Huixiao Zhang

Two-phase quasi-equilibrium in β-type Ti-based bulk metallic glass composites.

The microstructural evolution of cast Ti/Zr-based bulk metallic glass composites (BMGCs) containing β-Ti still remains ambiguous. This is why to date the strategies and alloys suitable for producing such BMGCs with precisely controllable volume fractions and crystallite sizes are still rather limited. In this work, a Ti-based BMGC containing β-Ti was developed in the Ti-Zr-Cu-Co-Be system. The glassy matrix of this BMGC possesses an exceptional glass-forming ability and as a consequence, the volume fractions as well as the composition of the β-Ti dendrites remain constant over a wide range of cooling rates. This finding can be explained in terms of a two-phase quasi-equilibrium between the supercooled liquid and β-Ti, which the system attains on cooling. The two-phase quasi-equilibrium allows predicting the crystalline and glassy volume fractions by means of the lever rule and we succeeded in reproducing these values by slight variations in the alloy composition at a fixed cooling rate. The two-phase quasi-equilibrium could be of critical importance for understanding and designing the microstructures of BMGCs containing the β-phase. Its implications on the nucleation and growth of the crystalline phase are elaborated.

Glass-forming ability and thermal stability of Nd70−xFe20Al10Yx alloys

Abstract The glass-forming ability (GFA) and thermal stability of Nd 70− x Fe 20 Al 10 Y x (0≤ x ≤15) alloys produced by melt spinning and copper-mold casting have been investigated. Ribbon samples in the composition range show a fully amorphous structure. Except Y=5 at.%, bulk amorphous alloys at least 2 mm in diameter were obtained. Both the amorphous ribbon and the bulk cylinder for the Nd 55 Fe 20 Al 10 Y 15 alloy show a distinct glass transition temperature and a wide supercooled liquid region before crystallization. It is suggested to use a new parameter to characterize both the GFA and the thermal stability of Nd–Fe–Al–Y alloys, showing that Nd 60 Fe 20 Al 10 Y 10 alloy is the best one both in GFA and thermal stability. The remanence ( J r ), magnetization ( J 1500 ), and coercivity ( H c ) for the as-cast Nd 70− x Fe 20 Al 10 Y x amorphous cylinders depend strongly on the composition. The enhanced GFA and thermal stability as well as the reduction of magnetic properties are discussed.

Temperature dependence of the Raman-active modes in the nonpolar a-plane GaN film

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the nonpolar a-plane (or [11 (2) over bar0]-oriented) GaN thin film are analyzed in the range from 100 to 570 K. The nonpolar a-plane GaN film is grown on an r-plane [or (1 (1) over bar 02)-oriented] sapphire substrate by metal organic chemical vapor deposition. The spectral features of the Raman shifts, intensities, and linewidths of the active phonons modes A(1)(TO), E-1(TO), and E-2(high) are significantly revealed, and corresponding temperature coefficients are well deduced by the empirical relationships. With increasing the measurement temperature the Raman frequencies are substantially redshifted and the linewidths gradually broaden. The compressive-strain-free temperature for the nonpolar a-plane GaN film is found to be at about 400 K. Our studies will lead to a better understanding of the fundamental physical characteristics of the nonpolar GaN film. (c) 2007 American Institute of Physics.

Preparation of bulk Zr55Al10Ni5Cu30 metallic glass ring by centrifugal casting method

In this work, a conventional centrifugal casting method was used to produce bulk amorphous Zr55Al10Ni5Cu30 rings. The 1-mm thick amorphous ring of the Zr55Al10Ni5Cu30 alloy was successfully prepared by this method. With increasing thickness of the rings, no crystalline phase can be seen on the transverse section near the outer surface of the rings. However, Zr2Cu and Zr2Ni particles precipitated from the amorphous matrix near the inner surface for the rings with thicknesses of 1.5 or 2 mm. For the I mm thick ring, the hardness showed no evident variation along the whole cross section. The hardness increased with increasing distance from the outer surface to the inner surface for the rings with thicknesses of 1.5 and 2 mm. Centrifugal casting is a useful process to produce amorphous alloy parts, such as amorphous tube and graded amorphous matrix composites

Atomic interaction mechanism for designing the interface of W/Zr-based bulk metallic glass composites

