Geun Woo Lee

Icosahedral-phase formation in as-cast Ti-Zr-Ni alloys

Previously, the formation of the thermodynamically stable icosahedral Ti-Zr- Ni i-(Ti-Zr-Ni) phase has been observed in rapidly quenched alloys, or by a low-temperature solid-state transformation from crystal phases that form at higher temperatures. Here we report the nucleation and growth of the icosahedral (i) phase directly from the liquid in as-cast Ti 33 Zr 46 Ni 21 and Ti 37 Zr 42 Ni 21 alloys. Diffraction patterns obtained by transmission electron microscopy and X-ray diffraction confirm the phase identity. Differential scanning calorimetry measurements show an endothermic transformation from the i-phase to a phase mixture of a C14 Laves phase and a solid-solution phase, demonstrating that this i-phase is also stable. The short time that the liquid remains in the Laves-phase-forming field and the higher nucleation rate for the i-phase, owing to strong similarities in the local structures of the i-phase and liquid, allow i-phase nucleation and growth directly from the liquid.

Abnormal drop in electrical resistivity with impurity doping of single-crystal Ag

Resistivity is an intrinsic feature that specifies the electrical properties of a material and depends on electron-phonon scattering near room temperature. Reducing the resistivity of a metal to its potentially lowest value requires eliminating grain boundaries and impurities, but to date few studies have focused on reducing the intrinsic resistivity of a pure metal itself. We could reduce the intrinsic resistivity of single-crystal Ag, which has an almost perfect structure, by impurity doping it with Cu. This paper presents our results: resistivity was reduced to 1.35 μΩ·cm at room temperature after 3 mol% Cu-doping of single-crystal Ag. Various mechanisms were examined in an attempt to explain the abnormal behavior.

Interfacial Free Energy Controlling Glass-Forming Ability of Cu-Zr Alloys

Glass is a freezing phase of a deeply supercooled liquid. Despite its simple definition, the origin of glass forming ability (GFA) is still ambiguous, even for binary Cu-Zr alloys. Here, we directly study the stability of the supercooled Cu-Zr liquids where we find that Cu64Zr36 at a supercooled temperature shows deeper undercoolability and longer persistence than other neighbouring compositions with an equivalent driving Gibbs free energy. This observation implies that the GFA of the Cu-Zr alloys is significantly affected by crystal-liquid interfacial free energy. In particular, the crystal-liquid interfacial free energy of Cu64Zr36 in our measurement was higher than that of other neighbouring liquids and, coincidently a molecular dynamics simulation reveals a larger glass-glass interfacial energy value at this composition, which reflects more distinct configuration difference between liquid and crystal phase. The present results demonstrate that the higher crystal-liquid interfacial free energy is a prerequisite of good GFA of the Cu-Zr alloys.

Dynamic pressure-induced dendritic and shock crystal growth of ice VI

Crystal growth mechanisms are crucial to understanding the complexity of crystal morphologies in nature and advanced technological materials, such as the faceting and dendrites found in snowflakes and the microstructure and associated strength properties of structural and icy planetary materials. In this article, we present observations of pressure-induced ice VI crystal growth, which have been predicted theoretically, but had never been observed experimentally to our knowledge. Under modulated pressure conditions in a dynamic-diamond anvil cell, rough single ice VI crystal initially grows into well defined octahedral crystal facets. However, as the compression rate increases, the crystal surface dramatically changes from rough to facet, and from convex to concave because of a surface instability, and thereby the growth rate suddenly increases by an order of magnitude. Depending on the compression rate, this discontinuous jump in crystal growth rate or “shock crystal growth” eventually produces 2D carpet-type fractal morphology, and moreover dendrites form under sinusoidal compression, whose crystal morphologies are remarkably similar to those predicted in theoretical simulations under a temperature gradient field. The observed strong dependence of the growth mechanism on compression rate, therefore, suggests a different approach to developing a comprehensive understanding of crystal growth dynamics.

Multiple pathways of crystal nucleation in an extremely supersaturated aqueous potassium dihydrogen phosphate (KDP) solution droplet

Significance We successfully achieve unprecedentedly deep levels of supersaturation (S ∼ 4.1) with KH2PO4 (KDP) solutions by using a newly developed device that combines electrostatic levitation with Raman and X-ray scattering. Our study reveals two interesting phenomena. One is an existence of two different solution states, that is, low-concentration KDP solution (LCS) and high-concentration KDP solution (HCS). The other is an emergence of different crystallization paths that depend on the degree of supersaturation: (i) LCS to stable KDP crystal (tetrahedral structure), (ii) LCS to HCS to a metastable KDP crystal (monoclinic structure) to stable KDP crystal. This is a direct in situ observation of multiple pathways of nucleation in aqueous solution. Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH2PO4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.

