Papers by Keyword: Metallic Glass

Abstract: The temperature dependence of the diffusion coefficient in metallic glass-forming systems do not follow the Arrhenius behavior over a wide temperature range. Instead, it exhibits a kink behavior at around the glass transition temperature. Some researchers associate this behavior to the difference in the diffusion mechanism operating in the glassy and the supercooled liquid state, whereas others do not support this view. In addition, usually, the temperature dependence of the diffusion coefficient is analyzed by splitting the temperature range into two regions, above and below the glass transition temperature. In the present study, we developed an analytical theory that describes the continuous variation of the diffusion coefficient across a temperature where the kink behavior is observed. According to the theory, the kink behavior arises from the freezing of free volume available for diffusion by lowering the temperature. A connection to the vacancy mechanism of diffusion has been also pointed out.

Abstract: The relation between the annealing temperature dependence of the structural inhomogeneity and the diffusion coefficient in a metallic glass forming system Zr-Ti-Cu-Ni-Be is studied by using reported experimental data. It is shown that the diffusion coefficient increases with the increase of the correlation length of the structural inhomogeneity. Interestingly, the result found resembles the behavior known in superionic glasses. A discussion on the found relationship is given by exploiting the model for the superionic glasses proposed by the author. Based on the model, an inhomogeneity dependent diffusivity maximum is predicted.

Abstract: Depending on the composition and structure of metallic glasses cells with the dimensions in the range from tenths nanometers to tenths micrometers were observed on the ductile fracture surface. The variation in dimple size was compared with the serrations presented on the loading curve at the nanoindentation of the metallic glasses with different compositions. Higher instantaneous deformation can be connected with simultaneous shearing at more suitable shear band configurations. The cell morphology with the various cell sizes is observed at the failure of the metallic glasses. At the failure of high strength metallic glasses, the cells are formed in short time due to the release of high amount of stored elastic energy. In this case the uniform cell morphology with the cell size of about 20 nm is observed.

Abstract: High temperature oxidation of Al₈₆Ce₁₀Fe₄ amorphous alloy at 630°C in static air for 15, 60, 300, 600, 1,200, 3,000, 6,000 and 12,000 minutes was studied. The morphology, composition and microstructure of the oxide films were investigated using SEM, EDS and XRD. The hardness and corrosion resistance of the oxide films were determined by micro-indentation and polarization technique. The results indicate oxidation at 630 °C instigates nucleation, growth and coarsening of fcc-Al, Al₁₁Ce₃ and Al₁₃Fe₅ crystalline phases inside the Al₈₆Ce₁₀Fe₄ matrix, reducing the hardness of the alloy, and oxidation of the alloy surface, forming crystallized Al₂O₃ and AlFeO₃ chemical compounds. Active anodic dissolution behavior and diffusion-controlled cathodic polarization of the oxidized alloy were observed. The corrosion resistance of the oxide films rates in the level of 10^-7 ~10^-5 [A/cm²]. The results demonstrate Al₈₆Ce₁₀Fe₄ amorphous alloy, as a new-developed high temperature high strength glass, exhibits potential application for aerospace and national defense in terms of its high mechanical strength, high temperature endurance, and satisfactory oxidation and corrosion resistance.

Abstract: In this present study, molecular dynamics (MD) simulation has been performed to investigate the influence of applied hydrostatic compressive and tensile pressure on glass forming process of Ni₆₂Nb₃₈ bimetallic glass using embedded atom method (EAM). During fast cooling (~10 K ps^-1), tensile and compressive pressure has been applied having 0.001 GPa,0.01 GPa and 0.1 GPa magnitude. The glass transition temperature (T_g) for each pressurized (Tensile and Compressive nature) cooling case has been calculated and T_g is found to be dependent on both magnitude and nature of the pressure applied during cooling process.Voronoi cluster analysis has also been carried out to identify the structural evaluation during hydrostatically pressurised fast cooling process. In case of both hydrostatic tensile and compressive pressurised cooling processes, T_g increases with the increase of pressure from 0.001 GPa to 0.1 GPa in magnitude.

Abstract: Molecular dynamics (MD) simulations of metal-metallic glass (Al-Cu₅₀Zr₅₀) multilayer during nanoindentation is carried out to investigate the load-displacement response, mechanical properties and deformation mechanisms. The indentation study is carried out at temperatures in the range of cryogenic to room temperature (10 K-300 K). The indenter speeds are varied between 0.5-5 Å/ps to study the effect of loading rate. The interaction between Al-Cu-Zr atoms are defined by EAM (Embedded Atom Method) potential. A sample size of 200 Å × 200 Å × 200 Å (in x y z-direction) comprising of 538538 atoms is used for nanoindentation. P P S boundary condition (BC) in x y z direction and NVT ensemble are used. We observed a peak load of 117 nN, at a temperature of 10 K with a loading rate of 5 Å/ps. We found that as the loading rate increase, the peak load also increases. As anticipated, the increase in temperature decreases the strength of the multilayer. The atomic displacement vector plots reveal that MG act as hurdles to the movement of dislocations nucleated at the interface.

