Papers by Keyword: Metallic Glass

Abstract: The non-isothermal crystallization kinetics of Mg₆₁Zn₃₅Ca₄ glassy alloy prepared via melt-spinning were studied by using isoconversion method. The crystalline characterization of Mg₆₁Zn₃₅Ca₄ was examined by X-ray diffraction. Different scanning calorimeter was used to investigate the non-isothermal crystallization kinetics at different heating rates (3-60 K/min). The calculated value of Avrami exponent obtained by Matusita method indicated that the crystalline transformation for Mg₆₁Zn₃₅Ca₄ is a complex process of nucleation and growth. The Kissinger-Akahira-Sunose method was used to investigate the activation energy. The activation energy of crystallization varies with the extent of crystallization and hence with temperature. The Sestak-Berggren model was used to describe the non-isothermal crystallization kinetics.

Abstract: The thermal stability, glass-forming ability, soft-magnetic properties and mechanical properties of Co₄₆Fe_19+xB_22.5Si_5.5Nb_7–x (x=0–2) bulk metallic glasses were investigated. The 5.5 at% Nb addition was found to be effective in approaching alloy to a eutectic point, resulting in an increase in glass-forming ability. By copper mold casting, bulk metallic glass rods with diameters up to 5 mm were produced. Except for high glass-forming ability, the bulk metallic glasses also exhibit good soft-magnetic properties, i.e., low coercive force of 1.34–2.14 A/m, high effective permeability at 1 kHz of 2.26–3.06×10⁴, and high fractures strength (σ_f) of 4010–4460 MPa. This Co-based bulk metallic glass system with high strengths and excellent soft-magnetic properties is promising for future applications as a new functional material.

Abstract: The crystallization kinetics of melt-spun Cu_64.5Zr_35.5 amorphous alloy ribbons was investigated using differential scanning calorimetry (DSC) at different heating rates. Besides, the Kissinger and isoconversional approaches were used to obtain the crystallization kinetic parameters. As shown in the results, the activation energies for glass transition and crystallization process at the onset, peak and end crystallization temperatures were obtained by means of Kissinger equation to be 577.65 ± 34, 539.86 ± 54, 518.25 ± 20 and 224.84 ± 2 kJ/mol, respectively. The nucleation activation energy E_nucleation is greater than grain growth activation energy E_growth, indicating that the nucleation process is harder than grain growth. The local activation energy E_α decreases in the whole crystallization process, which suggests that crystallization process is increasingly easy.

Abstract: Thermoplastic forming is a promising method for fabricating metallic glass (MG) products with complex shapes. This method can avoid the difficulties encountered in other manufacturing processes, such as very high cooling rate required by casting and catastrophic cracking in machining. However, during thermoplastic forming the adhesion between dies and MGs restricts the production. It is therefore important to explore the underlying adhesion mechanisms during forming and establish guidelines for selecting proper die materials. In this paper, we comprehensively studied the adhesion between La-based MG and some widely-used die materials (electroless Ni-P, Si, alumina and silicon nitride) in the thermoplastic forming process. It was found that, among these die materials, alumina has the best performance, which is attributed to its strong chemical bonds and low surface energy. The study concludes that the surface energy and the type of chemical bonds can be proper indicators for selecting die materials.

Abstract: Development of liquid-phase separated bulk metallic glasses is retarded due to difficulties in finding of immiscible systems with high glass-forming ability (GFA) of coexistent liquids. Zr-Ce alloy is a typical liquid immiscible system characterized by a liquid miscibility gap. We added Co and Cu into the Zr-Ce immiscible system and optimized the composition of the designed Zr-Ce-Co-Cu immiscible alloys. The solidification experiments were carried out for the quaternary alloys. The result indicates that the melt separated into ZrCo-rich and CeCu-rich liquids upon cooling through the miscibility gap. By optimizing the relative atomic ratio of Co:Cu, the coexistent ZrCo-rich and CeCu-rich liquids automatically assembled eutectic compositions during the liquid-liquid phase separation (LLPS). Under the condition of fast quenching, the two liquids subsequently undergo liquid-to-glass transition, resulting in the formation of composite structure with two glasses in the samples. We successfully developed phased-separated metallic glasses based on the Zr-Ce-Co-Cu immiscible alloys. This work not only strengthens the understanding in the LLPS but also provides a new strategy on the design of the dual glassy composites.

