Icosahedral Short-range Order Research Articles

Possible influence of quenched-in nuclei on quasicrystal formation in mechanically alloyed Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 glassy powder

The influence of quenched-in nuclei of icosahedral short range order, proposed as a possible explanation for quasicrystal formation, has been investigated by producing a melt-spun ribbon from previously mechanically alloyed Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 glassy powder. A ribbon prepared from highly pure materials forms a quasicrystalline phase during the first stage of the crystallization process, whereas no quasicrystal formation has been detected in the devitrification of the ribbon produced from the mechanically alloyed powder. This finding indicates that the absence of quenched-in nuclei may not be the only reason for the lack of quasicrystal formation in mechanically alloyed powders.

Difference in icosahedral short-range order in early and late transition metal liquids.

New short-range order data are presented for equilibrium and undercooled liquids of Ti and Ni. These were obtained from in situ synchrotron x-ray diffraction measurements of electrostatically levitated droplets. While the short-range order of liquid Ni is icosahedral, consistent with Frank's hypothesis, significantly distorted icosahedral order is observed in liquid Ti. This is the first experimental observation of distorted icosahedral short-range order in any liquid, although this has been predicted by theoretical studies on atomic clusters.

Surface Tension and Viscosity of Quasicrystal-Forming Ti–Zr–Ni Alloys

Recent X-ray scattering measurements show that icosahedral short-range order in Ti–Zr–Ni alloys is responsible for a change in phase selection from the stable C14 Laves phase to the quasicrystalline icosahedral phase, and that icosahedral short-range order increases at deeper undercoolings. This change in short-range order should be reflected in changes in the thermophysical properties of the melt. The surface tension and viscosity of quasicrystal-forming Ti–Zr–Ni alloys were measured over a range of temperature, including both stable and undercooled liquids, by an electrostatic levitation (ESL) technique. ESL is a containerless technique which allows processing of samples without contact, greatly reducing contamination and increasing access to the metastable undercooled liquid. The measured viscosity is typical of glass-forming alloys of similar composition to the quasicrystal-forming alloys studied here; however, the surface tension shows an anomaly at deep undercoolings.

Icosahedral short-range order in amorphous alloys.

We have characterized the icosahedral short-range order in amorphous solids using local environment probes. Such topological local order is pronounced even in an amorphous alloy that does not form quasicrystalline phases upon crystallization, as demonstrated by the extended x-ray absorption fine structure and x-ray absorption near-edge structure of a Ni-Ag amorphous alloy analyzed through reverse Monte Carlo simulations.

High-resolution TEM study of the microstructure of Zr 65Ni 10Cu 7.5Al 7.5Ag 10 bulk metallic glass

Zr 65Ni 10Cu 7.5Al 7.5Ag 10 bulk metallic glass with a diameter of 2 mm was prepared by copper-mould casting. The microstructures of the bulk alloy were examined by X-ray diffraction, optical microscopy and high-resolution electron microscopy. It was found that the structure of the bulk metallic glass formed by copper-mould casting is actually not homogeneous, but composed of three different regions: a purely amorphous portion which makes up the major part of the bulk sample; a second portion containing a few of very small particles in the amorphous matrix, and a third portion consisting of the quasicrystals in the amorphous matrix. The formation of quasicrystal during casting of the melt involved the inward diffusion of Zr and outward diffusion of Ni and Cu. The icosahedral short-range order is likely to have already existed in the glassy state of Zr 65Ni 10Cu 7.5Al 7.5Ag 10 alloy after casting.

Short-range order in undercooled metallic liquids

Abstract The containerless processing technique of electromagnetic levitation was combined with elastic neutron scattering in order to study the short-range order (SRO) of stable and deeply undercooled liquids of the pure elements Ni, Fe and Zr and of the quasicrystal-forming alloy Al65Cu25Co10. The results deliver experimental evidence for an icosahedral short-range order (ISRO) prevailing in the investigated metallic melts.

Crystallization of liquids and glasses to quasicrystals

The local structures of metallic glasses and undercooled liquid metal alloys are presumed to contain a high degree of icosahedral short-range order, incompatible with translational periodicity and creating a barrier to the nucleation of crystal phases. The barrier should be lower for the nucleation of icosahedral quasicrystals (i-phase). A brief overview is provided of studies in liquids and glasses that support this conclusion. Recent X-ray diffraction and undercooling results in a TiZrNi alloy that forms an i-phase are presented, correlating an icosahedral local order in the liquid with a lower nucleation barrier for the i-phase. For a growing number of alloys that can be prepared as glasses, a nanosized i-phase microstructure forms upon glass crystallization (devitrification), consistent with the expected low nucleation barrier. However, since glasses that do not form quasicrystals have similar devitrification nanostructures, other crystallization mechanisms must also be operative. These are reviewed. A new model for homogeneous nucleation that couples the interfacial and the long-range diffusion fluxes, is advanced as one way to explain nanostructure formation in these metallic glasses.

