New amorphous alloys as micromaterials and the processing technologies

Local electronic structures of Zr-based metallic glasses have been calculated using the discrete variational Xcluster molecular orbital method. The cluster models are constructed for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses on the base of local structure parameters determined by extended X-ray absorption fine structure analysis. The valence-band X-ray photoelectron spectra computed from the local electronic structures agree well with experimental spectra. Based on the feature of chemical bonds in those glasses evaluated from the electronic structures, we discuss the relationship between the chemical bond and the stability of supercooled liquid state. The strength of the Zr-Ni bond is larger than that of Zr-Cu bond, while the Cu-Cu bond strength is nearly equal to the Ni-Ni bond. Moreover, the Zr-Zr bond strength is larger in Zr-Cu glass than in Zr-Ni glass. This indicates that the formation of primary crystalline phase is more difficult in Zr-Cu glass than in Zr-Ni glass. The difficulty contributes to the relative stabilization of supercooled liquid state in Zr-Cu glass compared with that in Zr-Ni glass. It is also shown that local atomic structures of Zr-Pd glass are well described by the icosahedral atomic configuration. Recently, bulk metallic glasses have attracted much attention because of the importance in materials science and the extension of their application fields. 1) Bulk metallic glasses are characterized by a wide supercooled liquid region, � Tx which is defined as the temperature interval between glass transition temperature, Tg and crystallization temper- ature, Tx. The appearance of the wide supercooled liquid region implies the high resistance against crystallization, leading to large glass-forming ability. The stable supercooled liquid state enables the production of bulk metallic glasses. It is therefore quite important to understand the reason for the stabilization of the supercooled liquid state from the view- point of interests in materials science as well as development of new bulk metallic glasses. The three empirical rules have been proposed by Inoue 2) in order to explain the origin of the wide supercooled liquid region and good glass-forming ability in bulk metallic glasses. It is also suggested that the stability of the supercooled liquid state is strongly correlated with their local structure. 3-6) Alamgir et al. 7,8) have inves- tigated the relation between the electronic structure and the glass-forming ability in Pd-Ni-P glassy alloys, and pointed out that the existing electron models are not fully adequate to explain the high glass stability of the Pd40Ni40P20 alloy. However, little is known about the relation of the chemical bond among constituent atoms with the stability of super- cooled liquid state in bulk metallic glasses. The present study is intended to investigate the local electronic structures and the chemical bonds among constit- uent atoms in Zr-based metallic glasses, and to clarify the relation between the chemical bond and the stability of supercooled liquid state. The local electronic structures are calculated for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses using the discrete variational (DV) Xcluster molecular orbital

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.

Previously published results have shown that viscosity greatly influences on the deformation behavior of the bulk amorphous alloy in supercooled liquid region during microforming process. And viscosity is proved to be a component of the evaluation index which indicating microformability. Based on the fluid flow theory and assumptions, bulk amorphous alloy can be regarded as the viscous materials with a certain viscosity. It is helpful to understand how the viscosity plays an important role in viscous materials with various viscosities by numerical simulation on the process. Analysis is carried out by linear state equation in FEM with other three materials, water, lubricant oil and polymer melt, whose viscosities are different obviously. The depths of the materials flow into the U‐shaped groove during the microimprinting process are compared in this paper. The result shows that the deformation is quite different when surface tension effect is not considered in the case. With the lowest viscosity, water can reach the bottom of micro groove in a very short time. Lubricant oil and polymer melt slower than it. Moreover bulk amorphous alloys in supercooled liquid state just flow into the groove slightly. Among the alloys of different systems including Pd‐, Mg‐ and Zr‐based alloy, Pd‐based alloy ranks largest in the depth. Mg‐based alloy is the second. And Zr‐based alloy is the third. Further more the rank order of the viscosities of the alloys is Pd‐, Mg‐ and Zr‐based. It agrees well with the results of calculation. Therefore viscosity plays an important role in the microforming of the bulk amorphous alloy in the supercooled liquid state.

The unique thermoplastic forming ability of metallic glasses in their supercooled liquid state makes them the ideal embossing materials for miniature fabrication. However, the understanding and controlling of micro filling process that is crucial for miniature fabrication and their applications remain fundamental, yet presently unresolved issues. Here, the mold filling kinetics of a model Pd-based metallic glass in supercooled liquid state is studied using different Si micro molds with different channels. A universal kinetic equation, which can describe the filling kinetics of viscous metallic supercooled liquid in micro molds with irregular shapes, is obtained.

Intrinsic or extrinsic size-dependent deformation behavior of a Zr-based bulk metallic glass in supercooled liquid state?

