Electromagnetic levitation apparatus for diffraction investigations on the short-range order of undercooled metallic melts

Abstract

One of the most fundamental questions in the research field of undercooled melts is concerned with the structural short-range order of melts, because the short-range order decisively influences the physical properties and the solidification behaviour of undercooled liquids. Following the pioneering work of Frank in 1952, an icosahedral short-range order should be energetically favoured in undercooled metallic melts. Although this hypothesis is nearly 50 years old, direct experimental information on the short-range order prevailing in undercooled metallic liquids has only been available for a short time. This is mainly due to the fact that undercooled melts are a metastable state of matter which is difficult to conserve for times long enough to perform diffraction experiments of good quality. In this paper an apparatus is described that allows us to investigate the short-range order of undercooled metallic melts by combining the undercooling technique of electromagnetic levitation with the diffraction techniques of elastic neutron scattering and energy dispersive x-ray diffraction (EDXD) with synchrotron radiation. The diffraction experiments were performed with this device at the Institut Laue-Langevin (ILL) and at the European Synchrotron Radiation Facility (ESRF). These experiments provided the first direct experimental proof of an icosahedral short-range order prevailing in a great variety of undercooled melts of pure metals and alloys forming quasicrystalline or polytetrahedral phases. The icosahedral short-range order exists already at temperatures, T, above the melting temperature, TL, and becomes more pronounced if the melts are undercooled.