Abstract: Metallic glasses (MGs) exhibit extremely high strength and superior resistance to corrosion. They are also supposed to be resistant against displacive irradiation due to their inherent disordered structure, and thereby are viewed as potential candidates for applications in irradiation environments. However, the structures and properties evolution of metallic glasses, especially bulk metallic glasses (BMGs), under irradiation has not been fully understood up to now. In this work, the structural stability and damage characteristics of a Zr-based BMG under helium ions irradiation environment were investigated. Meanwhile, the effect of structural relaxation and crystallization on the irradiation response of the BMG was also studied. Results show that the BMG reserves the amorphous structure within the studied range of fluence, and exhibits better irradiation resistance compared to that of the crystalline alloys. In our opinion, the initial free volume concentration affects the damage morphology of the BMG, while partial crystallization will lead to significantly embrittlement under irradiation.

Abstract: The influence of direct current pulse on–off patterns on spark plasma sintering of [Fe_0.8Co_0.2B_0.05Si_0.2]₉₆Nb₄ on kinetics competition and coordination mechanism of the interface bonding and crystallization has been investigated systematically. No crystallization of metallic glassy matrix and good bonding state between the particles were responsible for good mechanical properties of the fabricated Fe-based bulk glassy alloy at 2:9 (on: off) pattern. The higher local micro area’s instantaneous power and larger cooling time caused by low pulse duty ratio played a vital role in consolidation of amorphous alloy powder and desired structural properties.

Abstract: The chip formation mechanism during precision cutting of metallic glass (Zr₅₅Cu₃₀Ni₅Al₁₀ at%) was investigated around the glass transition temperature (673 K). Orthogonal cutting of metallic glass are conducted on a fly-cutting machine at various cutting speeds. The new surface of the chips was slightly shiny while the free surface exhibited lamellar slip structure. The cutting temperature was a proportional to the cutting ratio and chip shear angle. The surface integrity worsened with an increased flow of cutting chip due to an increase in the cutting speed. An increase in the cutting temperature caused the chips formation to change from flow type chips to discontinuous chips. When the cutting speed exceeded 300 m/min, the shear angle increased while the shear pitch of the chips decreased. It appears that when the cutting temperature exceeded the glass transition temperature, the strength of the metallic glass decreased and the ductility mode changed due to viscous flow.

Abstract: In this paper the change of electrical resistivity of the amorphous alloy Fe₈₀Ni₁₀Si₅B₅ during heat treatment is investigated. Parallel studies by magnetometry technique, X-ray diffraction and calorimetric analysis were used to interpret the nature of the changes in the structure and properties of the alloy during annealing. These features are related with magnetic nature of the alloy and crystallization at a higher temperature.

Abstract: The amorphousization of Zr₆₅Nb₃₅ alloy was performed. The Zr-Nb based alloys contained Al and Co elements were fabricated by arc-melting and melt-spinning methods. The superconducting property of the Zr_(65-x)Nb_35-xAl_x (x = 0~15 at%) and Zr_(65-x)Nb₂₀Al₁₅Co_x alloys (x = 3~10 at%) was investigated by magnetic susceptibility measurements. The Zr_(65-x)Nb₂₀Al₁₅Co_x metallic glasses (x = 6~10 at%) with superconducting nanocrystalline particles dispersed in an amorphous matrix exhibited a superconductivity below about 3.5 K. The addition of Co element led drastically to the amorphousization of the superconducting Zr₆₅Nb₂₀Al₁₅ alloy.