Materials Science Forum Vols. 539-543

Abstract: Nanoscale structural change in (Fe0.5Co0.5)72B20Si4Nb4 bulk glassy alloy on annealing has been investigated using transmission electron microscopy. On annealing at temperatures above 773K, electron diffraction intensity analysis showed a clear structure change for a Cr23C6-type local atomic ordering. The local structure formation of Cr23C6-type nanophase was confirmed by nanobeam electron diffraction. A development process of dense precipitates of the Cr23C6-type nanophase was further studied by high-resolution electron microscopy. It was found that the formation of the highly-dense nanoprecipitates provides an increase in Vickers hardness.

Abstract: Fe- and Co-based bulk glassy alloys (BGAs) with diameters up to 5 and 4 mm were formed respectively in (Fe,Co)-B-Si-Nb system by copper mold casting. The Fe-based glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system exhibit saturation magnetization (Is) of 0.84-1.13 T and low coercive force (Hc) of 1.5-2.7 A/m. Its BGAs exhibit superhigh strength ( σ f) of 3900-4250 MPa, Young’s modulus (E) of 190-210 GPa, elastic strain (εe) of 0.02 and plastic strain (εp) of 0.0025. For the Co-based glassy alloys in [(Co1-xFex)0.75B0.2Si0.05]96Nb4 system, in addition to superhigh σ f of 3980-4170 MPa, they also exhibit excellent soft magnetic properties, i.e., Is of 0.71-0.97 T, Hc of 0.7-1.8 A/m, high effective permeability (μe) of 1.48-3.25×104 at 1 kHz under a field of 1 A/m, and extremely low saturation magnetostriction (λs) of 0.55-5.76×10-6.

Abstract: This paper introduces a technique for fabricating nano-structures through super plastic nano-forging of metallic glass using nano-scale dies that are fabricated by a focused-ion beam (FIB). FIB-machining characteristics of glassy carbon and Zr-based metallic glass have been studied and are useful for fabricating nano-scale dies because of the isotropic homogeneity of their amorphous structures. We used the dies to nano-forge Pt48.75Pd9.75Cu19.5P22 metallic glass. The thin foil specimens were heated in a small furnace and compressively loaded in a small vacuum chamber. Dies, a die-forged 1μm-diameter micro-gear, and both 800 and 400nm periodic nano-structures for optical applications were demonstrated. We observed the effects of thermal expansion and contact angle between the working material and the die materials on nano-formability. Metallic glasses are highly useful as materials for nano-imprinting and as die materials for FIB nano-machining.

Abstract: Behaviors of fracture and deformation in a Zr-Al-Ni-Cu bulk metallic glass(BMG) was investigated by using three-point bending tests. Apparent fracture toughness obtained by bending test was 40MPam1/2 which is comparable to the value of ductile crystalline metals. This high toughness of the BMG should be understood by the crack-tip plasticity as well as crystalline metals. It is well known that plastic deformation occurs very inhomogeneously when BMGs are deformed at room temperature. Such inhomogeneity is manifested by the appearance of surface steps caused by localized shear deformation. In the present study, the surface steps due to the localized shear bands near a fracture surface have been examined in detail by using SEM and AFM, where much attention has been paid on the variation of the surface step height measured along the localized shear band. The variation of the step height indicates the gradient of plastic shear deformation, and it can be understood, in principle, as the introduction of elastic singularities corresponding dislocations in the case of crystalline materials.

Abstract: In order to examine mechanical properties of a metallic glass Zr50Cu40Al10 in low temperature below room temperature, the temperature T dependence of mechanical resonance of ultrasonic wave are measured. The mechanical resonance frequency in an as-quenched sample shows an abrupt increase at 200K for longitudinal wave and 160 K for transverse wave with decreasing T. After this abrupt increase, the sound propagation cannot be detected below these temperatures but the wave propagation is restored with increasing T and there is an abrupt decrease at 260K for the both wave modes. The similar hysteresis is observed in temperature dependence of the electrical resistivity. These suggest a kind of structure instability of Zr50Cu40Al10 in low temperature region.