Molecular-dynamics study of the local symmetry changes in metallic liquids

It has been conjectured that the local structure of simple monoatomic liquids is mainly of five-fold symmetry. Experimental evidence for such icosahedral building blocks has recently been found in liquid droplets of lead adjacent to a silicon wall and in deeply undercooled melts of metallic elements like bcc iron, fcc nickel and bcc zirconium. We have used molecular dynamics in order to investigate supercooling effects in the melt of another element (aluminum) on the basis of a common neighbour analysis. In the simulation, we have employed an embedded atom method potential optimized with the help of ab initio calculations for aluminum. The simulations confirm the recent experimental results that, independent of the system, the icosahedral short-range order strongly increases with the degree of undercooling.

Mechanical spectroscopy of hydrogen-absorbing quasicrystals

The H-induced damping peak in icosahedral quasicrystals, generally attributed to a Snoek-type reorientation mechanism, was studied in comparison between Zr69.5Cu12Ni11Al7.5 and Ti53Zr27Ni20 using the vibrating-reed technique. Whereas the peak in Zr69.5Cu12Ni11Al7.5 is practically the same as in the amorphous phase (in accordance with icosahedral short-range order in metallic glasses), significant differences in shape and position of the peak are found between the two quasicrystalline alloys. These might possibly be used as a probe to distinguish between icosahedral structure types. Another interesting result is a higher intrinsic damping of quasicrystalline Zr 69.5Cu12Ni11Al7.5, compared to the amorphous phase, between 450 and 650 K. © 2003 Elsevier B.V. All rights reserved.

Evidence of icosahedral short-range order in Zr70Cu30 and Zr70Cu29Pd1 metallic glasses

Change in local atomic environment during crystallization of Zr-based glassy alloys was studied by extended x-ray absorption fine structure (EXAFS) spectroscopy. The formation of icosahedral quasicrystalline phase followed by crystallization of tetragonal CuZr2 has been observed in the Zr70Cu29Pd1 glassy alloy during annealing up to 850 K. On the other hand, the binary Zr70Cu30 alloy shows a single glassy to crystalline CuZr2 phase transformation. The local atomic environment of as-quenched Zr70Cu30 alloy is matched to an icosahedral local atomic configuration, which is similar to that of the as-quenched Zr70Cu29Pd1 alloy and the alloy annealed at 593 K containing icosahedral phase. Considering that the supercooled liquid region appears prior to crystallization in the Zr70Cu30 glassy alloy, the observed results support the theory claiming a strong correlation between the existence of local icosahedral short-range order and stability of the supercooled liquid state.

Local order of liquid and supercooled zirconium by ab initio molecular dynamics.

It has been suggested that icosahedral short-range order (SRO) occurs in deeply undercooled melts of pure metallic elements. We report results of first-principles molecular dynamics simulations for stable and undercooled zirconium liquids. Our results emphasize the occurrence of a local order more complex than the icosahedral one. For stable liquid, the local order is interpreted on the basis of a competition between a polytetrahedral SRO and a bcc-type SRO. We also demonstrate that a bcc-type SRO increases with the degree of undercooling.

Influence of Icosahedral Short-Range Order in Undercooled Melts on the Nucleation Barrier Directly Observed

Influence of Icosahedral Short-Range Order in Undercooled Melts on the Nucleation Barrier Directly Observed

First x-ray scattering studies on electrostatically levitated metallic liquids: demonstrated influence of local icosahedral order on the nucleation barrier.

To explain the unusual stability of undercooled liquids against crystallization, Frank hypothesized that the local structures of undercooled liquids contain a significant degree of icosahedral short-range order, which is incompatible with long-range periodicity. We present here the first direct experimental demonstration of Frank's complete hypothesis, showing a correlation between the nucleation barrier and a growing icosahedral short-range order with decreasing temperature in a Ti39.5Zr39.5Ni21 liquid. A new experimental facility, BESL (Beamline Electrostatic Levitation), was developed to enable the synchrotron x-ray structural studies on deeply undercooled, reactive liquids.

