We studied the glass forming ability of Ni-Nb binary alloys and found that some of the alloys can be prepared into bulk metallic glasses by a conventional Cu-mold casting. The best glass former within the compositional range studied is off-eutectic Ni 62 Nb 38 alloy, which is markedly different from those predicted by the multicomponent and deep eutectic rules. The glass formation mechanism for binary Ni-Nb alloys was studied from the thermodynamic point of view and a parameter ␥ * was proposed to approach the ability of glass formation against crystallization. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2158130͔ Bulk metallic glasses ͑BMGs͒ as a prominent class of functional and structural materials with unique properties have attracted intensive interests due to their considerable significance in science and technology. [1][2][3][4][5][6][7][8][9][10][11] In order to make the best use of these noncrystalline materials, the key problem is to develop BMGs with improved properties and excellent glass forming ability ͑GFA͒. It is commonly regarded that the formation of metallic glasses is controlled by two factors, i.e., the cooling rate and the composition of the alloys. The critical cooling rate, which is the most effective gauge for GFA of the alloys, is hard to be measured experimentally. Hence, a great deal of efforts have devoted to the investigation on the composition of glass forming alloys. Inoue et al. 11 and Johnson 1 framed the empirical rules to predict the element selection and compositional range of glass forming alloys. These rules have played an important role as a guideline for synthesis of BMGs for the last decade.Binary alloys are usually considered to have a lower GFA due to their lack of complicated structure with atomic configuration according to the "confuse principal." 2,3 However, recent experimental results have shown that binary Zr-Cu alloys can also be vitrified into BMGs. [12][13][14] In contrast with the deep eutectic rule, the better glass formers in Zr-Cu binary alloy system such as Zr 35.5 Cu 64.5 and Zr 50 Cu 50 are off eutectic. Therefore, the empirical rules for glass formation, that is, multicomponent alloys with composition near deep eutectic, could be no longer the major concern for designing BMGs.In this work, we reported that some of the Ni-Nb binary alloys can be prepared into fully glassy rods up to 2 mm in diameter by a conventional Cu-mold casting method. The best glass former within the compositional range studied is off-eutectic Ni 62 Nb 38 alloy, which is different from those predicted by the multicomponent and deep eutectic rules. The glass formation mechanism of the binary alloys was studied from the thermodynamic point of view based on Miedema's calculation model.Ingots of Ni-Nb binary alloys with different compositions were prepared separately by arc-melting of 99.9% ͑at. %͒ pure Ni and Nb in titanium-gettered argon atmosphere. The rods of 2 mm in diameter were prepared by suction casting under argon atmosphere. The structure of the samples was charact...

Zr 48.5 Cu 46.5 Al 5 bulk metallic glass matrix composites with diameters of 3 and 4 mm were produced through water-cooled copper mold casting. Micrometer-sized bcc based B2 structured CuZr phase containing martensite plate, together with some densely distributed nanocrystalline Zr 2 Cu and plate-like Cu 10 Zr 7 compound, was found embedded in a glassy matrix. The microstructure formation strongly depends on the composition and cooling rate. Room temperature compression tests reveal significant strain hardening and plastic strains of 7.7% and 6.4% before failure are obtained for the 3-mm-and 4-mm-diam samples, respectively. The formation of the martensite phase is proposed to contribute to the strain hardening and plastic deformation of the materials.

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Magnetic domain structure of Nd 60 Al 10 Fe 20 Co 10 bulk metallic glass ͑BMG͒ has been studied by using magnetic-force microscopy. In the magnetic-force images it is shown that the exchange-interaction-type magnetic domains with a period of about 360 nm do exist in the BMG, which is believed to be associated with the appearance of hard-magnetic properties in this system. The existence of the large-scale domains demonstrates that the magnetic moments of a great deal of short-scale ordered atomic clusters in the BMG have been aligned by exchange coupling. Annealing at 715 K leads to partial crystallization of the BMG. However, the exchange coupling is stronger in the annealed sample, which is considered to arise from the increase of transition-metal concentration in the amorphous phase due to the precipitation of Nd crystalline phase.Nd-Fe-Al-based bulk metallic glasses ͑BMG͒ with a diameter up to 12 cm have been recently prepared. 1 This BMG system has evoked an extensive attention due to its hard magnetic properties at room temperature. 2-8 The high coercivity of these amorphous alloys is striking because no structural anisotropy exists in the disordered packing solid. It is supposed that the BMG is an ensemble of short-scale ordered magnetic atomic clusters in uniform distribution. These clusters consist of Nd and transition elements and possess large random anisotropy. It is the magnetic-exchange coupling among them that causes the high coercivity of this magnetic system. Most of the consequent results supported this model. 7-9 However, no direct experimental evidence for the existence of exchange-coupling interaction in this system has been presented so far. Therefore, it is of a special interest to study the magnetic domain structure of the BMG. In this paper, the magnetic domain structure of Nd 60 Al 10 Fe 20 Co 10 was investigated by magnetic-force microscopy ͑MFM͒.Ingots with compositions of Nd 60 Al 10 Fe 20 Co 10 were prepared by arc melting from elemental Nd, Fe, Al, and Co with a purity of 99.9% in a titanium-gettered argon atmosphere. Cylindrical specimens of around 3 mm in diameter and 50 mm in length were prepared from the ingots by die casting into a copper mold under argon atmosphere. The details of preparation can be seen in Ref. 5. The structure of the as-cast cylinder was characterized by x-ray diffraction ͑XRD͒. 5 Magnetic measurements were performed using a vibrating sample magnetometer with a maximum applied field of 1592 kA m Ϫ1 . The study of domain structure was carried out by using Digital Instruments NanoScope IIIa D-3000 MFM. It allows the topographic and magnetic-force images to be collected separately and simultaneously in the same area of the sample by using Tapping/Lift modes. The magnetic tips used were microfabricated Si cantilevers with a pyramidal tip coated with magnetic Co-Cr thin film of 40 nm thickness and a coercivity of about 32 kA m Ϫ1 . For the mode of dynamic detection, the cantilever is vibrated and its resonant frequency f 0 and phase will be modulated by the magnet...

