Characteristics of granule solidification in gas atomization of molten beryllium

Abstract

Experimental and analytical studies on gas atomization of the molten beryllium and the production of beryllium granules are presented. The impact of various factors, including the choice of gas (nitrogen or helium), the cooling gas flow rate (ranging from 300 to 650 m/s), melt temperature, and droplet size (<500 µm), on the cooling rate and granule properties, is demonstrated. It has been determined that the solidification of beryllium granules can occur through two distinct mechanisms depending on the atomization process. These mechanisms include crystallization and amorphization (glass transition). When beryllium melt is atomized with nitrogen, granules with diameters less than 100 µm solidify via the amorphization mechanism (glass transition), while those with diameters exceeding 300 µm solidify through crystallization. In such cases, a portion of granules with sizes ranging from 100 to 300 µm undergoes a mixed mechanism solidification. In this process, the surface becomes amorphous, while the central part crystallizes, resulting in the formation of a “shell” on the surface, marking the transition from the glass transition mechanism to the crystallization mechanism. The thickness of this “shell” depends on the granule diameter, measuring 10–15 µm for 300 µm granules and 20–25 µm for 100 µm granules. The findings from this research align well with the hypothesis of a glass-crystalline mechanism of beryllium granule solidification, which leads to their separation at the interfacial boundary. Such solidification through a mixed mechanism results in the creation of a removable “crust” on the granule, which is typically more contaminated with impurities. Understanding this effect opens up possibilities for practical applications in the production of specialized materials from beryllium. The ability to separate the “crust” from the “core” provides the conditions for obtaining specialized sintered beryllium grades suitable for use in nuclear reactors and foil production, where a beryllium microstructure with “clean” boundaries is essential.