Precipitation of an icosahedrally symmetric ordered phase in Zr–Ti–Cu–Ni–Al metallic glasses

Results from a study of the precipitation of an icosahedrally symmetric ordered phase in Zr62−xTixCu20Ni8Al10 amorphous alloys are presented. The data indicate the development of icosahedral short-range order (similar to that in the ordered phase) in the undercooled melt during quenching, and in the amorphous phase with annealing in the supercooled liquid region. Precipitation is controlled by long-range diffusion in alloys made with less than 3 at. % Ti. The resulting lower nucleation rates and longer induction times lead to a fine microstructure that contains icosahedral phase (i phase) grains ranging from 50 to 100 nm in diameter. In the glasses made with 5–7.5 at. % Ti, the composition of the precipitating phase is more similar to that of the glass, lowering the kinetic nucleation barrier and leading to an extremely fine nanostructured devitrified material, with i phase grain sizes from 2 to 5 nm. The x-ray diffraction pattern from this phase is similar to that of the i phase. However, because the grain size is nearly the same as the edge length of the unit cell of a high-order crystal approximate phase, no distinction can be made between the i phase and crystal approximate phases in such nanostructured materials.

Phase transformation in aZr41Ti14Cu12.5Ni10Be22.5bulk amorphous alloy upon crystallization

The phase precipitation, evolution, and transformation of the ${\mathrm{Zr}}_{41}{\mathrm{Ti}}_{14}{\mathrm{Cu}}_{12.5}{\mathrm{Ni}}_{10}{\mathrm{Be}}_{22.5}$ bulk amorphous alloy (BAA) are investigated upon prolonged annealing by means of in situ neutron diffraction, x-ray diffraction, and acoustic measurements. Icosahedral quasicrystalline (I phase), ${\mathrm{Be}}_{2}\mathrm{Zr}$ (hex, $a=0.38\mathrm{nm},$ $c=0.32\mathrm{nm}),$ and Laves phase ZrTiNi are observed at the first crystallization step. The I phase and ${\mathrm{Be}}_{2}\mathrm{Zr}$ phases transfer into stable crystalline phases at higher temperatures. The phase transitions induce marked microstructural and properties changes. The relation between the first precipitation of the temporary phases and the local microstructures of BAA is discussed. The precipitation of the I phase is suggested to be a structural heredity of the local icosahedral short-range order of BAA. The fundamental structural discontinuity between the crystalline and amorphous states results in an excellent glass forming ability and a high stability of the glass-forming alloy.

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 Frank's idea of an icosahedral SRO prevailing in undercooled metallic liquids.

X-ray and electrostatic levitation undercooling studies in Ti–Zr–Ni quasicrystal forming alloys

The first undercooling measurements on electrostatic-levitated droplets of TiZrNi alloys that form the icosahedral quasicrystal phase are presented. The reduced undercooling for crystallization decreases with an increasing polytetrahedral order of the primary solidifying phase, suggesting the development of icosahedral short-range order in the undercooled liquid. X-ray diffraction measurements made at the advanced photon source on liquid droplets of these alloys, aerodynamically levitated and heated to near their liquidus temperature, however, show only weak evidence for increased icosahedral order. This suggests that significant ordering occurs below the melting temperature.

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.

Short-range order in undercooled melts forming quasicrystals and approximants

For the first time, the short-range order of deeply undercooled liquids of alloys forming quasicrystalline and polytetrahedral phases (Al 13Fe 4 and Al 74Co 26) 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.

Undercooling And X-Ray Structural Studies Of Ti-Zr-Ni Liquids

ABSTRACTMaximum undercooling results for the icosahedral phase (i-phase), polytetrahedral C14 Laves phase, and solid solution phases are presented as a function of composition in Ti-Zr-Ni liquids. Containerless processing was achieved using the electrostatic levitation facility located at NASA/Marshall Space Flight Center. The maximum reduced undercooling decreases with increasing icosahedral short-range order in the ordered phase. The first synchrotron x-ray diffraction data from aerodynamically levitated liquids of Ti-Zr-Ni alloys suggest an icosahedral short-range order in the liquids, supporting Frank's hypothesis, correlating icosahedral order in the liquid with the nucleation barrier to the crystal phase. The strong negative heats of mixing between Ti/Zr and Ni and their relative atomic sizes likely favor the formation of this local icosahedral order.