液体金属合金の均一液相の過冷却: Ni-Nb系均一液相の共晶組成近傍の小さな過冷却現象

摘要

液体の過冷却現象は金属学や物質科学において重要である．この現象はこの分野の基礎過程である凝固や結晶化現象へも密接に関係している．さらに，多くの分野から注目を集めている金属ガラスにも関係している．その理由は，金属ガラスが金属液体の過冷却状態を経て，たとえば，急速冷却法により作成されているからである．過冷却研究は不均質核生成の影響を排除して均質核生成を実現する努力の歴史である．%To study the supercooling of many liquid metals with high melting temperatures, the investigation was performed for the cooling curves of electrostatic levitation (ESL) experiments, which had been originally obtained for the measurements of many physical properties. The largest supercooling over the literature values was found for liquid Ru (428 K), Ta (721 K), W (601 K), and Ir (438 K), where temperatures in the parentheses mean the supercooling of respective liquid metals. This indicates the validity of ESL for the supercooling experiments of liquid metals and alloys because of being rather free from the heterogeneous nucleation. This ESL was applied to the study of supercooling of homogeneous liquid phase in the composition range from 29 at % Nb to 71 at% Nb of eutectic Ni-Nb system, whose eutectic point is present at 40.5 at% Nb and 1448 K. The experimental result shows a poor supercooling tendency of homogeneous liquid phase around the eutectic composition in spite of large supercooling far apart from this eutectic composition. This characteristic feature was discussed based on the classical nucleation theory coupled with the knowledge recently found, the existence of concentration fluctuations in the homogeneous liquid phase near the eutectic point.