冷却速率对锆合金氢化物析出的影响

Effect of Cooling Rate on Hydride Precipitation in Zirconium Alloys

氢化物是影响核燃料固有性能与核结构材料服役安全的关键问题,而冷却速率显著影响氢化物的宏观形貌和微观特性。本工作借助OM、BSE-SEM以及EBSD等表征手段,对不同冷却条件下析出的锆氢化物进行了系统研究。实验结果发现,fcc结构δ相是锆合金中形成的主要氢化物结构,其在锆合金板材轧向与横向平面内呈条状分布,随冷却速率提高,亚稳态面心四方(fct)结构γ氢化物数量明显增多。2种结构的氢化物在α-Zr母体晶粒内取向一致,与基体保持{0001}//{111}、<11-20>//<110>取向关系。证实氢化物尖端的强应变能够诱导新的纳米氢化物优先在已有氢化物尖端位置形核、生长,然后通过氢化物堆垛、排列构成条状氢化物。此外,锆合金中Zr(Fe,Cr)_(2)第二相粒子作为微区氢陷阱与形核位点,对氢化物条状形貌的形成具有促进作用。

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-structuredδphase,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 fccstructuredγ-hydrides increased in number density with rapid cooling rate.The two types of hydrides exhibited the same crystallographic orientation while sharing oneα-parent grain,both holding an orientation relationship of{0001}//{111}and<11-20>//<110>with theα-Zr matrix,independent of the cooling rate.Prior to complete transformation into theδphase,theγ-hydride is proposed as a transitional phase during the initial stage of hydride precipitation,given that theγ-phase requires a lower hydrogen concentration for the phase transformation and exhibits lower precipitation strain than theδphase.{111}<11-2>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.