Materials Science Forum Vols. 539-543

Abstract: XAFS measurements of the Cu, Ni and Zr K-edges for the melt-quenched Zr67Cu33 and Zr67Ni33 metallic glasses were curried out using synchrotron radiation at 20K. Fitting calculations for the EXAFS results reveal that local structure around Ni and Zr in Zr67Ni33 is well represented by those for the crystalline Zr2Ni, while local structure around Zr in Zr67Cu33 is better fitted by an icosahedron rather than crystalline Zr2Cu. Such differences of the local structure attribute to the differences of the stability of super-cooled liquid state and glass formability between Zr67Cu33 and Zr67Ni33 metallic glasses.

Abstract: The thermomechanical processing consisting in severe cold rolling (true strain 0.7–1.9) followed by a post-deformation annealing (200-700oC) is applied to Ti-50.0 and 50.7at%Ni alloys. The thermal stability of the amorphous phase as well as the influence of post-deformation annealing on the structure, substructure and temperature range of martensitic transformations are studied using TEM and DSC techniques. For a given level of cold work, the equiatomic alloy has a higher volume fraction of amorphous phase than the nickel-rich one. For both alloys, the higher the volume fraction of the amorphous phase, the higher the thermal stability. For a given post-deformation annealing temperature, the DSC martensitic transformation peaks from the material subjected to amorphization cold work are sharper and the hysteresis between the direct and reverse transformations is narrower than those for a material subjected to strain hardening cold work. This observation confirms the absence of the well-developed dislocation substructure in the severely deformed alloy subjected to nanocrystallization heat treatment, which is consistent with TEM results.

Abstract: The degree of amorphousness of the cast glassy sample is evaluated by free volume, which can be roughly estimated by density. The glass structural difference of cast glassy samples is mainly originated to the difference of those cooling rates. In order to clarify the degree of amorphousness, we use the volume change during annealing just below glass transition temperature (Tg) in this study. Therefore the volume change has close relation with free volume. We tried to control the excess free volume in Zr-TM-Al (TM: Cu, Ni and Co) bulk glassy alloys (BGAs) using small additive element. We found the elements; Pd, Ag, Pt and Au, whose outer d-electrons are filled up, are effective to increase the volume change/free volume in Zr-Cu-Al BGAs. The relationship between the Young’s modulus and volume change indicates the existence of two groups. The difference between them might be originated to the difference in glass structures.

Abstract: Crystalline precipitates in a bulk-metallic-glass (BMG) braze were investigated with an intense x-ray microbeam. The precipitates were found in the Pd40Cu30P20Ni10 BMG braze matrix after joining crystalline Ti-6Al-7Nb. However, the role (if any) played by the precipitates in improving the mechanical bond of the BMG/crystalline joint is unknown. X-ray microdiffraction and microfluorescence measurements from small sample volumes were made with an ~ 0.5 x 0.5 μm2 beam. Spatially-resolved Laue diffraction and x-ray fluorescence measurements were made on several second-phase crystals within the BMG matrix. Although precipitate crystals with the observed compositions were anticipated to be predominantly hexagonal, one of the crystals was found to be cubic or tetragonal. The instrumentation includes capabilities for 3D depth-resolved measurements of crystal structure and for fluorescence analysis of elemental composition. Depth profiling gave information about the grain distribution and morphology in the BMG matrix.

Abstract: The relationships of optimal compositions of bulk metallic glasses (BMGs) have been analyzed with a set of thermodynamic quantities of mismatch entropy normalized with respect to Boltzmann's constant (Sσ!/kB) and mixing enthalpy (Hmix) and with assistance of classification result of BMGs. The analysis shows clear dependence of the location of Sσ!/kB and Hmix values for optimal compositions of ternary BMGs in Sσ!/kB-Hmix chart in each alloy system. The ternary BMGs are summarized into four types: (1) Cu-based, (2) La-, Zr- and Ca-based, (3) Pd-based and (4) Fe- and Mg-based BMGs according to their maximum value of Sσ!/kB ((Sσ!/kB)max) and the largest and negative value of Hmix ((Hmix)L.N.) or their half values which are obtained for each system. These types of BMGs are related with the local arrangements of BMGs, which are widely accepted as an icosahedral structure for Metal-Metal type, a transformed tetragonal dodecahedrons and triacontrahedrons structure for Pd-Metalloid type, and a network-like structure for Metal-Metalloid type.

Abstract: Effects of Fe, Co and Al addition to Zr55Al10Ni5Cu30 and Zr70Cu30 metallic glass ribbons were studied. 20 at.% of Fe addition prevented Zr55Al10Ni5Cu30 molten alloys from being supercooled and resulted in nanocrystallization, while Zr55Al10Ni5Cu30 alloys containing 20 at.% Co could be quenched into a supercooled liquid region. Fe addition also degraded Zr70Cu30 metallic glass, while Al addition improved both glass phase stability and mechanical properties. Degradation of Zr-based metallic glass by Fe addition originates in the large negative enthalpy of mixing Fe with Cu.

Abstract: Anomalous small and wide angle scattering measurements on electron-beam welded joint has been carried out at Zr absorption edge. We preformed an anomalous and simultaneous small- and wide-angle scattering (A-SWAXS) analysis on the welded joint with and without crystallization. For the wide angle measurements, we extend the SAXS measurements up to 13 nm-1, where q is still below the first halo, but is large enough for several Bragg peaks to be observed. In the present analysis, the ASAXS analysis at the HAS/beads interface is interpreted in conjunction with the corresponding AWAXS analysis. The origin of SAXS and WAXS was found to be different.

Abstract: Two topics are described for structure analyses of glass and liquid using a combination of conical nozzle levitation (CNL) technique and diffraction experiments. The structure of high-purity bulk forsterite (Mg2SiO4) glass synthesized by a CNL technique has been determined by a combination of high-energy x-ray, neutron diffraction, and reverse Monte Carlo (RMC) modeling technique. The 3-dimensional atomic configuration derived from RMC modeling revealed that unusual network structure. In order to study structures of high-temperature and undercooled liquids, a CNL system has been developed and integrated with the two-axis diffractometer for glass, liquid, and amorphous materials at SPring-8, which is one of the third-generation synchrotron source. High-energy x-ray diffraction experiments were performed to obtain reliable diffraction data for the liquid phase of metallic glass-forming Zr-Cu binary alloys.