Papers by Keyword: Supercooled Liquid Region

Abstract: In the present study, Cu₆₀Zr₃₀Ti₁₀ metallic glass powders were prepared by mechanical alloying of pure Cu, Zr, and Ti powder mixtures. Cu₆₀Zr₃₀Ti₁₀ metallic glass composite powders were obtained after 5 h of milling as confirmed by X-ray diffraction and transmission electron microscopy. The metallic glass powders were found to exhibit a supercooled liquid region before crystallization. Cu₆₀Zr₃₀Ti₁₀ bulk metallic glass were synthesized by vacuum hot pressing the as-milled Cu₆₀Zr₃₀Ti₁₀ metallic glass powders at 723 K in the pressure range of 0.72 ~ 1.20 GPa. Cu₆₀Zr₃₀Ti₁₀ BMG with nanocrystalline precipitates homogeneously embedded in a highly dense glassy matrix was successfully prepared under applied pressures. It was found that the pressure could enhance the thermal stability and prolong the existence of amorphous phase inside Cu₆₀Zr₃₀Ti₁₀ powders.

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: 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.

Abstract: Bulk metallic glasses (BMGs) are amorphous alloys that exhibit unique mechanical properties such as high strength due to their liquid-like structure in the vitreous solid state. While they usually exhibit low ductility, they can be toughened by incorporating secondary phase particles within the amorphous matrix via composite fabrication to generate amorphous metal matrix composites (MMCs). Traditional MMCs are fabricated at high temperature in the liquid state with tedious blending processes. This high temperature processing route often leads to unwanted reactions at the reinforcement/matrix interface, creating brittle intermetallic by-products and damaging the reinforcement. In the present work, novel bulk metallic glass composites (BMGCs) were fabricated at low processing temperatures via thermoplastic forming (TPF) above the glass transition temperature of the amorphous matrix. Here, the unique thermophysical features of the matrix material allow for TPF of composites in non-sacrificial moulds incorporating various types of reinforcement, via processing in the solid state at low temperatures (less than 200 °C), within a short timeframe (less than 10 minutes); this avoids the formation of brittle phases at the reinforcement/matrix interface leading to efficient bonding between particles and matrix, thereby creating a tough, low density composite material.

Abstract: High-strength Cu-based bulk glassy alloys with a large supercooled liquid region in Cu-Zr-Ti-Ni systems were prepared by means of copper mold casting. The Cu-based bulk glassy alloys samples were tested by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Instron testing machine. The result indicates, the maximum diameter was 5.0 mm for the Cu₅₅Zr₂₅Ti₁₅Ni₅ bulk glassy alloy. The temperature interval of supercooled liquid region (ΔT_x) is as large as 45.48-70.98 K for the Cu-Zr-Ti-Ni alloy. The Cu-based glassy alloys rods exhibited the very high mechanical properties and the distinct plastic strains. The compressive fracture strength is 2155 MPa, 2026 MPa and 1904 MPa respectively for Cu₅₀Zr₂₅Ti₁₅Ni₁₀, Cu₅₅Zr₂₅Ti₁₅Ni₅ and Cu₅₄Zr₂₂Ti₁₈Ni₆ bulk glassy alloys. The Vickers hardness is respectively 674, 678 and 685 for the Cu₅₀Zr₂₅Ti₁₅Ni₁₀, Cu₅₅Zr₂₅Ti₁₅Ni₅ and Cu₅₄Zr₂₂Ti₁₈Ni₆ bulk glassy alloys. The addition Co element to Cu-Zr-Ti-Ni alloy expand the ΔT_x, the ΔT_x is 74.5 K for Cu₅₀Zr₂₂Ti₁₈Ni₆Co₄ bulk glassy alloys.