Abstract: The Mg65Cu25Gd10-xNdx (x=0 ~ 10) amorphous alloy rods with 3~6 mm in diameter were prepared by Cu-mold injection method. The thermal properties and mechanical properties of these amorphous alloys have been investigated by DSC, SEM with EDS capability, X-ray diffractometry (XRD) and Vickers hardness test. The XRD revealed that these entire as-quenched Mg65Cu25Gd10-xNdx alloy rods exhibit a broaden diffraction pattern of amorphous phase. A clear Tg (glass transition temperature) and supercooled region (about 30~60 K) were revealed for all of those Mg65Cu25Gd10-xNdx alloys. In addition, the single stage crystallization of the Mg65Cu25Gd10 alloy was found to change into two stages crystallization when the Nd element was added into this alloy. In parallel, the crystallization temperature (Tx) and supercooled region (
Tx) presents a decreasing trend with increasing Nd content. The lowest liquidus temperature (Tl, about 721 K) occurs at the Mg65Cu25Gd8Nd2 alloy. In addition, The Mg65Cu25Gd8Nd2 alloy exhibits the high γ value (0.416, defined as γ= Tx/Tg+Tl), a relatively high Trg (0.59, defined as Trg = Tg/Tl) and the highest hardness in these alloys.

Abstract: The thermal properties of a Zr76.11Ti4.20Cu4.51Ni3.16Be1.49Nb10.53 bulk metallic glass (BMG) have been investigated by using a differential scanning calorimeter (DSC). The composition of dendrite phase was then subsequently analyzed by using an EPMA, XRD, and TEM. The glass transition and crystallization onset temperatures were determined as 339.7 °C and 375.8 °C for this BMG, respectively. The Zr-Ti-Nb dendrite phase was found to have a BCC structure. Mechanical properties have also been examined by conducting a series of uniaxial compression tests at various temperatures within supercooled liquid region under the strain rates between 10-4 /s and 3×10-2 /s. The hardness of matrix and dendrite was then measured separately. The glassy matrix appears to play major role on the elongation, while dendrite phase on the strength of this BMG composite at high temperatures within supercooled liquid region.

Abstract: The oxidation behavior of Y56Al24Co20 bulk metallic glass containing ~4.2% Y-rich particles was studied over the temperature range of 325-450oC in dry air. The results showed that the oxidation kinetics of the alloy followed the parabolic rate law at T≤375oC, with rate constants generally increased with temperature. Conversely, para-linear kinetics were observed at higher temperatures (T≥400oC). The scales formed on the Y-base amorphous composite were temperature dependent, consisting of exclusively yttrium oxides (Y2O3) at T≤375oC and of mostly Y2O3 and alumina (Al2O3) at temperatures. No evidence of any cobalt-containing oxide was found in the scale.

Abstract: A method for making interatomic potentials is proposed and is applied to Cu-Zr-Hf-Ni- Al bulk-metallic-glass systems. The method consists of three steps. Firstly, potential function form is determined so that bonding nature can be described. Secondly, materials properties used for fitting are selected so that the potential has enough robustness. Here, it is noted that materials properties must be added in accordance with the purpose of the study. Finally, potential parameters have been optimized using global-search procedure. Developed potential well reproduces material properties of them.

Abstract: Micro-forming is considered to be a suited technology to manufacture very small metallic parts (several μm~mm). Zr-based bulk metallic glass, Zr62Cu17Ni13Al8, has been expected to be a promising metallic material for micro-forming process due to their isotropy, low flow stress in a wide supercooled liquid region and good stability of amorphous matrix. Therefore, one can expect that micro-forming of Zr62Cu17Ni13Al8 might be feasible at a relatively low stress in the supercooled liquid state without any crystallization during hot deformation. In this study, micro-formability of Zr62Cu17Ni13Al8 bulk metallic glass was investigated for micro-forging of U-shape pattern. Microformability was estimated by comparing Rf values (=Af/Ag), where Ag is corss-sectional area of U groove, and Af the filled area by material. Micro-forging process was also simulated and analyzed by applying the finite element method. The micro-formability of Zr62Cu17Ni13Al8 was increased with increasing load and time in the temperature range of the supercooled liquid state. In spite of the similar trend in the variations of Rf values, FEM simulation results showed much higher Rf values than the experimental Rf values. This disagreement was analyzed based on the stress overshoot phenomena of bulk metallic glasses in the supercooled liquid region. FEM simulation of the microstamping process was applicable for the optimization of micro-forming process by carefully interpreting the simulation results.