Nanoindentation studies of zirconium hydride

https://doi.org/10.1016/j.jallcom.2003.08.094Get rights and content

Abstract

In order to study such nanomechanical properties of the zirconium hydride as nanohardness, nanoindentation tests have been carried out in the present study. As reference data, macroscopic mechanical properties of the zirconium hydride such as Vickers hardness have also been measured. The zirconium hydride specimens were fabricated directly from pure zirconium metal in a modified UHV Sieverts apparatus, and had the hydrogen contents with 1.5–1.7 H/Zr. The nanohardness of the zirconium hydride increased with decreasing maximum indentation load, which has been referred to as indentation size effect (ISE). The zirconium hydride had higher nanohardness than crystal-bar zirconium and pure zirconium, which was qualitatively in good agreement with the results of the Vickers hardness tests. The work hardening surface layer due to polishing the samples strongly affected the nanohardness, but barely affected the Vickers hardness. It was considered that the difference between the values of the nanohardness and the Vickers hardness were caused by not only the ISE, but also the difference of the indenter geometry and the testing method between the nanoindentation and the Vickers hardness test.

Introduction

Zirconium alloys such as Zircaloy and Zr-2.5Nb have been employed as the fuel claddings and the pressure tubes of light water reactors (LWRs) and CANDU nuclear reactors. In the core of these reactors, corrosion and irradiation effects are the most important problems. In zirconium alloys, hydrogen is generated by the corrosion reaction as follows:Zr+2H2OZrO2+2H2

Hydrogen in zirconium alloys has an extremely low terminal solid solubility (TSS) at room temperature [1], which leads to formation of zirconium hydride. The hydride is brittle and degrades the mechanical properties of the zirconium alloys. Therefore, it is important to understand the mechanical properties of the zirconium hydride for the prevention of hydride-induced fracture. In our previous studies, the macroscopic mechanical properties of the hydride such as elastic moduli, microhardness and stress–strain curve have been examined [2], [3], [4]. By using the data of the macroscopic mechanical properties, analysis based on the finite element method (FEM) and fracture mechanics has been performed to elucidate the fracture behavior of the hydrided fuel cladding tubes [4], [5], [6]. However, it is also important to elucidate the nanomechanical properties of the hydride since the mechanical properties of the most materials are inhomogeneous from a nanoscale point of view. Therefore, in the present study, nanohardness of the zirconium hydride, which is one of the important nanomechanical properties, has been estimated by nanoindentation tests. As reference data of the macroscopic mechanical properties, Vickers hardness of the zirconium hydride has also been measured. The results of the nanohardness and the Vickers hardness were compared, and the difference between the nanomechanical properties and the macroscopic mechanical properties were discussed.

Section snippets

Materials

The zirconium hydride specimens were directly fabricated from pure zirconium metal with 99.9% purity in a modified ultra high vacuum (UHV) Sieverts apparatus. The hydrogen content of the specimen ranged from 1.45 to 1.70 H/Zr (δ-ZrH2−x). The hydride specimen consisted of many platelets inside large grains without any microcracks and pores. The specimen surface was polished mechanically with emery papers down to 1200 grade, and then with aluminum oxide powder down to 0.05 μm. These specimens were

Results and discussion

The value of the Vickers hardness for the polished specimens of the zirconium hydride measured in the present study is shown in Fig. 2. The hardness value of the zirconium hydride is much higher than that of crystal-bar zirconium and pure zirconium, and slightly decreases with increasing hydrogen content.

Fig. 3 reveals the nanohardness of these polished specimens as a function of the maximum indentation load. From this diagram, the value of the nanohardness slightly tends to increase with

Conclusion

In order to study such nanomechanical properties of the zirconium hydride as nanohardness, nanoindentation tests have been carried out in the present study. As reference data, macroscopic mechanical properties of the zirconium hydride such as Vickers hardness have also been measured. The nanohardness of the zirconium hydride increased with decreasing the maximum indentation load, which has been referred to as the indentation size effect (ISE). The zirconium hydride had higher nanohardness than

Acknowledgements

The present authors would like to thank Mr. T. Sugahara who was an MSc student in Osaka University for his experimental assistance. This work has been carried out under the auspices of the Japan Atomic Energy Research Institute.

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