The effect of cooling rate and grain size on hydride microstructure in Zircaloy-4

doi:10.1016/j.jnucmat.2018.11.011

Journal of Nuclear Materials

Volume 513, January 2019, Pages 221-225

https://doi.org/10.1016/j.jnucmat.2018.11.011 Get rights and content

Abstract

We explore the distribution, morphology and structure of zirconium hydrides formed using different cooling rates through the solid state Zr+[H] → Zr + hydride transus, in fine and blocky alpha Zircaloy-4. We observe that cooling rate and grain size control the phase and distribution of hydrides formed. The blocky alpha (coarse grain, > 200 μm) Zircaloy-4, has a smaller grain boundary area to grain volume ratio and this significantly affects nucleation and growth of hydrides as compared to fine grain size (∼11 μm) material.

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Main body

Zirconium alloys are used in the nuclear industry as fuel cladding, as it has a good strength to neutron absorption cross section ratio and reasonable corrosion resistance. One concern when using zirconium alloys in high temperature water reactors is corrosion, as during service it can react with high temperature water to generate an oxide scale and pick up hydrogen [1]. For service conditions (∼350 °C) this hydrogen may exist in solution where it is highly mobile [2,3]. The hydrogen travels

Summary

We observe that the structure and population of hydrides is strongly determined by the cooling rate when hydrides are precipitated in zirconium. The relative grain size, and thus grain boundary area vs grain interior, can significantly change the hydride population. Importantly in terms of understanding the performance of nuclear fuel, the population of hydrides may influence the failure of the zirconium cladding during delayed hydride cracking and/or long-term storage.

Acknowledgements

TBB acknowledges funding from the Royal Academy of Engineering for his research fellowship. TBB and VT acknowledge funding from EPSRC through the HexMat programme grant (EP/K034332/1). Electron microscopy was performed within the Harvey Flower Electron Microscopy Suite and the Quanta was purchased within the Shell AIMS UTC. We would like to thank Alex Foden for assistance with the EBSD pattern matching.

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Hydride precipitation in zirconium alloys leads to embrittlement, making it essential to understand their prevalence and stability in the microstructure. Dictionary indexing of Kikuchi patterns, along with orientation relationship analysis and x-ray diffraction, confirmed the presence of both delta and gamma hydride phases in Zircaloy-4. Both phases were found to be stable in recrystallised zirconium, with the gamma phase exhibiting a distinct orientation relationship with the matrix. Delta hydride morphology and orientation were influenced by local stresses, resulting in a change in orientation during precipitation. By analysing the orientation relationships, the evolution of hydride phases could be visualised, providing insights into the room temperature stability of both delta and gamma hydrides.
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Hydrogen pick-up and hydride precipitation can lead to embrittlement and fracture strength reduction of nuclear fuel cladding tubes made of Zircaloy. Plastic deformation of hydride packets and its interaction with local plasticity in the zirconium matrix is a key linkage of microstructure feature to structural integrity of hydrided polycrystalline bulk Zircaloy. This work focuses on explicit representation of hydride packets from high spatial resolution electron backscatter diffraction onto a crystal plasticity finite element model for capturing and understanding slip localisation near hydride-matrix phase boundaries, based on the extracted material property of hydrides. The mechanisms behind slip evolution including slip nucleation, slip transfer, and slip inhibition are studied by combined high-resolution digital image correlation and crystal plasticity results. Through assessing various slip transfer parameters, new slip transfer criterion is proposed for α/δ phase boundaries. Prior to slip transfer criterion, local micromechanical quantities, specifically shear stress and stored energy density, are necessary to drive and provide pathway for subsequent slip transfer at α/δ phase boundaries.
Characterization of local deformation around hydrides in Zircaloy-4 using conventional and high angular resolution electron backscatter diffraction
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Zircaloy-4 is used as a fuel cladding material for water reactors, as it has good mechanical properties, corrosion resistance, and a low thermal neutron absorption cross section. However, the mechanical performance of Zircaloy-4 can be reduced during service due to hydrogen uptake and hydride formation. These hydrides are brittle, and often reduce the strength and toughness of materials as well as increase susceptibility to delayed hydride cracking (DHC). In this work, large grain Zircaloy-4 with hydrides was prepared and then cross sectioned using cryo-ion beam polishing, using plasma focused ion beam (pFIB) and broad ion beam (BIB) approaches to enable the preparation of a very high quality flat surface with no preferential etching of either the hydride or zirconium metal (typically metallographic polishing preferentially removes hydrides). Conventional and high angular resolution electron backscatter diffraction (EBSD) analysis were then used to explore morphology, deformation fields, and orientation relationships between the zirconium matrix and hydrides. Four maps were collected for analysis which included hydrides near grain boundaries: (a) where the hydride smoothly decorates across two of the connecting boundaries near a triple junction; (b) where the hydride smoothly decorates the boundary; (c) a mixture of smooth decoration of the interface and protrusion into the grains; (d) fine scale hydride that protrudes into one grain. This work highlights that incompatibility of the hydride within the zirconium matrix is strongly linked to the orientation relationship of the hydride and matrix, and the grain boundary character. These results may enable enhanced understanding of the role of hydrides in fracture as well as stress-induced hydride reorientation and DHC susceptibility.
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Short communicationThe effect of cooling rate and grain size on hydride microstructure in Zircaloy-4

Abstract

Section snippets

Main body

Summary

Acknowledgements

Nucl. Eng. Des.

Prog. Nucl. Energy

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Int. J. Hydrogen Energy

Scripta Mater.

Short communication
The effect of cooling rate and grain size on hydride microstructure in Zircaloy-4