Materials Science Forum Vols. 475-479

Abstract: One-dimensional GaN nanostructure films were successfully synthesized by the recently developed sputtering post-nitridation technique. The morphology and structure of GaN nanowires are investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results indicate that the crystalline GaN nanostructures have a hexagonal wurtzite structure, and there is not any other phase such as Ga2O3 or Ga in the specimen. It also confirms that high quality crystal was obtained in the resulting sample due to the lattice defects decreased and the crystallinity improved in the process of nitridation at high temperature. The growth mechanism of the GaN nanostructures is briefly discussed.

Abstract: The upper bound for the production of uniform amorphous ribbons during free-jet melt spinning is predicted by coupling a mass balance condition for the melt-pool with a simple boundary layer model for momentum transfer. The relationships between melt-pool length, ribbon thickness and wheel speed are investigated, and a criterion is developed for the onset of unsteady melt-pool behavior, which has previously been associated with increased surface roughness, porosity, and the formation of crystalline phases at high wheel speeds.

Abstract: We have demonstrated the production of gallium oxide thin films on various substrates such as Si(111), SiO2, and sapphire by metalorganic chemical vapor deposition using the trimethylgallium (TMGa) as a precursor in the presence of oxygen. The XRD data revealed that the as-deposited gallium oxide films were fully amorphous but very small crystallites with monoclinic structures were found with the thermal annealing at a sufficiently high temperature, regardless of substrate materials. The AFM analysis indicated that the surface roughness increased by the thermal annealing.

Abstract: The present paper covers in a comprehensive manner the formation of the Cu-based Cu-Zr-Al bulk metallic glasses (BMGs). Composition optimization for BMG formation is realized by using an “e/a-variant criterion”. This criterion is incorporated into the ternary phase diagram by a straight composition line, which is defined by linking a specific binary composition to the third element. There e/a-variant composition lines are constructed: (Cu9Zr4)1-xAl, (Cu61.8Zr38.2)1-xAlx and (Cu56Zr44)1-xAlx, where Cu9Zr4, Cu61.8Zr38.2 and Cu56Zr44 are specific cluster compositions in the Cu-rich Cu-Zr binary system. No Cu-based BMGs are obtained in the composition line (Cu9Zr4)1-xAlx using our suction casting techniques, while BMGs are obtained within an e/a span from 1.24 to 1.3 and from 1.28 to 1.36 respectively along the other two lines (Cu61.8Zr38.2)1-xAlx and (Cu56Zr44)1-xAlx. Thermal analysis results indicate that the BMGs on every composition line manifest increased thermal stability and glass forming ability (GFA) with increasing e/a ratios. The maximum appears in Cu58.1Zr35.9Al6 with the e/a value of 1.3, which belongs to the (Cu61.8Zr38.2)1-xAlx series. The characteristic thermal parameters of this BMG are Tg = 760K, Tg/Tm = 0.659 and Tg/Tl = 0.648, which are all superior to those reported for the known Cu55Zr40Al5 BMG.

Abstract: The crystallization reaction of Zr65Ni25Ti10 (at. %) metallic glass was studied by using differential scanning calorimetry (DSC) at constant heating rate. Two distinct exothermic peaks were observed from the continuous heating DSC curves, implying the crystallization process undergoes two different stages. The crystallization kinetic parameters, including activation energy (E), Avrami exponent (n) and frequency factor (K0), were determined with non-isothermal analysis method based on the DSC data. The values of E are 267.353 KJ/mol and 224.293 KJ/mol, n 4.0±0.1 and 6.8±0.1, K0 4.4±0.05E+20 and 2.0±0.08E+15, for the first and second crystallization stage, respectively. The results suggest that the crystallization mechanism is governed dominantly by interface controlled growth with constant nucleation rate for the first crystallization stage and with increasing nucleation rate for the second stage.

Abstract: Zr-based metallic-glass matrix composites, which are reinforced by continuous tungsten fibers, were prepared by melt infiltration casting. The interface structure was analyzed by using X-Ray diffraction, SEM and EPMA. The results illustrate that for the Zr55Al10Ni5Cu30 matrix composite, in addition to the interface diffusion, an interface eutectoid reaction between W fiber and Zr in the liquid state takes place and forms W5Zr3 phase at the interface during casting, the interface is included in Class Ⅲ system; but for (Zr55Al10Ni5Cu30)0.98Nb2 and Zr57Al10Ni12.6Cu15.4Nb5 matrix composite, the addition of Nb restrains the interface eutectoid reaction effectively, which may be attributed to reducing diffusion coefficient of Zr through the interface, and there is only a thinner diffusion layer at the interfaces, the type of interface is changed to the Class Ⅱ system

Abstract: The glass-forming ability, thermal stability and magnetic properties of the Nd60-xDyxFe30Al10 (x=0, 2, 5) bulk amorphous alloys were investigated by x-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and the vibrating sample magnetometer (VSM). The results show that the glass forming ability of the Nd60-xDyxFe30Al10 (x=0, 2, 5) alloys decrease with increasing Dy content. The as-cast Nd60-xDyxFe30Al10 (x=0, 2, 5) alloys show hard magnetic behavior at room temperature. With increasing Dy content, the intrinsic coercivity of the alloys increase significantly while the saturation magnetization and remanence of the alloys decrease monotonously. With increasing annealed temperature, the intrinsic coercivity of the Nd55Fe30Al10Dy5 alloy decreased significantly, while the saturation magnetization and remanence decrease monotonously. The Nd55Fe30Al10Dy5 alloy shows soft magnetic behavior after annealed at 773K for 30 min.

Abstract: Glassy Fe77Ga3P9.5C4B4Si2.5 alloy powders were synthesized in the particle size range below 125 µm by Ar gas atomization. A glassy alloy disc of 10 mm in diameter and 3 mm in thickness with a high relative density of 99.7 % was synthesized by spark plasma sintering. The resulting glassy magnetic core exhibits good soft magnetic properties, i.e., 1.35 T for saturation magnetization, 5 A/m for coercive force and 9600 for maximum permeability.

Abstract: Small-angle scattering measurements utilizing anomalous dispersion effect just below the Zr and Ni K edge have been made for welded Zr-based amorphous materials. The measurements were carried out at a bending magnet beam-line 40B of SPring8. A small and parallel X-ray beam generated at the third generation synchrotron is suitable for nanostructure mapping of heterogeneous materials, whose structure varies with less than a millimeter scale. The remelted and heat-affected zones were examined for electron-beam welded Zr-Ni-Cu-Al alloys. Merits and the characteristics, as well as its present limitations of scanning small-angle scattering with a use of anomalous dispersion effect is discussed as a tool to examine microstructure in the spatially inhomogeneous materials in macroscopic scale as well as microscopic scale.

Abstract: The effects of structural relaxation and partial crystallization on the mechanical property of the Ti40Zr29Cu9Ni8Be14 bulk metallic glass (BMG) have been investigated. The atomic structure of the as-cast Ti40Zr29Cu9Ni8Be14 metallic glass transforms into a more relaxed state at the temperature region of 452 –585 K, below the crystallization onset temperature of 631 K. Stable icosahedral phase forms in the amorphous matrix by growth of the pre-existing nuclei in the amorphous matrix during first crystallization step. The compressive plastic strain of the as-cast Ti40Zr29Cu9Ni8Be14 BMG is 6.7 %, and decreases when the structural relaxation occurs. However, the plastic strain increases when a few nanometer size icosahedral phase particles form in the amorphous matrix by the partial crystallization treatment.