To meet the demand of modern acoustic absorbing material for which acoustic absorbing frequency region can be readily tailored, we introduced woodpile structure into locally resonant phononic crystal ͑LRPC͒ and fabricated an underwater acoustic absorbing material, which is called locally resonant phononic woodpile ͑LRPW͒. Experimental results show that LRPW has a strong capability of absorbing sound in a wide frequency range. Further theoretical research revealed that LRPC units and woodpile structure in LRPW play an important role in realization of wide band underwater strong acoustic absorption.

The structural evolution and property changes in Nd 60 Al 10 Fe 20 Co 10 bulk metallic glass ͑BMG͒ upon crystallization are investigated by the ultrasonic method, x-ray diffraction, density measurement, and differential scanning calorimetry. The elastic constants and Debye temperature of the BMG are obtained as a function of annealing temperature. Anomalous changes in ultrasonic velocities, elastic constants, and density are observed between 600-750 K, corresponding to the formation of metastable phases as an intermediate product in the crystallization process. The changes in acoustic velocities, elastic constants, density, and Debye temperature of the BMG relative to its fully crystallized state are much smaller, compared with those of other known BMGs, the differences being attributed to the microstructural feature of the BMG. Recently, many glass forming alloys were discovered. 1,2Previous studies show that the crystallization in various bulk metallic glasses ͑BMGs͒ is characterized by the appearance of intermediate metastable phases. The formation of icosahedral phases as an intermediate product of the crystallization process in BMGs indicates that there is a structural relationship between the BMGs and quasicrystalline.3,4 The Ndbased BMGs have also evoked intensive interests due to their unique magnetic properties and anomalous crystallization behavior. 1,[5][6][7] It is found that the microstructural change induced by relaxation and crystallization has a very sensitive effect on the thermal and magnetic properties of the BMGs. This implies that the hard magnetic property of the BMGs may be related to the formed intermediate phases prior to the crystallization. In this letter, the microstructural and property changes in the Nd 60 Al 10 Fe 20 Co 10 BMG upon crystallization are investigated by using ultrasonic method and density measurement, which are effective and sensitive tools for studying the structural and vibrational characteristics of BMGs. 8-11The structural and property features, prior to and in the crystallization process, are connected.Nd 60 Al 10 Fe 20 Co 10 BMG was prepared by the die casting method. 6 The BMG rod was cut to a length of about 10 mm and its ends were polished flat and parallel. Afterward, the sample was stepwise isothermally annealed at various temperatures for 1.0 h in a vacuum of 10 Ϫ3 Pa, respectively. After each annealing, the rod was cooled to room temperature, and the acoustic velocities and density were measured. The acoustic velocities were measured by a pulse echo overlap method using a MATEC 6600 ultrasonic system with a 10 MHz frequency. 8,9 The density was measured by the Archimedean technique and the accuracy is 0.1%. Elastic constants ͑e.g., Young's modulus E, shear modulus G, bulk modulus K, and Poisson's ratio ͒ and Debye temperature ⌰ D of the BMG were derived from the acoustic velocities and densities. 12 The structure of the samples was characterized by x-ray diffraction ͑XRD͒ using a MAC M03 diffractometer with Cu K␣ radiation. Differential scanning calorimet...

Cu 47.5 Zr 47.5 Al 5 was prepared by arc melting and solidified in situ by suction casting into 2-5-mm-diameter rods under various cooling rates (200-2000 K/s). The microstructure was investigated along the length of the rods by electron microscopy, differential scanning calorimetry and mechanical properties were investigated under compression. The microstructure of differently prepared specimens consists of macroscopic spherical shape chemically inhomogeneous regions together with a low volume fraction of randomly distributed CuZr B2 phase embedded in a 2-7 nm size clustered "glassy-martensite" matrix. The as-cast specimens show high yield strength (1721 MPa), pronounced work-hardening behavior up to 2116 MPa and large fracture strain up to 12.1-15.1%. The fracture strain decreases with increasing casting diameter. The presence of chemical inhomogenities and nanoscale "glassy-martensite" features are beneficial for improving the inherent ductility of the metallic glass.

The thermal stability of Nd 60 Fe 20 Co 10 Al 10 bulk metallic glass (BMG) has been studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), isochronal dilatation and compression tests. The results show that the glass transition of the BMG takes place quite gradually between about 460 and 650 K at a heating rate of 0.17 K/s. Several transformation processes are observed during continuous heating with the first crystallization process beginning at about 460 K, while massive crystallization takes place near the solidus temperature of the alloy. The positive heat of mixing between the two major constituents, Nd and Fe, and, consequently, a highly inhomogeneous composition of the attained amorphous phase are responsible for the anomalous thermal stability in this system. 