Deformation and fracture of zirconium hydrides during the plastic straining of Zr-4
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.145
Deformation and fracture of zirconium hydrides during the plastic straining of Zr-4 Luca Reali1*, Saïd El Chamaa2*, Daniel S. Balint 3, Catrin M. Davies3, Mark R. Wenman2 1
Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2
Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom
3
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
*
Equal authorship
ABSTRACT Crack initiation in zirconium alloys is an important issue for the safety of watercooled fission reactors. Zirconium hydrides that precipitate in service are potential crack nucleation sites. In this work, the deformation and cracking of zirconium hydrides was studied during room temperature deformation of a Zircaloy-4 tensile sample up to fracture. The sample contained a hydrogen concentration of 100േ20 ppm. The main aims of this study were to better understand the mechanisms behind the hydride fracture in a polycrystalline matrix, and to identify at which point in the deformation of the Zr matrix the first hydrides break. Cracks thus nucleated may coalesce and propagate through the hydrided Zr-alloy. Scanning electron microscopy (SEM) images of a number of hydrides, both intergranular and intragranular, were taken at discrete increments of deformation during an interrupted tensile test. The results show that cracks in hydrides tend to always occur normal to the applied load, signalling the importance of the external stress. However, evidence is also provided to support the hypothesis that internal stresses generated by microstructural constraints may lead to the fracture of some intergranular hydrides.
INTRODUCTION Zirconium alloys are the material of choice for cladding the nuclear fuel in pressurised water reactors. This is due to the combination of a low neutron absorption cross section and good mechanical and corrosion resistance. An important issue for the safety of the Zr-alloys is the fact that they are prone to the precipitation of hydrides. This leads to alloy embrittlement and under certain circumstances a time-dependent fracture mechanism known as delayed hydride cracking (DHC). As hydrogen tends to accumulate 559
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in regions under tensile stress, hydrides consequently form at the root of notches or flaws. If the stress is sufficiently high, they fracture and open a crack whose tensile field attracts more hydrogen, enabling the process to repeat itself [1]. The length scale of the stress raisers present in the cladding, of the macrohydrides that are visible under the optical microscope and of the DHC cracks is up to millimetres. However, at the microscale hydrides possess a finer structure. Individual micro-hydrides about 1-10 Pm in length and a fr
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