Dirk Holland-Moritz

Short-range order in undercooled Co melts

It was suggested in 1952 by Frank that an icosahedral short-range order (SRO) should be energetically favourable in undercooled melts of systems consisting of atoms with a sphere-like geometrical symmetry. Although this idea is nearly 50 years old, there is still a lack of direct experimental information on the SRO prevailing in undercooled metallic liquids. In this work the results of investigations of the SRO of deeply undercooled liquids of Co by energy dispersive diffraction of synchrotron radiation are presented. The experimentally determined structure factors were simulated by assuming that the liquid consists of clusters with different short-range structures. The best fit of the experimental data is obtained if an icosahedral SRO is assumed. Thus, our investigations strongly support Franks idea of an icosahedral SRO prevailing in undercooled metallic liquids.

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.

Density and atomic volume in liquid Al-Fe and Al-Ni binary alloys

Abstract The density of liquid Al – Fe and Al – Ni binary alloys have been determined over a wide temperature range by a non-contact technique combining electromagnetic levitation and optical dilatometry. The temperature and composition dependences of the density are analysed. A negative excess volume correlates with the negative enthalpy of mixing, compound forming ability and chemical short-range ordering in liquid Al – Fe and Al – Ni alloys.

X-ray diffraction study of undercooled molten silicon

The short-range order of molten silicon was investigated in a wide temperature range from 1893 K down to 1403 K, corresponding to an undercooling of 290 K. Energy-dispersive x-ray diffraction was used in combination with electromagnetic levitation. The structure factor and the pair correlation function were determined as a function of temperature from the experimental data. A small hump on the higher wave vector side of the first peak in the structure factor was observed at all temperatures. The position of the first peak in the pair distribution function shifted to shorter distances and its height increased gradually with decreasing temperature. No discontinuous behavior was observed in the entire temperature range investigated.

Short-range order in undercooled and stable melts forming quasicrystals and approximants and its influence on nucleation

An icosahedral short-range order has been predicted to prevail in undercooled metallic melts. For melts of pure metals, this hypothesis was recently experimentally confirmed by diffraction experiments. This paper presents results of neutron scattering experiments on the short-range order of stable and undercooled liquids of alloys forming quasicrystals and polytetrahedral crystals. The studies indicate that an icosahedral topological short-range order prevails in all of the investigated melts. The influence of this icosahedral short-range order on the nucleation behaviour of solid phases from the undercooled liquid is discussed.

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.

Evidence of the transition from ordered to disordered growth during rapid solidification of an intermetallic phase

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni50Al50 undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ΔT*≈250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ΔT*. The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.

Atomic diffusion mechanisms in a binary metallic melt

The relation between static structure and dynamics as measured through the diffusion coefficients in viscous multicomponent metallic melts is elucidated by the example of the binary alloy Zr64Ni36, by a combination of neutron-scattering experiments and mode-coupling theory of the glass transition. Comparison with a hard-sphere mixture shows that the relation between the different self diffusion coefficients strongly depends on chemical short-range ordering. For the Zr-Ni example, the theory predicts both diffusivities to be practically identical. The kinetics of concentration fluctuations is dramatically slower than that of self-diffusion, but the overall interdiffusion coefficient is equally large or larger due to a purely thermodynamic prefactor. This result is a general feature for non-demixing dense melts, irrespective of chemical short-range order.

Disorder trapping and grain refinement during solidification of undercooled Fe–18 at% Ge melts

The electromagnetic levitation technique has been used to systematically study microstructure evolution and growth rate as a function of undercooling in concentrated Fe–18 at% Ge alloy. The samples are undercooled to a maximum of 240 K. Growth-rate analysis and transmission electron microscopy reveal that, beyond an undercooling of 120 K, the primary phase to solidify is disordered. Microstructural investigations show a decrease in grain size with increasing undercooling. Orientation-imaging microscopy using electron back-scattered diffraction (EBSD) and microhardness measurements have been used to show that recovery and recrystallization play a significant role in the evolution of final microstructure. Microstructural evolution has also been discussed in light of current models of dendrite growth and grain refinement.

Short-range order in undercooled melts forming quasicrystals and approximants

Abstract For the first time, the short-range order of deeply undercooled liquids of alloys forming quasicrystalline and polytetrahedral phases (Al13Fe4 and Al74Co26) is investigated by combining the containerless processing technique of electromagnetic levitation with elastic neutron scattering. The experimentally determined structure factor, S(Q), was simulated by assuming that clusters with different structures exist in the liquid. The best fit of the experimental data is obtained under the assumption of an icosahedral short-range order. This supports the nearly 50 year-old prediction of Frank that an icosahedral short-range order prevails in undercooled metallic liquids.