Materials Science Forum Vols. 539-543

Abstract: New bulk metallic glasses (BMG) have been synthesized in the Mg-Cu-RE systems (RE= Y, Gd) with high critical diameter. In order to determine the glass forming range of these ternary systems, the calculation of the onset of driving forces (ODF) has been performed and has shown a large composition range for BMG formation. The synthesis of particular compositions in this domain has shown that all the produced alloys present a high glass forming ability (GFA) measured by DSC experiments. The ODF model, synthesized alloys by mould casting, characterization by X-ray diffraction and DSC techniques are presented.

Abstract: Bulk metallic glass (BMG) composites with dual amorphous phases were fabricated by spark plasma sintering of a mixture of Cu-based and Zr-based amorphous powders in their overlapped supercooled liquid region. The Zr-based amorphous phases are well distributed homogeneously in the Cu-based metallic glass matrix after consolidation. The BMG composite still remains as an amorphous structure after consolidation. The BMG composite with dual amorphous phases shows macroscopic plasticity after yielding, and the plastic strain increased to around 3.4% in the BMG composite containing 30 vol% Zr-based amorphous phase. The successful consolidation of BMG composite with enhanced plasticity was achieved by introducing a second amorphous phase in the metallic glass matrix.

Abstract: Formation of the icosahedral order in supercooled liquids and glassy phases is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the short-range icosahedral order grows in the supercooled liquids as well as in the glassy phases. Structural analyses reveal that the icosahedral clusters form a network in which the clusters are connected via the pentagonal-bicap sharing. Geometrical property of the network formed by the icosahedral clusters is an origin of medium-range order in the glassy phases

Abstract: The effect of partial crystallization on the mechanical properties of a Zr based bulk metallic glass (Vitreloy 1) is investigated. Viscoelastic properties are studied by mechanical spectroscopy in large frequency and temperature ranges, both below or above the glass transition temperature (Tg), whereas viscoplastic properties are investigated by compression tests . To study the interaction between crystallization and mechanical properties at high temperature, nanocomposites are produced thanks to appropriate heat treatments. Formation of nanocrystalline particles induces an increase of the storage elastic modulus, especially in the glass transition range, where this modulus is very low in the amorphous material. It also results in a decrease of the loss elastic modulus, corresponding to a decrease of the atomic mobility. Finally, partial crystallization induces very large hardening revealed by the compression tests but the hardening extent depends strongly on the applied strain rate.

Abstract: The atomistic structure of Czochralski-grown SixGe1-x binary mixed semiconductor was studied experimentally and theoretically. By extended X-ray absorption fine structure (XAFS) studies it was found that bulk SiGe semiconductor is a random mixture and that the Ge-Ge, Ge-Si and Si-Si bond lengths maintain distinctly different lengths and vary in a linear fashion against the alloy composition across the whole composition range 0 < x < 1, in good agreement with expectations derived from the ab-inito electronic structure calculations. The result indicates that SiGe is a suitable model for a disorder mixed material and that the bond lengths and bond angles are distorted with the composition.

Abstract: The electronic structure and phase stability of the Zr-Ni-Al ternary glassy alloys, have been systematically investigated experimentally and theoretically. Thermal stability and crystallization process have been clarified by the DSC and XRD measurements. Composition dependence of the stability parameter; Tg/Tl has been clarified. The electronic structure around the Fermi level has been investigated by using the high resolution UPS. It is found that there is a sizable pseudogap at the Fermi level of the electronic structure of the Zr65Ni20Al15 glassy alloy. The DVXα calculations for the charateristic clusters in the Zr-Ni-Al glassy alloy having high glass forming ability have been also performed. It is found that these clusters formation reduces the internal energy and can contribute to the phase stability. The LMTO-ASA band calculations for the Zr6NiAl2 compound which is the most relevant compound to the Zr60Ni25Al15 glassy alloy have been also performed. It is found that there is a sizable pseudogap at the Fermi level in its electronic structure, suggesting exiting of the pseudogap of the glassy alloy.

Abstract: Monolithic Zr65Al7.5Ni10Pd17.5 and Zr65Al7.5Ni10Cu17.5 hardly had plastic deformability in tension at room temperature, although Zr65Al7.5Ni10Pd17.5 showed a large plastic strain in compression. A depressed zone that is the evidence of viscous flow deformation (so-called viscous flow depression zone; VFD-zone) was observed for Zr65Al7.5Ni10Pd17.5 around the edge at the point of failure on its specimen surfaces. However, no VFD-zone was seen for Zr65Al7.5Ni10Cu17.5. Thus, a clear difference in tensile plastic behavior was found between Zr65Al7.5Ni10Pd17.5 and Zr65Al7.5Ni10Cu17.5. A structural change in the vicinity of the fracture surface of Zr65Al7.5Ni10Pd17.5 was measured by micro-beam X-ray diffractometry (MB-XRD). We may be able to discuss the room temperature plasticity of Zr65Al7.5Ni10Cu17.5-xPdx bulk metallic glasses correlating with the phase stability of the amorphous and icosahedral quasi-crystalline phases.

Abstract: The free volume relaxation just under the glass transition region was investigated by the high-resolution density measurement using the bulk metallic glasses with the compositions of Pd40Ni40P20 and Zr55Cu30Ni5Al10. The relaxation process was well described by a stretched exponential function with Kohlrausch exponent values less than unity. The reduced free volumes in an as-quenched state were estimated as 0.0117 and 0.0353 for Pd40Ni40P20 glass and Zr55Cu30Ni5Al10 glass, respectively. The specific heat curves Cp(T) for Pd42.5Cu30Ni7.5P20 alloy were obtained for the supercooled liquid, the equilibrium liquid and the crystallized alloy. The isenthalpic Kauzmann temperature TKH and isentropic Kauzmann temperature TKS were estimated as 471 K and 522 K, respectively, from the specific heat data.

Abstract: Viscosity of a Zr55Cu30Al10Ni5 supercooled liquid alloy having bulk metallic glass forming ability has been measured by using a penetration viscometer with a cylindrical probe under high speed heating conditions at rates between 20°C/min and 400°C/min in the temperature range from the glass transition temperatures (Tg) up to above the crystallization temperatures (Tx). The viscosity of the alloy decreased with increasing the heating rate and tended to saturate at the heating rate of 200°C/min and above. Corresponding measurements of the differential thermal calorimetry (DSC) under the high-speed heating conditions have been also done.