Chemical short-range order in undercooled Cu-Ni melts

Abstract

For liquid Cu-Co, which shows a rich phase diagram, it has been shown that Cu and Co demix, if the liquid is undercooled below its binodal line. In contrast Ni-Cu shows a comparatively simple phase diagram of a solid solution and no phase separation in liquid state has been reported so far. On the other hand, measurements of the electrical resistivity show a non-linear temperature dependence for melts of the Cu-rich alloys Ni40Cu60 and Ni20Cu80 [1]. A possible explanation for this non-linear temperature dependence might be concentration fluctuations that increase with increasing undercooling of the liquid, hence indicating a tendency for demixing. Also the slightly positive enthalpy of mixing of Ni-Cu points towards a demixing nature of this system. While these are indirect arguments, in this work we present direct investigations on the short-range order in Ni42.5Cu57.5 alloy melts by elastic neutron diffraction that have been performed on the D20 diffractometer at the Institute Laue-Langevin. An electromagnetic levitation furnace was used as sample environment allowing to undercool the melts below the melting temperature [2]. Partial structure factors have been determined by application of an isotopic substitution technique using samples of the four different isotopic compositions 58Ni42.5natCu57.5, 60Ni42.5natCu57.5, natNi42.5natCu57.5 and 62Ni42.563Cu57.5. The Bhatia-Thornton partial structure factor SNN that describes the topological short-range order of the melt closely resembles the structure factors observed for melts of the pure elements Ni [3] and Cu, indicating a similar topological structure. For the Bhatia-Thornton partial structure factor SCC that describes the chemical short-range order of the melt a rise in intensity is observed at low momentum transfer, q, on top of a relatively large flat background with some smaller oscillations towards larger q. The rise of SCC at low q increases with decreasing temperature. It directly points to the demixing nature of the system with concentration fluctuations on large length scales that become more pronounced with increasing undercooling. Interestingly, the generic behavior of phase separation has recently been studied by means of molecular dynamics simulations on binary symmetric Lennard-Jones mixtures [4,5]. A demixing behavior has been found for attractive Lennard-Jones liquids, if the interaction parameters of the unlike atomic pairs are smaller than those of the like atomic pairs. The shape of SCC and its temperature dependence calculated by the molecular dynamics simulations closely resemble those we observed experimentally for liquid in Ni42.5Cu57.5 showing that main predictions on the dependence of the chemical short-range order on the atomic interactions inferred from this simple generic model are found in real metallic systems like Cu-Ni. References 1) T. Richardsen, G. Lohöfer, Int. J. Thermophys. 23, 1207 (2002). 2) D. Holland-Moritz et. al., Meas. Sci. Techn. 16, 372 (2005). 3) T. Schenk et al., Phys. Rev. Lett. 89, 075507 (2002). 4) S.K. Das et al., J. Chem. Phys. 125, 024506 (2006). 5) S. Amore et al., J. Chem. Phys. 134, 044515 (2011).