Papers by Keyword: Bulk Metallic Glass

Abstract: The gold alloys are used in jewelry for their aesthetic appearance and inertia to the environment. In jewelry a hardness superior to 300 HV is desirable to facilitate the final machining and reduce the wear of the final product. The maximum hardness that can be obtained with the standard 18 carats gold (Au₇₅-Ag_12.5-Cu_12.5 (wt%)) through a combination of heat treatment and cold working is about 290 HV. Gold-based bulk metallic glasses (BMGs) are an alternative as they present unique properties in comparison with crystalline counterparts, especially easy thermoplastic processing combined with a high hardness. Pd-base BMGs are another solution, due also to their attractive features.An Au₄₉Cu_26.9Si_16.3Ag_5.5Pd_2.3 (% at.) and a Pd₄₀Cu₃₀Ni₁₀P₂₀ (% at.) bulk metallic glasses were fabricated by a copper mould suction casting technique in an argon atmosphere. In the as-cast state hardness, shear modulus and hardness are high (HV_0.3= 360 and 530 in the Au-and Pd-base BMGs, respectively). Various heat treatments have been performed to modify the microstructural state. Formation of crystalline particles induces an increase of both shear modulus and hardness but a drastic decrease in toughness and therefore this formation should be absolutely avoided during casting or thermo processing.

Abstract: The effect of B₂O₃ fluxing on the glass-forming ability (GFA), the structure and the soft magnetic properties of Fe (-Co)-B-Si-Nb bulk metallic glasses (BMGs) has been investigated. The large Fe-Co-B-Si-Nb BMG specimens with diameters up to 7.7 mm, which is approximately 1.5 times as large as that of the maximum diameter of the copper mold cast one (= 5 mm), were prepared by the fluxing and water quenching. Thus the GFA of the Fe-Co-B-Si-Nb BMG are improved by the fluxing. It was confirmed that the soft magnetic properties of the Fe-Co-B-Si-Nb BMG are also improved by the fluxing. On the other hand, it was found for the Co-free Fe-B-Si-Nb BMG that the B₂O₃ fluxing promotes the precipitation of the α-Fe (-Si) and Fe₂B phases in the central region of the specimens; i.e., the GFA of the Fe-B-Si-Nb BMG is decreased by the fluxing. The Fe-B-Si-Nb BMG specimens show a flat hysteresis loop, indicating a good linear relationship between the magnetic induction and the applied magnetic field. These results of the Fe-B-Si-Nb BMG show that it is possible to develop a new soft magnetic material that exhibits constant permeability, which is necessary for producing inductors and choke coils.

Abstract: The relation between fragility and cooperativity of atomic motion in bulk metallic glass forming liquids is studied based on the bond strength-coordination number fluctuation model. The model describes the temperature dependence of the viscosity in terms of the mean values of the bond strength, coordination number and their fluctuations of the structural units that form the melt. According to the model, the cooperativity increases with the increase of fragility. The model estimates that the magnitude of the cooperativity N_B extends approximately from 7 to 60 structural units, depending on the material. The temperature dependence of N_B for different metallic glass forming systems reveals that N_B increases with the decrease of temperature. The relation between N_B and diffusivity of atoms is discussed briefly.

Abstract: Bending tests have been used extensively to assess the plastic deformation behavior of bulk metallic glasses (BMGs), however, a detailed experimental investigation of the deformation evolution of BMGs during such bending tests have been rarely reported. In the present work, the deformation evolution of a Zr-based BMG during three-point bending has been studied, and four distinct deformation evolution stages have been observed. After the elastic stage (stage I), the specimen starts to undergo some local plastic deformation while the bending curve still manifests an "elastic" state (stage II). In stage III, the specimen undergoes relatively stable plastic deformation. Finally, in stage IV, more plastic deformation occurs on the tensile side of the bending specimen resulting in the final fracture of the specimen. The current findings provide a fundamental understanding of the deformation mechanisms of BMGs under bending tests, which lays down a good foundation for further investigation of the deformation behavior of BMGs under more complex stress states.

Abstract: Bulk Fe₇₃Nb₃Al₂Ge₂P₉C₆B₄Si₁ metallic glass with the diameter of 2 mm was prepared by copper mold casting. The Fe-based metallic glass exhibits good magnetic properties with high saturation magnetization and low saturated magnetostriction. The saturation magnetization and coercive force remains almost unchanged with annealing temperature below the crystallization temperature. The crystallization leads to the decrease in the softness of magnetic properties. The Fe-based glass exhibits a good correspondence between large glass-forming ability and good soft magnetic properties.

Abstract: Firstly, this paper reminds the reader of some basic facts about the glassy state, then of the various ways to produce amorphous metals with particular emphasis on the route of vitrification from the melt. Vitrification of an undercooled melt is the most important route from the viewpoint of the application of metallic glasses. We compare diffusion in some metallic glasses with related crystalline metals. Glassy metals, also called metallic glasses, comprise conventional [1] and bulk metallic glasses [2,3]. We remind the reader of the major experimental techniques for diffusion studies in metallic glasses. The paper then reviews our current understanding of diffusion in glassy metals (see also [4,5,6]), including conventional as well as bulk metallic glasses and undercooled melts. We cover the temperature dependence of diffusion in metallic glasses and discuss the spectrum of activation parameters of glassy metals and its difference to the corresponding one of crystalline metals. We mention the pressure dependence and the isotope effect and we discuss tracer diffusion and viscosity diffusion for a bulk metallic glass and its undercooled melt. Finally we mention computer simulations of atomic jump processes. The diffusion mechanism in metallic glasses differs from that in crystalline metals and involves thermally activated, highly collective (chain-like or caterpillar-like) diffusion jumps. Finally, we mention diffusion along shearbands in a plastically deformed glassy metal.

Abstract: Structural relaxation through isothermal annealing below the glass transition temperature was conducted on a Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass. Differential scanning calorimetry was used to quantify enthalpy differences between the as-cast and relaxed samples, which were then related to average free volume differences. The influence of structural relaxation on plasticity was examined. While the free volume decreasement can be clearly observed between the as-cast and relaxed samples, structural relaxation is not accompanied by severe embrittlement.

Abstract: Linear and nonlinear viscoelastic behaviors of a Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass are investigated through experiments and described by the fictive stress model. Systematic deviations between the predicted stress-strain curves by fictive stress model and by the experimental results were found. In order to describe the flow stress curves of the Zr₅₅Al₁₀Ni₅Cu₃₀ BMG at different temperatures and strain rates in the supercooled liquid region more precisely, the fictive stress model was modified. The parameters of the model were optimized by the genetic algorithm, and a time relaxation factor Z' was introduced. The comparisons of the predicted compressive stress-strain curves and extrusion load-punch stroke curve by the modified fictive stress model with the experimental data show good agreements.