette base experimentale a permis de d'identifier de nouvelles lois de fluage pour les phases α et β. Ces lois ont ete ensuite implementees dans un modele elements finis afin de simuler le comportement du materiau biphase. La morphologie 3D des phases (en particulier la germination de la phase β aux joints de grains α) est introduite explicitement dans les simulations afin de mettre en evidence son effet sur le comportement macroscopique. M5® est une marque deposee d'AREVA NP.

Comportement en fluage à haute température dans le domaine biphasé (α + β) de l'alliage M5®

Biaxial creep tests on HANA-4 tubes, Nb-added Zircaloy-4 were conducted using internal pressurization of closed-end tubes to investigate the rate-controlling mechanisms over a range of hoop stresses, $$ 8.38 \times 10^{ - 5} E - 2.87 \times 10^{ - 3} E $$
 , at three different temperatures 673 K, 723 K, and 773 K (400 °C, 450 °C, and 500 °C). The mechanistic creep parameters such as stress exponent (n) and activation energy (Q
 C) were then determined from steady-state creep rates. Based on the variance in stress exponent with respect to the applied stress, three regimes have been identified: a stress exponent close to 1 at low stresses that increased to 3 at the intermediate stresses, which became 4.5 at high stresses. An activation energy value of 226 kJ/mol was evaluated for the n = 3 and n = 4.5 regimes, which lies close to the activation energy for self-diffusion (Q
 L) in α-Zr alloys. Further, TEM analyses of crept microstructures and comparison of experimental results with standard models were undertaken to find out the rate-controlling mechanisms. Coble creep, climbing of dislocations to bypass β-Nb precipitates, and dynamic recovery by edge dislocation climb are proposed as the rate-controlling mechanisms in the n = 1, n = 3, and n = 4.5 regimes of HANA-4, respectively.

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Coble, Orowan Strengthening, and Dislocation Climb Mechanisms in a Nb-Modified Zircaloy Cladding

We present a unified model for calculation of zirconium alloy fuel cladding rupture during a postulated loss-of-coolant accident in light water reactors. The model treats the Zr alloy solid-to-solid phase transformation kinetics, cladding creep deformation, oxidation, and rupture as functions of temperature and time in an integrated fashion during the transient. The fuel cladding material considered here is Zircaloy-4, for which material property data (model parameters) are taken from the literature. We have modelled and simulated single-rod transient burst tests in which the rod internal pressure and the heating rate were kept constant during each test. The results are compared with experimental data on cladding rupture strain, temperature, and pressure. The agreement between computations and measurements in general is satisfactory. The effects of heating rate and rod internal pressure on the rupture strain are evaluated on the basis of systematic parameter variations of these quantities. In the α-phase ...

https://www.tandfonline.com/doi/pdf/10.1080/18811248.2011.9711677

Modelling and Simulation of Reactor Fuel Cladding under Loss-of-Coolant Accident Conditions

Tensile creep behavior of the Zr-2.5Nb alloy has been studied through tests under constant load (stress range ~ 137–371 MPa) between 275 and 375 °C. The minimum creep rate of the alloy is found to vary with applied stress and temperature following a power-law relation. The values of stress exponent (n) obtained in the range of 5.3–6.8 in the temperature interval of 300–375 °C; whereas it is calculated as 1.2 and 7.0 under low and high stresses, respectively at 275 °C. The apparent activation energy of creep ( Q C ) and stress exponent have been determined by analyzing the experimental data. The obtained Q C (~198.5 ± 10.7 kJ/mol) is found to be higher than the lattice self-diffusion activation energy of pure zirconium (113 kJ/mol). Using this value, stress exponent ~5.6 ± 0.23 is obtained by the temperature-compensated power law. Microstructural characterization by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) analysis has confirmed coarsening of β-(Nb, Zr) precipitates with compositional changes during creep. The dislocation-precipitate interaction as evidenced by TEM observations is considered to be the origin of threshold stress, which decreases with increasing temperature. Considering the threshold stress, true activation energy of creep ( Q t ) and true stress exponent ( n t ) are found as ~160.4 ± 6.9 kJ/mol and ~4.8 ± 0.22, respectively. Analysis of creep data has confirmed the role of dislocation climb as the rate-controlling mechanism, along with validity of Monkman-Grant and modified Monkman-Grant relations. Scanning electron microscopy (SEM) of creep fracture surfaces has revealed evidence for prominent ductile fracture. Further, the obtained creep damage tolerance factor value of 1.9, indicating the predominance for cavitation during creep.

Tensile creep and rupture behavior along with evolution of microstructure in a Zr-2.5Nb alloy

At the present time, the N-1 alloy (which is used as a jacket material in the power reactors working with an aqueous coolant) is studied intensively for establishing the mechanisms controlling the plastic deformation process during creep and for giving a mathematical description of this process. This paper deals with a study of the creep process of the N-1 alloy at higher temperatures that are characteristic of emergency situations. Statistical processing of the results shows that in the 10-150 MPa stress range, the activation energy for creep does not depend on the magnitude of stress and is equal to (240 +/- 10) kJ/mole. This value is close to the activation energy for self-diffusion of ..cap alpha..-zirconium. The actual activation energy for creep (that was obtained taking the temperature dependence of the modulus of elasticity into account) is equal to (235 +/- 10) kJ/mole and it virtually coincides with the apparent activation energy (considering the experimental scatter).

Steady-state creep of the N-1 alloy in the 650–870°K range

N-1 alloy creep at 1170–1370 K

At present time, the deformation behavior of the N-I alloy is being studied extensively since it forms the material of the fuel element jackets of the thermal reactors Figure 1 shows the results of the creep tests carried out on the alloy in the 880-I170~ range Describing the plastic deformation process of the alloy in the region of the polymorphic transformation (880-I150~ is extremely difficult since the structure of the material changes abruptly It was previously suggested [I] that the temperature range of polymorphic transformation of zircaloy-2 can be divided into two regions and it was considered that at a heating rate of 25~ and at a temperature below 1200~ the deformation of the material is determined by the a-phase and that at higher temperatures, it is controlled by the B-phase Nevertheless, when predicting the deformation behavior of the N-I alloy in the region of polymorphic transformation, it is desirable to consider the actual ratio of the aand the F-phases

Creep of the N-1 alloy within the region of polymorphic transformation

Creep of fully recrystallized and cold-worked Zr-1% Nb alloy upon rapid heating

Metallographic studies at different stages of strain showed that the structure for which superplasticity is realized is formed in the regime of both active stretching and creep after a strain of approximately 5%. In situ electron-microscopic studies under conditions of heating established that at 1120/sup 0/K part of the ..beta.. phase is dispersed over the volume of the ..cap alpha.. grain. The dispersed separations of the ..beta.. phase remain when the strain is small. Further strain leads to coagulation of the formations of the ..beta.. phase, primarily at locations of stress concentration (at the joints of grains), and reduction of the grains.

E. N. Pirogov

Papers

Steady-state creep of the N-1 alloy in the 650–870°K range

N-1 alloy creep at 1170–1370 K

Creep of the N-1 alloy within the region of polymorphic transformation

Creep of fully recrystallized and cold-worked Zr-1% Nb alloy upon rapid heating

Mechanism of superplasticity in N-1 zirconium alloy