S. J. Poon

Electron Localization in Metallic Quasicrystals

Bulk icosahedral-quasicrystalline aluminum-palladium-rhenium alloys of high structural quality and thermal stability are found to exhibit low-temperature electrical resistivities that are four orders of magnitude larger than those found in disordered metals and metallic glasses. Experiments suggest that these quasiperiodic alloys, which have a semimetallic electron density, are insulators at low temperature. The findings are discussed in light of theories on electron localization and band-gap formation in ordered metallic systems.

Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases

Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.

CaAl-based bulk metallic glasses with high thermal stability

We report that binary Ca–Al alloys can be readily cast into amorphous rods of 1 mm in diameter. Upon further alloying to depress the liquidus temperature, the amorphous rod diameter is increased to 3 mm. The high glass transition temperature Tg∼210 °C and crystallization onset temperature Tonset∼240 °C or higher observed are attributed to the covalent bonding trend noted in Ca–Al alloys that exhibit complex network structures. Along with a high microhardness value of 200–235 DPH (diamond pyramid hardness) and low mass densities of 2 gm/cm3, these thermally stable light-metal alloys are recognized as potential structural amorphous metals.

Ball milling-induced nanocrystal formation in aluminum-based metallic glasses

Various experimental techniques have been used to investigate the effect of mechanical milling on the structural stability of rapidly solidified aluminum-based metallic glasses. Using transmission electron microscopy (TEM) and X-ray diffraction methods, the formation of nanocrystalline Al particles in some ball-milled Al-rich metallic glasses (such as Al90Fe5Gd5 and Al90Fe5Ce5) is clearly observed. For other compositions with lower Al concentration such as Al85Ni5Y10, no such phase transformation can be detected by TEM or X-ray. However, differential scanning calorimetry (DSC) measurements show that the crystallization peaks of the ball-milled Al85Ni5Y10 metallic glass shifted to higher temperatures, while the crystallization enthalpy associated with the first exothermic peak decreased to a lower value, revealing that certain structural changes have taken place as a result of mechanical deformation. The compositional dependence of the structural stability of Al-based metallic glasses against mechanical deformation suggests that the nanocrystal formation induced by a deformation process is different from that caused by a thermal process. The large plastic strain induced atomic displacements and the enhancement of atomic mobility during the deformation process, are the possible mechanisms of mechanical deformation-induced crystallization. Our results demonstrate a new way of obtaining nanophase glassy composite alloy powders which are suitable for engineering applications upon further consolidation processing.

Mg–Ca–Zn Bulk Metallic Glasses with High Strength and Significant Ductility

The development of Mg–Ca–Zn metallic glasses with improved bulk glass forming ability, high strength, and significant ductility is reported. A typical size of at least 3–4 mm amorphous samples can be prepared using conventional casting techniques. By varying the composition, the mass density of these light metal based bulk amorphous alloys ranges from 2.0 to 3.0 g/cm 3 . The typical measured microhardness is 2.16 GPa, corresponding to a fracture strength of about 700 MPa and specific strength of around 250–300 MPa cm 3 /g. Unlike other Mg- or Ca-based metallic glasses, the present Mg–Ca–Zn amorphous alloys show significant ductility.

Electrical transport properties of TiCoSb half-Heusler phases that exhibit high resistivity

Electrical transport measurements have been performed on doped and undoped TiCoSb half-Heusler phases. The semiconducting properties are found to be more robust than those reported for MNiSn (M = Ti, Zr, Hf ). Undoped TiCoSb phases exhibit large n-type Seebeck coefficients and high resistivities that reach -500 µV K-1 at 300 K and ~1500 Ω cm at 4.2 K, respectively. A tendency towards carrier localization is seen in several disordered phases. The effects due to n-type and p-type dopants are readily manifested in the thermopower, from which moderately heavy electron and hole band masses are inferred. The unusual properties measured are consistent with the prediction of a wide bandgap for the TiCoSb phase. A resistivity minimum is observed at 500-600 K for undoped and V-doped TiCoSb. Consequently, the semiconducting gap has not been determined.

Formation of bulk metallic glasses in neodymium-based alloys

Abstract Bulk amorphous Nd-A1-TM (TM [tbnd] transition metal) alloy rods with diameters up to 6 mm have been produced using a mould-casting method. X-ray and electron diffraction experiments reveal that these rods are in an amorphous state. Differential scanning calorimetry (DSC) measurements show that the DSC curves of the amorphous rods are similar to those of the rapidly quenched ribbon samples with the same composition. The low liquidus temperatures and relatively high reduced glass temperatures T rg, combined with the large atomic-size variation in these alloys, are important factors in helping to explain their superior glass-forming ability.

Short range ordering in amorphous Al90FexCe10-x

Abstract The atomic structure of liquid-quenched amorphous Al 90 Fe x Ce 10− x ( x = 5, 7) was studied by diffraction method using X-rays from a synchrotron source. The diffraction data were Fourier-transformed to obtain the atomic pair-density function, and were compared with those obtained by the pulsed neutron scattering to determine the AlFe and AlCe coordination and the extent of compositional short range ordering. The accuracy of the pair-density function, limited by both statistical and systematic errors, is discussed. It was found that both Fe and Ce atoms show strong compositional short range ordering, amounting to 70–80% of the maximum ordering possible. While Ce atoms form a dilute dense random packing substructure, the substructure of Fe atoms and the surrounding Al was found to be substantially different from the random packing.

Unique metallic glass formability and ultra-high tensile strength in AlNiFeGd alloys

Abstract The metallic glass formability of aluminum-rich AlNiFeGd alloys has been systematically investigated. The critical cooling rate required to form an amorphous state in this system is generally low, and comparable to that of some of the best metallic glass formers, such as PdCuSi. Amorphous ribbons up to 0.25 mm thick can easily be produced by the single-roller melt-spinning technique. Tensile strengths as high as 1280 MPa and Youngs modulus of 75 GPa have been obtained. Bulk amorphous alloys with good mechanical properties are optimized in Al 85 Ni 6 Fe 3 Gd 6 . DSC and DTA studies reveal that the glass formability is unique for Al-based alloys because the reduced glass temperature T rg for AlNiFeGd can be as low as 0.44. This is much lower than conventional theory would suggest for easy glass forming systems. A mechanism for the unusual glass formability is suggested.

Submicrometer Superconducting YBa2Cu3O6+x Particles Made by a Low-Temperature Synthetic Route.

Evidence suggests that superconducting, orthorhombic YBa2Cu3O6+x+ (x ≳ 0.5) is always produced by oxidation of the oxygen-deficient, tetragonal form (x ≲ 0.5) of this phase (commonly referred to as 123). A synthetic route whereby solution-derived, carbon-free precursors are decomposed at 650� to 700�C in inert atmosphere to yield tetragonal 123 is now available. Appropriate precursors include hydrated oxides derived from the hydrolysis of organometallic solutions and aqueous solution-derived hyponitrites. Subsequent oxidation of the tetragonal phase at 400�C results in submicrometer particles of orthorhombic 123. Superconductivity (Tc onset ≈87 K) has been confirmed in these materials by both Meissner effect and specific-heat measurements.