Dong Yuan-Da

Structure and properties of bulk nano-structured WC-CO alloy by mechanical alloying

Abstract Mixtures of elemental powders of nominal composition WC–6wt%Co and WC–6wt%Co–1wt%VC were prepared using 99.5% purity tungsten, graphite, cobalt and vanadium powders with particle sizes smaller than 75 μm. Mechanical alloying (MA) was performed in a QM-1 planetary ball mill. The structural evolution and the crystallite size changes of the powders during MA were monitored by X-ray diffraction. The results show that cemented carbides of WC–Co powder with crystalline sizes of about 10 nm were directly synthesized from elemental powders by mechanical alloying. Cold compacting was carried out at a pressure of 800 MPa using a manual uniaxial press with carbide insert dies. The hardness and sintered density of sintered samples were measured. The effects of small VC additions on the grain size, density and hardness of sintered samples were also investigated.

Glass Formation Ability and Kinetics of the Gd55Al20Ni25 Bulk Metallic Glass

We report a new bulk glass-forming alloy Gd55Al20Ni25. The bulk sample of the alloy is prepared in the shape of rods in diameter 2 mm by suction casting. The rod exhibits typical amorphous characteristics in the x-ray diffraction pattern, paramagnetic property at 300 K, distinct glass transition and multi-step crystallization behaviour in differential scanning calorimetry traces. The glass formation ability of the alloy is investigated by using the reduced glass transition temperature Trg and the parameter γ. Kinetics of glass transition and primary crystallization is also studied. The fragility parameter m obtained from the Vogel–Fulcher–Tammann dependence of glass transition temperature Tg on ln ( is the heating rate) classifies the bulk metallic glasses into the intermediate category according to Angells classification.

Amorphization Induced High Magneto-Caloric Effect of Gd55Al20Ni25 Ternary Alloy

We report the amorphization induced high magneto-caloric effect (MCE) of recently developed Gd55Al15Ni30 bulk metallic glass (BMG). The magnetic properties of the Gd55Al15Ni30 BMG are investigated in comparison with that of its crystalline counterpart. It is found that amorphization can increase the saturation magnetization and decrease the hysteresis of Gd55 Al15 Ni30 alloys, which indicate the possible enhancement of MCE. The magnetic entropy changes and the refrigerant capacity of the BMG as well as the crystalline samples is calculated directly from isothermal magnetic measurements. The results show the amorphization induced high MCE of the alloy and the excellent refrigerant efficiency of Gd55Al15Ni30 bulk metallic glass.

Effect of Gallium Addition on Magnetic Properties of Nd2Fe14B-Based/α-Fe Nanocomposite Magnets

Abstract The influence of Ga addition on the crystallization behavior and the magnetic properties of nanocomposite Nd 2 Fe 14 B-based/α-Fe magnets was investigated. It was found that the addition of 0.2% did not change the crystallization temperature of amorphous alloy, but the magnetic properties were improved significantly because of the strong exchange coupling interaction between the hard and soft magnetic phases. The optimum magnetic properties with i H e = 600.3 kA·m −1 Br = 0.75 T, and ( BH ) max = 88.03 kJ·m −3 were obtained in bonded Nd 9.5 (FeCoZr) 83.8 Ga 0.2 B 6.5 magnet with 15 m·s −1 wheel speed and 670 °C annealing treatment. The apparent improvement of magnetic properties originates from the grain refinement calculated using the Scherrer formula from corresponding XRD patterns and the excellent rectangularity of the demagnetization curve.

Thermal Stability and Kinetics of Binary Cu65Hf35 Bulk Metallic Glass

We investigate the thermal stability and kinetics of Cu65Hf35 bulk metallic glass (BMG). Cu65Hf35 glassy rods in diameter up to 2 mm are prepared by a conventional copper mould suction casting. Kinetics of glass transition and crystallizations are investigated and the ideal glass transition temperature of the alloy is obtained. It is found that the dependence of crystallization temperature of the BMG on the heating rates follows the Vogel–Fulcher–Tammann (VFT) non-linear relationship rather than the Kissinger and Lasocka linear fittings. The long-term thermal stability of the BMG is investigated by means of continuous crystallization diagrams obtained from the extension of VFT analysis. It is suggested that ideal glass transition and crystallization temperatures can also be regarded as the long-term stability criteria of the BMG.

Long-Term Thermal Stability of Binary Cu50.3Zr49.7 Bulk Metallic Glass

We investigate the kinetics and thermal stability of a simple Cu50.3Zr49.7 binary bulk metallic glass (BMG). The long-term thermal stability of Cu50.3Zr49.7 BMG is evaluated by a newly developed method from an extension of Vogel-Fulcher–Tammann analysis. The method has been proven to be valid in Cu50.3Zr49.7 or even other BMGs.

The phase transition of syndiotactic polystyrene induced by electrostatic field

The crystal structure and morphology of syndiotactic polystyrene (sPS) melt-crystallized in various electrostatic fields have been investigated by using wide angle X-ray diffraction (WAXD), differential scanning calorimeter (DSC) and scanning electron microscope (SEM). WAXD and DSC analyses show that sPS is gradually transformed from a phase to β phase with increase of electrostatic intensity. The SEM observations indicate that the morphology of sPS micro-crystals is strongly dependent on electrostatic intensity applied to the sample during solidification. The variation of the micro-crystals orientation from disordered lamellae to ordered lamellare have been observed with increase of electrostatic intensity.

Curie temperature of the intergranular amorphous phase in nanocrystalline Fe89Zr7B4 alloy

The FeZrB amorphous alloys for simulating the intergranular amorphous phase in the nanocrystalline Zr7B4 soft magnetic materials were obtained by mechanical alloying of a mixture of elemental Fe, Zr and B powders for 25 h. It is shown that the Curie temperature of the simulated intergranular phase alloy is much lower than that of the intergranular phase with the same chemical composition in the nanocrystalline Fe89Zr7B4 alloy. The possible mechanism is mainly due to the strong ferromagnetic exchange force among the nanocrystalline α-Fe grains.

Effect of Electrostatic Reduced-Gravity Environment on the Crystallographic Parameters of Melt-Crystallized Polypropylene

An electrostatic reduced-gravity device was developed and used specially for the study of the melt crystallization of polypropylene. The crystal structure of melt-crystallized polypropylene prepared under the reduced-gravity environment was investigated by using X-ray diffraction. The experiment results show that the crystal structure of the polypropylene is strongly dependent on the gravity applied to the sample during solidification. It is found that the crystallographic parameters a and b increase markedly with reduced-gravity ratio, while the value of c increases mildly.