C. Prioul

Hydride embrittlement in ZIRCALOY-4 plate: Part I. Influence of microstructure on the hydride embrittlement in ZIRCALOY-4 at 20 °C and 350 °C

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, andβ-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 1500 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.

Hydride embrittlement in ZIRCALOY-4 plate: Part II. interaction between the tensile stress and the hydride morphology

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 our results and the existing studies were explained by the major self-accomodation 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.

Isothermal martensitic transformation in Fe-Ni and Fe-Ni-C alloys at subzero temperatures

A survey of experimental work on the isothermal martensitic transformation in the presence of prior martensite is given in Fe-Ni and Fe-Ni-C alloys having subzero Ms (Mb) temperatures. Low temperature dilatometric measurements are correlated with internal friction measurements in the 5 to 200 K temperature range. This study will show that this transformation can be discussed in terms ofC-curve behavior as in Fe-Ni-Mn alloys. Nevertheless, in Fe-Ni and Fe-Ni-C alloys internal stresses created by the austenite-to-martensite transformation during cooling play an important part in the development of the isothermal transformation. Furthermore, internal friction is shown to be proportional to expansivity thus indicating that the internal friction technique can be applied successfully for the study of phase transformations.

EFFECT OF HYDRIDES ON THE MECHANICAL PROPERTIES OF ZIRCALOY-4

ABSTRACT In order to better understand the embrittlement of Zircaloy-4 by hydrides and the ductile-brittle transition on this alloy, Zircaloy-4 sheet tensile specimens in the stress-relieved, recrystallized and β treated states were hydrided (10 to 1500 ppm wt H) and then tested at two temperatures (20°C, 350°C). Metallographic and fractographic analyses were carried out to determine the fracture micro-mechanisms. The results showed that, at 20°C, Zircaloy-4 undergoes a significant ductile to brittle transition for high hydrogen contents. Heat treatment shifts this transition (to zero elongation) considerably, from 1050 ppm wt H for the stress-relieved state to less than 250 ppm wt H for the β treated state. However, at 350°C, Zircaloy-4 remains ductile up to hydrogen content higher than 1100 ppm wt. At 20°C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents, and by intergranular crack and decohesion through the continuous hydride network for high hydrogen content. A model based on image analysis and hydride embrittlement micro-mechanism observations is used to calculate the upper-limit hydrogen content which makes Zircaloy-4 totally brittle. The difference between the mechanical behaviors of stress-relieved and recrystallized states is also explained.

Isothermal Martensitic Transformation in Fe-Ni-C Alloys at Subzero Temperatures in the Presence of Hydrogen

ABSTRACT Dilatometric and internal friction results show that hydrogen induces a stabilization of the γ phase and an increase in the intensity of the internal stresses created during cooling by the martensitic transformation.

Internal Friction Spectra Modifications Related to Carbon in Acicular Martensite

ABSTRACT In this study we present an analysis of the behaviour of low Ms iron-nickel alloys containing different amounts of carbon. Influence of parameters like carbon content, test frequency, cycling rate and strain amplitude are analysed during cooling (austenite to martensite transformation) and heating (first stages of rearrangement). Results are discussed in comparison with phase transformation models on one hand and with carbon-dislocation interaction on the other hand.