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Acta Metall Sin  2024, Vol. 60 Issue (9): 1155-1164    DOI: 10.11900/0412.1961.2023.00166
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Effect of Cooling Rate on Hydride Precipitation in Zirconium Alloys
GONG Weijia1,2, LIANG Senmao1,2, ZHANG Jingyi3, LI Shilei4, SUN Yong5, LI Zhongkui1,2(), LI Jinshan1
1.School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
2.Clean Energy Research Center, Yangtze River Delta Research Institute of Northwestern Polytechnical University, Taicang 215400, China
3.Queen Mary University of London Engineering School, Northwestern Polytechnical University, Xi'an 710072, China
4.State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
5.Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China
Cite this article: 

GONG Weijia, LIANG Senmao, ZHANG Jingyi, LI Shilei, SUN Yong, LI Zhongkui, LI Jinshan. Effect of Cooling Rate on Hydride Precipitation in Zirconium Alloys. Acta Metall Sin, 2024, 60(9): 1155-1164.

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Abstract  

Zirconium alloys have been used as nuclear fuel claddings for decades, owing to their low thermal neutron absorption cross-section, good thermal conductivity, suitable mechanical properties, and excellent corrosion resistance. During in-reactor service, zirconium alloy cladding undergoes a corrosion reaction with the coolant and absorbs part of the hydrogen produced due to corrosion, resulting in the formation of brittle zirconium hydrides. Hydrides impose great risk to the mechanical integrity of the fuel claddings during reactor operation and even during storage and transportation of spent fuel rods. Hydride morphological features such as size, distribution, and growth direction are closely related to the cooling rate, which also affects the microstructural characteristics of hydrides, including nucleation sites, crystal structure, and precipitation strain. These factors further influence the mechanical properties and corrosion resistance of zirconium alloy cladding. Therefore, investigation of the influence of cooling rate on hydride precipitation is crucial to develop a theoretical study that can aid in the prevention of hydride embrittlement in nuclear fuel claddings. Herein, multiscale characterization techniques including OM, BSE-SEM, and EBSD were used to systematically investigate the morphology and microstructure of hydride precipitation under various cooling conditions in a Zr-4 plate material. The fcc-structured δ phase, well aligned in the plane of rolling and transverse directions, is the predominant hydride formed in zirconium alloys was found. With rapid cooling rates, the thickness and spacing of the hydrides decreased, forming finely dispersed plate-like distribution morphology. Intragranular hydrides and metastable fcc-structured γ-hydrides increased in number density with rapid cooling rate. The two types of hydrides exhibited the same crystallographic orientation while sharing one α-parent grain, both holding an orientation relationship of {0001}//{111} and <112¯0>//<110> with the α-Zr matrix, independent of the cooling rate. Prior to complete transformation into the δ phase, the γ-hydride is proposed as a transitional phase during the initial stage of hydride precipitation, given that the γ-phase requires a lower hydrogen concentration for the phase transformation and exhibits lower precipitation strain than the δ phase. {111}<112¯> twinning structures were identified within the hydrides, which are expected to favor alleviating hydride precipitation strains. High angular resolution EBSD revealed that strong tensile strains induced by the volume expansion of hydride precipitation are present in the vicinity of the hydride tip, which might act as the preferential nucleation site for new hydride precipitation, promoting the formation of hydride plate morphology. Furthermore, nanohydrides were identified precipitating at the boundary of Zr(Fe, Cr)2 second-phase particles, which is expected to play a role in the morphologic development of plate hydrides.

Key words:  zirconium alloy      hydride      cooling rate      microstructure      precipitation strain     
Received:  13 April 2023     
ZTFLH:  TG146.4  
Fund: National Natural Science Foundation of China(U2230124,12005170,U2067217);State Administration of Science, Technology and Industry for National Defense
Corresponding Authors:  LI Zhongkui, professor, Tel: 13991818960, E-mail: lizhangyi@nwpu.edu.cn

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00166     OR     https://www.ams.org.cn/EN/Y2024/V60/I9/1155

Fig.1  EBSD microstructure and texture of the Zr-4 plate after hydrogenation and homogenization annealing
(a) microstructure in the plane of normal direc-tion (ND) and rolling direction (RD)(b) {0001} <112¯0> pole figures (PFs) in the plane of RD and transverse direction (TD)
Fig.2  OM images of hydrides in Zr-4 alloy at various cooling conditions
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.3  High magnification OM images of hydrides in Zr-4 alloy at various cooling conditions
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.4  BSE-SEM images of hydrides in Zr-4 alloy at various cooling conditions
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.5  Phase distribution maps of hydrides in Zr-4 alloy at various cooling conditions (The red hexagonal units indicate the crystallography orientation of corresponding zirconium grain)
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.6  Inverse pole figures (IPFs) of hydrides in Zr-4 at various cooling conditions (The blue and green cubes represent the crystallographic orientations of δ- and γ-phase hydrides, respectively)
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.7  PFs of Zr grains and hydrides in Zr-4 alloy at various cooling conditions (The red, green, and blue unit cells represent the crystallographic orientations of α1, γ1, and δ2, respectively)
(a) 0.5oC/min (b) furnace cooling (c) air cooling (d) water cooling
Fig.8  Phase distribution map (a), IPF (b), and BSE-SEM (c-e) images of Zr(Fe, Cr)2 second phase particles and their surrounding hydrides in Zr-4 alloy at 0.5oC/min cooling condition (Yellow cubes show the orientation of the second phase particles)
Fig.9  Schematic illustrations of hydride and hydride twins formation in zirconium alloy
(a) three lattice occupations of atoms in hcp structure
(b) phase transformation of hcp-Zr into fcc-hydride
(c) 3D unit cells of α-Zr grain and hydride twins
(d) δ-hydride twin formation
Fig.10  Stress and strain fields of hydride precipitation in Zr-4 alloy under 0.5oC/min cooling condition characterized by high-resolution electron backscattering diffraction (HR-EBSD) (εxx, εyy, and εzz are the normal stresses in three directions; εxy, εyz, and εzx are the shear stresses in three planes)
(a) phase map (b) HR-kernel average misorientation (KAM)
(c) von Mises stress (d-f) normal strains (g-i) shear strains
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