Precipitation characteristics and growth mechanism of Fe and Cr from liquid lead-bismuth eutectic by quenching in liquid nitrogen

Abstract

The compatibility of structural steels with liquid lead-bismuth eutectic (LBE) restricts the development of the lead-cooled fast reactor (LFR) and the accelerator driven sub-critical (ADS) system. During the reactor operation, alloying elements in steel can dissolve into LBE. The dissolved alloying elements could precipitate in cold parts of the reactor due to the decrease in solubility, which can cause pipe clogging and worsen heat transfer. Therefore, it is necessary to study their precipitation characteristics to provide a reference for the filtration process. In this paper, Fe, Cr, and a mixture of Fe and Cr were added to liquid LBE at 1173 K and quenched in liquid nitrogen to study their precipitation characteristics. The scanning electron microscope (SEM) and field-emission scanning electron microscope (FESEM) were used to analyze the morphology, and the phase structure was identified by an X-ray diffractometer. The chemical compositions of precipitates were analyzed by an energy dispersive spectrometer (EDS, Esprit Compact, Bruker) and an electron probe microanalyzer (EPMA, EPMA-8050 G, Shimadzu). The solid-liquid interface structure and growth mechanism of precipitates were also discussed. The difference in growth mechanisms resulted in various morphologies for the precipitates. There were three morphologies of precipitates: faceted dendrite, polyhedron (cube, octahedron, and hopper), and seaweed dendrite. All the precipitates in LBE have a faceted growth character except the seaweed dendrites, irrespective of their morphology. The growth of Fe, Cr, and Cr-Fe faceted dendrites originated from continuous growth followed by lateral growth. On the contrary, the cubic and octahedral precipitates were controlled by the spiral growth mechanism, and the growth mechanism of hopper crystals was two-dimensional nucleation. The formation of seaweed dendrites was caused by a strong local undercooling of nonfaceted dendrites.