[en] The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350 C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, and β-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably. In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20 C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350 C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1,500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect

[en] Studies results concerning the ageing of zirconium alloys (zircaloy 2 and 4, Zr Nb alloys, and Zr Fe alloys) and its effects on mechanical properties, are presented: after a review of the static and dynamic ageing phenomena and the Portevin Le Chatelier effect, the case of zirconium-oxygen is more especially examined, with emphasis on the tensile and creep properties: interstitial oxygen appears to be the source of a rapid bonding of dislocations which leads to important effects on mechanical properties. Irradiation effects on ageing are also studied. (A.B.)

[en] C-Mn steels and associated welds can be susceptible to dynamic strain aging (DSA). In this case, fracture toughness passes through a minimum when the temperature increases from 20 to 300 deg. C. Since Charpy V-Notch data are not affected by the DSA phenomenon, the method for predicting the evolution of J_0.2 and dJ/da in the temperature domain where DSA occurs is still an open question. The purpose of the present study is the assessment of this decrease in fracture toughness using a local approach method. The results of J-R tearing resistance tests, characterized by crack initiation resistance J_0.2 and tearing modulus dJ/da relative to a base metal (A48 French standard steel) and manual metal arc deposited metals are presented and discussed. The local approach determination of J_0.2 was performed using the Rice and Tracey model. Comparisons between experimental results and the results of local approach modelling indicate that both J_0.2 and dJ/da decreases with DSA can be correctly predicted

[en] Plastic deformation in irradiated zirconium alloys occurs in a very heterogeneous manner at the grain scale by the clearing up of radiation-induced prismatic loops by gliding dislocations and the channeling of these dislocations inside narrow bands of the grain. A statistical TEM investigation of this mechanism has been performed on neutron irradiated recrystallized Zr alloys tested at 350 deg. C. Due to the strong anisotropy of plastic deformation, different loading conditions have been investigated. It is shown that for transverse tensile tests and closed end burst tests, only basal channels are observed, whereas for axial tensile tests, prismatic and pyramidal channels are observed. This phenomenon can be understood in terms of texture and interactions between dislocations and irradiation induced loops. From Schmid factor calculations, we have also been able to prove that irradiation leads to a higher increase of critical resolved shear stresses for prismatic and pyramidal slip systems than for basal slip system

[en] In weakly killed C-Mn steels, the dynamic strain aging (DSA) phenomenon induces an increase in the ultimate tensile strength associated with a ductility loss, usually measured in the 100-200 C temperature range. This phenomenon, which induces large toughness reductions, is not well characterized in the heat affected zones (HAZ) of the welds, since the sensitivity to DSA cannot be directly determined due to the very abrupt microstructure gradients existing in these zones. In order to study the influence of microstructure on DSA, tensile tests were performed on simulated HAZ, resulting from various quenching conditions of C-Mn steels. These results are interpreted through correlations with internal friction results on the same microstructures. It is concluded that the materials which have been submitted to the more severe quenching conditions appear to be less sensitive to DSA. This trend is attributed to the larger density of dislocations which can trap most of the interstitial atoms in these microstructures. Whatever the microstructure, the intensity of DSA, characterized by the ductility loss, is proportional to the Snoek peak height measured by internal friction. This result allows the evaluation of the sensitivity of C-Mn steels to DSA from a unique internal friction test. (orig.)

[en] The effect of an applied tensile stress on the hydrides morphology in ZIRCALOY-4 was studied. To this end, the residual stresses around the hydride caused by the hydride precipitation was first evaluated. Considering the disability to predict hydride transformation stresses by ordinary macroscopical mechanical calculation in previous studies, X-ray diffraction (XRD) profile analysis and transmission electron microscopy (TEM) observations were carried out to quantify the microstructural evolution in hydrided ZIRCALOY-4. The residual microstrains and microstresses in the matrix and around the hydride were thus estimated. The big discrepancy between the results and the existing studies were explained by the major self-accommodation of phase transformation deformation remaining inside the hydrides and the local plastic accommodation of ZIRCALOY-4. In order to study the stress effect on hydride orientation and to estimate the hydride orientation threshold stresses, hydrogen was introduced into the specimens under tensile stress. A quantitative technique was used to evaluate the susceptibility to perpendicular hydride formation under the influence of texture, residual stresses, and externally applied tensile stresses, following an improved approach that had been first developed by Sauthoff and then applied to Zr-H system by Puls. Both analytical and experimental results indicate that the threshold stress for producing perpendicular hydrides varies with the microstructural features, the yield strength, and the residual stresses

[en] A comparative experimental and FEM study has been carried out, in order to investigate dynamic and constraint aspects of the Charpy test. Standard V-notch Charpy specimens were tested under dynamic and static loading conditions. 2-D plane strain and 3-D models were employed in numerical analysis. In order to incorporate strain-rate effects, an elastic-viscoplastic constitutive equation has been applied, based on actual test data obtained for a low-alloy structural steel. Fully dynamic analysis clearly indicated inertial effects. Modal analysis enabled the confirmation of the origin of the oscillations on the load-displacement curve as beam vibration of the specimen resulting from interaction with the elastic striker. (orig.)

[en] Stress-strain behavior of 316 LN at 4 K is interpreted in terms of transformation induced plasticity and adiabatic shearing. This interpretation is supported by microscopy observations. Fatigue behavior at 4 K of wrought and weld metal is compared. This study shows that the mechanical properties of the TF coil casings of the TORE SUPRA tokamak are controlled by the mechanical properties of the 316 LN weldments

[en] This paper is concerned with the influence of δ-hydrides on the mechanical properties of three heat treated cold-rolled Zircaloy-4 sheets (stress-relieved, recrystallized and β treated), tested at room temperature and 350⁰C. Smooth tensile specimens of two thicknesses: 0.5 and 3.1 mm, containing different hydride volume fractions, up to 18% (about 1400 ppm H), have been tested. Metallographic and fractographic analysis were carried out in order to examine the fracture morphology near and on the fracture surface, and to determine the evolution of the fracture mechanism of hydrides as a function of temperature, hydride orientation and volume fraction

No abstract available