The interaction between active element Zr and W damages the W fibers and the interface and decreases the mechanical properties, especially the tensile strength of the W fibers reinforced Zr-based bulk metallic glass composites (BMGCs). From the viewpoint of atomic interaction, the W-Zr interaction can be restrained by adding minor elements that have stronger interaction with W into the alloy. The calculation about atomic interaction energy indicates that Ta and Nb preferred to segregate on the W substrate surface. Sessile drop experiment proves the prediction and corresponding in-situ coating appears at the interface. Besides, the atomic interaction mechanism was proven to be effective in many other systems by the sessile drop technique. Considering the interfacial morphology, Nb was added into the alloy to fabricate W/Zr-based BMGCs. As expected, the Nb addition effectively suppressed the W-Zr reaction and damage to W fibers. Both the compressive and tensile properties are improved obviously.

Strong room-temperature ferromagnetism in Cu-implanted nonpolar GaN films

Diluted magnetic nonpolar GaN:Cu films with ferromagnetic properties up to 380 K have been fabricated by implantation of Cu ions into nonpolar a-plane GaN films and a subsequent thermal annealing process. The nonpolar GaN:Cu films exhibit a strong saturation magnetization about 1.54 μB/Cu atom, while polar GaN:Cu films can only show a weak saturation magnetization of 0.36 μB/Cu atom. Moreover, according to the x-ray photoelectron spectroscopy results, the stronger ferromagnetism of nonpolar GaN:Cu films may be resulted from the higher Cu incorporation efficiency in nonpolar GaN films.

Improved Jc properties of Na-doped MTG YBCO

Abstract The effects of Na addition in the MTG YBCO (YBa 2− x Na x Cu 3 O y +40 mol% Y 2 BaCuO 5 ) system with x =0.0, 0.1, 0.2 on T c and J c properties have been investigated. It was observed that with increase in x ( x ≤0.2), T c,on is almost the same, but T c,off decreases. With certain Na addition, MTG YBCO showed an improved critical current density characteristic and a second peak in the magnetization curve. These might be connected with the inhomogeneous distribution of Na, which can become pinning centers against the motion of the flux. The formation of lower T c phase in Na-rich region induces the second peak with increasing magnetic fields.

Structure and magnetic characteristics of nonpolar a-plane GaN : Mn films

Diluted magnetic nonpolar GaN:Mn films have been fabricated by implanting Mn ions into unintentionally doped nonpolar a-plane (1 1 (2) over bar 0) GaN films with a subsequent rapid thermal annealing (RTA) process. The structure, morphology and magnetic characteristics of the samples were investigated by means of high-resolution x-ray diffraction (XRD), atomic force microscopy (AFM) and a superconducting quantum interference device (SQUID), respectively. The XRD analysis shows that the RTA process can effectively recover the crystal deterioration caused by the implantation process and that there is no obvious change in the lattice parameter for the as-annealed sample. The SQUID result indicates that the as-annealed sample shows ferromagnetic properties and magnetic anisotropy at room temperature.

First and second order Raman scattering spectroscopy of nonpolar a-plane GaN

Nonpolar a-plane [(1120)] GaN samples have been grown on r-plane [(1102)] sapphire substrates by low-pressure metal-organic chemical-vapor deposition. The room-temperature first and second order Raman scattering spectra of nonpolar a-plane GaN have been measured in surface and edge backscattering geometries. All of the phonon modes that the selection rules allow have been observed in the first order Raman spectra. The frequencies and linewidths of the active modes have been analyzed. The second order phonon modes are composed of acoustic overtones, acoustic-optical and optical-optical combination bands, and optical overtones. The corresponding assignments of second order phonon modes have been made. (c) 2007 American Institute of Physics.

The wetting of Fe78B13Si9 by liquid Sn

The wetting behavior of liquid Sn on three types of Fe78B13Si9 substrate, an amorphous substrate and two types of pre-annealed substrate, was studied by the sessile drop technique between 350 and 450 degreesC. The equilibrium contact angle of the amorphous Fe78B13Si9 substrate did not decrease monotonically with increasing temperature, which is evidently different from that of polycrystalline alloy substrate. Pre-annealing of Fe78B13Si9 substrate made its wettability very poor when the wetting temperature was lower than the pre-annealing temperature. However, when the wetting temperature was higher than the pre-annealing temperature, pre-annealing of substrate had little effect on the equilibrium contact angle. These were related to the effect of crystallization reaction of Fe78B13Si9 on interfacial tension and interfacial reaction