Topological and Chemical Short-Range Order in Undercooled and Stable Melts of Al 13 (Co,Fe) 4 Alloys
- PDF / 592,686 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 70 Downloads / 116 Views
LL6.5.1/MM4.5.1
Topological and Chemical Short-Range Order in Undercooled and Stable Melts of Al13(Co,Fe)4 Alloys Dirk Holland-Moritz*, Thomas Schenk*, Virginie Simonet** and Robert Bellissent*** * DLR, Institut für Raumsimulation, Linder Höhe, D-51170 Köln, Germany ** Laboratoire Louis Néel, CNRS, BP166, F-38042 Grenoble Cedex 9, France *** Centre d'Études Nucléaires de Grenoble, DRFMC/SPSMS/MDN, F-38054 Grenoble Cedex, France
ABSTRACT The short-range order of stable and deeply undercooled liquids of Al13(Co,Fe)4 alloys forming polytetrahedral phases is investigated by combining the containerless processing technique of electromagnetic levitation with elastic neutron scattering. Partial structure factors were inferred from the diffraction data which allow an analysis of both, the topological and the chemical short-range order of these alloy melts as a function of the temperature.
INTRODUCTION Following a consideration of Frank in 1952 [1], an icosahedral short-range order (SRO) should be energetically favorable in undercooled melts of systems consisting of atoms of spherelike geometrical symmetry. An icosahedron is a polytetrahedron consisting of 13 atoms which is characterized by 6 fivefold symmetry axes. If atomic interactions according to a Lennard-Jones potential are assumed, the energy of an icosahedral cluster is about 8.4% smaller than that of 13 atoms clusters with a fcc- or hcp-structure. While these considerations are concerned with small clusters, later the idea of a polytetrahedral SRO in undercooled melts was confirmed also for larger systems by molecular dynamic computer simulations on Lennard-Jones liquids [2,3], which exhibited an increasing degree of icosahedral SRO with increasing undercooling of the melt. Although the first theoretical considerations suggesting an icosahedral short-range order go back to the early 1950ies, from the experimental side the question of the short-range order in undercooled metallic melts remained unsolved for about fifty years. Recently, the structure of liquid Pb on a Si (001) substrate was studied at the solid (Si) -liquid (Pb) interface by X-ray diffraction at temperatures above TL [4]. The experiment provides evidence of a fivefold local symmetry of the liquid lead at the Si(001) interface and thus icosahedral short-range order. While these experiments were performed in the vicinity of a solid-liquid interface and at a temperature above the melting temperature also diffraction experiments with neutron- and synchrotron radiation performed by application of the containerless processing technique of electromagnetic levitation on bulk liquids of the pure metals Fe, Zr, Ni and Co in the stable and deeply undercooled regime give a direct proof of an icosahedral SRO [5,6]. This SRO of the liquid phase is independent on the structure of the corresponding solid phase and becomes more pronounced if the temperature of the melt is decreased. In the above mentioned work, monoatomic systems were investigated. For alloys, despite aspects of the topological SRO also those of a
Data Loading...