Formation of Zr-based bulk metallic glass with large amount of yttrium addition

The oxidation behavior of Zr55Cu30Al10Ni5 bulk metallic glass in air in the glassy state and the supercooled liquid state was studied using a thermogravimetric analyzer, X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. It was found that the isothermal oxidation kinetics of the glass in both states follows a two-step parabolic law. The oxidation process is governed by the inward diffusion of oxygen and the outward diffusion of Cu, with the first being dominant. The faster diffusion of atoms in the supercooled liquid state led to a network precipitation of crystalline Cu, and the crystallization that occurred in this state in the later stage of oxidation caused a reduction in the rate of oxidation. Two types of Zr oxides, i.e. t-ZrO2 and m-ZrO2, were formed in the oxidation process in both the glassy and supercooled liquid states. t-ZrO2 mainly formed in the outer layer of the oxide scale, while m-ZrO2 tended to form in the inner layer. The formation of m-ZrO2 is possibly activated by the crystallization of the glass near the interface of the oxide scale and the substrate alloy. In addition, oxidation also has a substantial effect on the formation of crystallized phases. The formation of a Cu-rich phase of Cu10Zr7 occurred in the oxidizing atmosphere. However, the formation of a Zr-rich phase of Zr2(Ni, Cu) mainly took place in a vacuum environment.

Geometrically complex, high aspect ratio microstructures have been successfully formed in Bulk Metallic Glass (BMG) via superplastic forming against silicon dies [1–3]. Although nanoscale features have been created in a similar fashion, there exists a demand to develop these metallic nanofeatures into high aspect ratio nanostructures with controlled geometries. In past research a process model was created to predict the achievable nanoscale feature sizes and aspect ratios through a flow model [4]. The flow model assumes force equilibrium with a viscous term to account for the required force to produce flow and a capillary pressure term required to overcome surface effects which are significant at the nanoscale. In this paper, a thin film model to predict the pressure distribution across the BMG during the forming process when it is in the supercooled liquid state is presented. Silicon molds with various nanofeatures were produced using Deep Reactive Ion Etching to achieve high aspect ratio dies over a relatively large area in order to validate these models.

The oxidation of the bulk amorphous alloy Zr65Cu17.5Ni10Al7.5 in air in its amorphous and the supercooled liquid states was studied in the temperature range 573–663 K using X-ray photoelectron spectroscopy (XPS). The oxide film mainly consisted of the oxides of Zr (as ZrO2) and Al (as Al2O3). No Cu or Ni was found in the oxide film formed on the amorphous state of the alloy while significant Cu (as CuO) was present in the oxide film formed on the alloy in its supercooled liquid state. The role of the various alloying elements during oxidation at high temperatures in air is discussed in the paper. The XPS data from oxide film support the previously suggested mechanism for oxidation of this alloy, i.e. the rate controlling process during oxidation of the alloy at low temperatures (in the amorphous state) is the back-diffusion of Ni and Cu, while the oxidation at high temperatures (in the supercooled liquid state) is dominated by the inward diffusion of oxygen.

The Zr-based bulk metallic glass matrix composites of a mixture of gas-atomized metallic glass powders and Fe-based nanostructured powders were fabricated by spark plasma sintering. The Fe-based nanostructured powders adopted for the enhancement of plasticity were well distributed in the matrix after consolidation, and the matrix remains as a fully amorphous phase. The successful consolidation of metallic glass matrix composite with high density was attributed to viscous flow in the supercooled liquid state during spark plasma sintering. Unlike other amorphous matrix composites, in which improved ductility could be obtained at the expense of their strength, the developed composite exhibited improvement both in strength and ductility. The ductility improvement in the composite was considered to be due to the formation of multiple shear bands under the presence of the Fe-based nanostructured particles.

The effect of phases, viz., a crystalline, a glassy, and a supercooled liquid state, on the cyclopolymerization of N‐substituted dimethacrylamides 1, initiated by ionizing radiation, was examined. In both the crystalline and supercooled liquid states, 1 was polymerized to give completely cyclized polymers, whereas no polymerization occurred in the glassy state. The structural investigations showed that the polymerization in the supercooled liquid state afforded polymers with the same structure as that of polymers formed in the liquid state, while the polymerization in the crystalline state yielded polymers with some difference in structure. Irrespective of the matrices where polymerizations were carried out, poly(1) obtained are amorphous, and the rate determining step was found to be an intramolecular cyclization reaction, by ESR spectra of propagating radicals. The results thus obtained suggest that the polymerizations of dimethacrylamides 1 in the solid and supercooled liquid states proceed through a virtually identical mechanism with that developed for the polymerization in the liquid state, but the matrices influence the polymerization process to some extent.

Strain-controlling mechanical behavior in noncrystalline materials: I: Onset of plastic deformation

A new method for heat capacity determination in supercooled liquid state using fast scanning calorimetry: Thermochemical study of 9,9'-bifluorenyl

Homogeneous deformation of Zr–Ti–Al–Cu–Ni bulk metallic glasses

Viscosity of the supercooled liquid and relaxation at the glass transition of the Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 bulk metallic glass forming alloy