冷却速率对锆合金氢化物析出的影响

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 <11^-20>//<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}<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.