Abstract: The effect of Nb addition on the glass-forming ability (GFA), soft-magnetic properties and mechanical properties of [(Co_0.65Fe_0.35)_0.9Ni_0.1]_73-xB_21.9Si_5.1Nb_x (x=36) alloy system were investigated. The results showed that by adjusting the content of Nb, the thermal stability of the supercooled liquid and the GFA increased effectively. With increasing the amount of Nb, the supercooled liquid region (ΔT_x) increased from 45 to 65 K and the reduced glass transition temperature (T_rg=T_g/T_l) was located in the range of 0.5840.644. As a result, the [(Co_0.65Fe_0.35)_0.9Ni_0.1]_73-xB_21.9Si_5.1Nb_x bulk glassy alloys (BGAs) with diameters up to 5.0 mm were produced by copper mold casting. In addition to the high GFA, the Co-based glassy alloys exhibited excellent soft-magnetic properties, i.e., saturation magnetization of 0.530.81 T, low coercive force of 0.511.75 A/m, and high effective permeability of (1.522.53)×10⁴ at 1 kHz under a field of 1 A/m. Besides, the Co-based BGAs also exhibited super high fracture strength of 42704490 MPa and vickers hardness of 11271182.

Abstract: In the current study, the amorphization behavior of mechanically alloyed Ni57Zr20Ti22Ge1 powder was examined in details. The conventional X-ray diffraction results confirm that the fully amorphous powders formed after 5 hours of milling. The thermal stability of the Ni57Zr20Ti22Ge1 amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the glass transition temperature (Tg) and the crystallization temperature (Tx) are 761 K and 839 K, respectively. The supercooled liquid region ΔT is 78 K. The appearance of wide supercooled liquid region may be mainly due to the Ge additions which cause the increasing differences in atomic size of mechanically alloyed Ni57Zr20Ti22Ge1 powders.

Abstract: Special remelting techincs including repeating the heating and quenching process for several times, electromagnetic mixing during the melting, increasing the superheating temperature, prolonging isothermal time can effectively decrease or remove the effect of impurity in raw materials on the glass-forming ability (GFA) of the alloy system. These technics were introduced to improve GFA of Fe60Co8Zr10Mo5W2B15 alloys (△Tx = 64 K, Trg = 0.61, Trg’= 0.59). Bulk metallic glasses of Fe60Co8Zr10Mo5W2B15 with the shape of rods (φ3×40) and plates (1×10×20) were casted into fully glassy state. It was proved the GFA of Fe60Co8Zr10Mo5W2B15 with industrial raw materials is very close to that with high purity raw materials. Fe60Co8Zr10Mo5W2B15 bulk metallic glass with industrial raw materials shows better potential appliance for their low cost.

Abstract: In the present study, Ti50Cu28Ni15Sn7 metallic glass and its composite powders reinforced with 4~12 vol% of SiC additions were successfully prepared by mechanical alloying. The as-milled Ti50Cu28Ni15Sn7 and composite powders were then consolidated by vacuum hot pressing into disc compacts with a 10 mm diameter and thickness of 2 mm. The structure of the as-milled powders and consolidated compacts was characterized by X-ray diffraction. While the thermal stability was examined by differential scanning calorimeter. In addition, the mechanical property of the consolidated bulk metallic glass and its composite was evaluated by Vickers microhardness tests. In the ball-milled composites, initial SiC particles were homogeneously dispersed in the Ti-based alloy glassy matrix. The presence of SiC particles did not dramatically change the thermal stability of Ti50Cu28Ni15Sn7 glassy powders. BMG composite with submicron SiC particles homogeneously embedded in a highly dense nanocrystalline/amorphous matrix was successfully prepared. A significant hardness increase with SiC additions was noticed for consolidated composite compacts.

Abstract: High-temperature plastic flow and tensile ductility in pre-annealed Zr65Al10Ni10Cu15 bulk metallic glass are examined in a supercooled liquid region. The specimen annealed at 673K for 1.8ks before the tensile test exhibits uniform deformation, and large elongation of 1100% is obtained in the tensile test at 673K and in a constant true strain rate of 1×10-2s-1. In the specimen, initial stress decreases in comparison with that of the specimen without annealing, but apparent strain hardening arise during deformation. SEM analysis after the deformation reveals that the cavitation is very limited even at the fracture tip. XRD and TEM analyses reveal that icosahedral phase is observed only in gage section but not in grip section after the deformation. The limited cavitation due to the accommodation of stress concentration through the grain boundary amorphous phase possibly makes a positive contribution to the large elongation in the annealed specimen, as well as the neck stability due to the strain hardening by the interaction between the icosahedral phase particles increasingly crystallized by the local strain.