Chemical short-range order in liquid Ni-Cu

Neutron diffraction in combination with isotopic substitution on the zero-scatterer62Ni4363Cu57shows indications for chemical short-range order in the stable liquid as evidenced by oscillations in the concentration-concentration structure factorSCC(q). This points towards a non-ideal solution behavior of Ni-Cu contrary to common believe but in agreement with measurements of free enthalpy of mixing. The temperature dependence ofSCCat small momentum transfer provides evidence of critical compositional fluctuations in Ni43Cu57melts.

Demonstration of the effect of stirring on nucleation from experiments on the International Space Station using the ISS-EML facility

The effect of fluid flow on crystal nucleation in supercooled liquids is not well understood. The variable density and temperature gradients in the liquid make it difficult to study this under terrestrial gravity conditions. Nucleation experiments were therefore made in a microgravity environment using the Electromagnetic Levitation Facility on the International Space Station on a bulk glass-forming Zr₅₇Cu_15.4Ni_12.6Al₁₀Nb₅ (Vit106), as well as Cu₅₀Zr₅₀ and the quasicrystal-forming Ti_39.5Zr_39.5Ni₂₁ liquids. The maximum supercooling temperatures for each alloy were measured as a function of controlled stirring by applying various combinations of radio-frequency positioner and heater voltages to the water-cooled copper coils. The flow patterns were simulated from the known parameters for the coil and the levitated samples. The maximum nucleation temperatures increased systematically with increased fluid flow in the liquids for Vit106, but stayed nearly unchanged for the other two. These results are consistent with the predictions from the Coupled-Flux model for nucleation.

Structure and dynamics of glass-forming alloy melts investigated by application of levitation techniques

In this work results of studies on the short-range order and on the atomic dynamics in different stable and undercooled glass-forming metallic melts are reviewed. In order to undercool the melts deeply below the melting temperature and to avoid chemical reactions of the melts with crucible materials, the samples are containerlessly processed utilizing the electromagnetic or the electrostatic levitation technique. The short-range structure of the melts is studied by neutron diffraction, while the atomic dynamics are investigated by quasielastic neutron scattering. The relationship between short-range structure and atomic dynamics is discussed within the mode coupling theory of the glass transition. We will show that taking the time- and space-averaged structural information provided by measured partial structure factors as an input, mode coupling theory is able to explain the experimental results concerning the activation energies for self-diffusion and the coupling/decoupling behavior of the self-diffusion coefficients of the different alloy components.

Phase separation in ternary Co-Gd-Ti liquids

The phase equilibria and the solidification behavior of ternary Co-Gd-Ti (Co ≤35 at.%) alloys have been investigated. The phase transformation and equilibria in the liquid phase were studied in situ for two alloys, Co30Gd35Ti35 and Co30Gd50Ti20, by combining electrostatic levitation of the samples with high-energy synchrotron x-ray diffraction (XRD) at elevated temperature. The XRD patterns with two diffuse maxima for molten Co30Gd35Ti35 give direct evidence for liquid-liquid phase separation in this composition. In contrast, no indication for phase separation in the Co30Gd50Ti20 alloy is detected. Coarsened microstructures, typical for the phase-separating systems, are observed for the Co30Gd35Ti35, Co25Gd37.5Ti37.5, Co10Gd45Ti45 and Co30Gd20Ti50 cast alloys. Our findings suggest that the stable miscibility gap of binary Gd-Ti extends into the ternary Co-Gd-Ti system (up to about 30 at.% Co). Thermodynamic calculations of the ternary Co-Gd-Ti system by the CALPHAD method are in good agreement with the experimental results.

Colloids as model systems for metals and alloys: a case study of crystallization

Metallic systems are widely used as materials in daily human life. Their properties depend very much on the production route. In order to improve the production process and even develop novel materials a detailed knowledge of all physical processes involved in crystallization is mandatory. Atomic systems like metals are characterized by very high relaxation rates, which make direct investigations of crystallization very difficult and in some cases impossible. In contrast, phase transitions in colloidal systems are very sluggish and colloidal suspensions are optically transparent. Therefore, colloidal systems are often discussed as model systems for metals. In the present work, we study the process of crystallization of charged colloidal systems from the very beginning. Charged colloids offer the advantage that the interaction potential can be systematically tuned by a variation of the particle number density and the salt concentration. We use light scattering and ultra-small angle x-ray scattering to investigate the formation of short-range order in the liquid state even far from equilibrium, crystal nucleation and crystal growth. The results are compared with those of equivalent studies on metallic systems. They are critically assessed as regards similarities and differences.

Phase behaviour of deionized binary mixtures of charged colloidal spheres

We review recent work on the phase behaviour of binary charged sphere mixtures as a function of particle concentration and composition. Both size ratios Γ and charge ratios Λ are varied over a wide range. Unlike the case for hard spheres, the long-ranged Coulomb interaction stabilizes the crystal phase at low particle concentrations and shifts the occurrence of amorphous solids to particle concentrations considerably larger than the freezing concentration. Depending on Γ and Λ, we observe upper azeotrope, spindle, lower azeotrope and eutectic types of phase diagrams, all known well from metal systems. Most solids are of body centred cubic structure. Occasionally stoichiometric compounds are formed at large particle concentrations. For very low Γ, entropic effects dominate and induce a fluid-fluid phase separation. Since for charged spheres the charge ratio Λ is also decisive for the type of phase diagram, future experiments with charge variable silica spheres are suggested.

Colloids as model systems for liquid undercooled metals

Charged colloidal particles interact via a hard core Yukawa potential, while isotropic Lennard-Jones-like potentials are frequently used as pair potentials in metals. We present measurements of the structure factor of shear molten monodisperse colloids and molten metals using ultrasmall-angle x-ray scattering and elastic neutron scattering, respectively. In both systems data analysis gives evidence of fivefold-symmetric short-range order becoming more pronounced with increasing deviations from equilibrium. The experiments demonstrate that in both systems topological effects control ordering in the melt state.

Icosahedral short-range order in deeply undercooled metallic melts

Experimental evidence of icosahedral short-range order in stable and deeply undercooled melts of pure metallic elements is obtained using the combination of electromagnetic levitation with neutron scattering. This icosahedral short-range order is shown to occur in the bulk metallic melt independently of the system investigated. It strongly increases with the degree of undercooling.

Direct determination of metastable phase diagram by synchrotron radiation experiments on undercooled metallic melts

The phase selection process during the crystallization of undercooled metallic melts is studied in situ by combining the electromagnetic levitation technique with energy dispersive x-ray diffraction of synchrotron radiation. The crystallization of metastable bcc phase in binary Ni-V alloys was identified. A metastable phase diagram of Ni-V alloy is constructed, which shows the primarily solidifying phase as a function of composition and undercooling. The analysis within nucleation theory emphasizes the important role of metal oxide as a heterogeneous nucleation site controlling the phase selection.