Abstract: The yield behavior of amorphous metals including the metallic glass shows intrinsic dependence on the hydrostatic stress, so that yield criterion models such as Mohr-Coulomb and Drucker-Prager are often used. Both the models can explain the asymmetry in the yield stress under uniaxial compression and tension conditions, while the asymmetry in the angle of fracture surface is not able to be determined based on any of those models. The free volume model is able to provide that foundation. Shibutani et al. proposed a new constitutive model for amorphous metals that was derived from some free volume models and the flow rule using the Drucker-Prager yield function as a plastic potential, and investigated the yield behavior and the formation of localized shear band under some temperature conditions using the implicit static FEM code. The formation of shear bands is an unstable phenomenon that is greatly affected by the initial imperfection. In this model, on the other hand, the temperature or the strain rate also affects the yield behavior considerably. In this study, the impact fracture of metallic glass was investigated by implementing the constitutive model proposed by Shibutani et al. into the explicit dynamic FEM code DYNA3D, with laying emphasis on reproducing asymmetry in the angle of fracture surface and the examination of effects of strain rate and temperature change.

Abstract: Ti-based amorphous alloys containing no harmful elements are desired. However, many Ti-based amorphous alloys contain toxic elements such as Al, Ni, V and Be. The presence of toxic elements within amorphous alloys is a concern when they are intended for use as a biomaterial. This problem has steered many researchers toward the development of Ti-based amorphous alloys without toxic elements. Our novel amorphous alloys were developed based on this principle. A series of Ti44Zr10Pd10Cu6+xCo23-xTa7 (x = 0, 4, 8) amorphous alloys were developed for biomedical application. A series of protocol tests were performed to check for biocompatibility and potential use of the novel alloys in humans. First, alloy ingots were alloyed by induction melting and then cast into copper molds. The cast rod was then used as the plasma cathode in a filtered cathodic vacuum arc deposition chamber to coat the 25-nm amorphous alloy thin film on the cover glass slides. These coated cover glass slides were then examined for biocompatibility. Cell proliferation and cell differentiation were investigated using Methylthiazol Tetrazolium assay test and by alkaline phosphatase assay on osteoblast like cells (SaOS-2), respectively. Field emission scanning electron microscopy was performed to evaluate the thin film surface characteristics. The thickness of thin film was analyzed using a Stylus profilometer. An adhesion scratch test was administered to evaluate the thin film adhesive strength and indirect hardness comparison. Electron Dispersive X-ray Spectrometry was performed to study the elemental composition. Lastly, a medical grade Ti-6Al-4V alloy was studied in parallel as a control material. Results indicated that all investigated Ti-based amorphous alloys were non-cytotoxic and were comparable to the Ti-6AL-4V. They also demonstrated an ability to support differentiation of osteoblast like cells. The adhesion and the hardness of the thin films on the substrates were superior to that of Ti-6Al-4V. The results suggested that the novel alloys in this study could be potentially utilized in biomedical applications.

Abstract: In this work, the heat-activated persulfate (PS) in the presence of Fe₇₈Si₉B₁₃ metallic glasses (MGs) shows an extremely difference in degradation of azo dye and triarylmethane dye, where Fe₇₈Si₉B₁₃ MGs exhibits a superior activation ability for PS with assistance of heat leading to the fast removal of two dyes. The structural features of Fe₇₈Si₉B₁₃ MGs are firstly characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), following analysis of surface topography by scanning electron microscope (SEM). The results show that with the addition of Fe₇₈Si₉B₁₃ MGs, the recalcitrant azo dye is completely removed within 5 min while only 6% of removal rate can be achieved without adding MGs, indicating that the refractory azo dye can be easily degraded by sulfate radical (SO₄•^–) from heat/MGs/PS. On the other hand, no big variation occurs between PS and MGs/PS under heat activation in degrading triarylmethane dye. Sole PS activated by heat results in a fast removal rate, indicating that triarylmethane dye can be easily degraded by PS itself compared to azo dye. The findings in this work present an in-depth understanding of heat/MGs/PS system in dyes degradation.

Abstract: Laser shock peening is a promising effective approach for improving mechanical properties of metallic glass. In this work, laser shock peening was employed to study the surface morphologies of metallic glasses with different toughness. Numerous localized circular-or arc-shaped structures, with the size of 5~20 μm, were observed in the shock treated surface. The number of these unique localized structures has a close correlation to the ability of metallic glass to accommodate plastic deformation. In addition, the surface morphology evolution of Zr-based metallic glasses with different crystalline degrees is also discussed, indicating that the circular-or arc-shaped structures only appear in fully amorphous system.