Uncertainty evaluation for density measurements of molten Ni, Zr, Nb and Hf by using a containerless method

We report on high-temperature density measurements of nickel (Ni), zirconium (Zr), niobium (Nb), and hafnium (Hf) in supercooled and stable liquid states by using an electrostatic levitator (ESL) and evaluation of their associated uncertainties. More specifically, this work demonstrates a detailed description of our non-contact measurement method (i.e. schematics of the instrumentation, levitation procedure and density calculation from droplet images). We find that the main contribution of the uncertainties come from measuring sample temperature and mass, aspect ratio of the sample shape, pixel-calibration factors for two-dimensional (2D) detector, and order of the fitting function for calculating the volume. The measurements are typically made with combined uncertainties less than 0.5% and 2.1% for two different types of pyrometers that are used in low temperature (600 K ~ 2800 K) and high temperature (1000 K ~ 3800 K) ranges each operating at a wavelength of 1.6 μm and 0.9 μm, respectively. At melting temperatures, the combined uncertainties of the density measurements of liquid metals are measured less than ± 1.4% for low temperature and ± 2.2% for high temperature cases.

Structural studies of a Ti-Zr-Ni quasicrystal-forming liquid

Employing the technique of electrostatic levitation coupled with high-energy x-ray diffraction, Ti39.5Zr39.5Ni21 liquids were shown previously to develop significant short-range icosahedral order with supercooling. However, that conclusion was based on the assumption of a single dominant cluster type in the liquid and the observed evolution of the high-q shoulder on the second peak in the structure factor, S(q). Here, new diffraction data that were obtained using more rapid data acquisition methods are presented. These allow structural studies to be made down to and through recalescence to the icosahedral quasicrystal. The liquid structures obtained from a Reverse Monte Carlo analysis of these data are characterized by their bond-angle distributions, Honeycutt and Andersen indices and bond orientational order parameters. These analyses indicate that while there are several different types of local order, the icosahedral short-range order is dominant and increases gradually with supercooling.

Simultaneous measurement of emittance, transmittance, and reflectance of semitransparent materials at elevated temperature

A two-substrate method is developed to simultaneously determine emissivity, transmittance, and reflectance of semitransparent materials with a single measurement under the same environment at elevated temperature. The three quantities can be obtained through the emissivities of substrates and the apparent emissivities resulting from the radiance of the sample heated by substrates. The two-substrate method is compared with the conventional method by measuring sapphire samples with various thicknesses, resulting in good agreements for all the samples. The present method will be useful to measure the temperature dependence of optical properties of porous ceramic materials.

Evolution of atomic structure in Al75Cu25 liquid from experimental and ab initio molecular dynamics simulation studies

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al(75)Cu(25) alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al(75)Cu(25) alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt-Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al(75)Cu(25) alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid-liquid transition appears in the bulk liquid Al(75)Cu(25) alloy in the studied temperature range.

A liquid-liquid transition can exist in monatomic transition metals with a positive melting slope

Liquid-liquid transitions under high pressure are found in many elemental materials, but the transitions are known to be associated with either sp-valent materials or f-valent rare-earth elements, in which the maximum or a negative slope in the melting line is readily suggestive of the transition. Here we find a liquid-liquid transition with a positive melting slope in transition metal Ti from structural, electronic, and thermodynamic studies using ab-initio molecular dynamics calculations, showing diffusion anomaly, but no density anomaly. The origin of the transition in liquid Ti is a pressure-induced increase of local structures containing very short bonds with directionality in electronic configurations. This behavior appears to be characteristic of the early transition metals. In contrast, the late transition metal liquid Ni does not show the L-L transition with pressure. This result suggests that the possibility of the L-L transition decreases from early to late transition metals as electronic structures of late transition metals barely have a Jahn-Teller effect and bond directionality. Our results generalize that a phase transition in disordered materials is found with any valence band regardless of the sign of the melting slope, but related to the symmetry of electronic structures of constituent elements.