Materials Science Forum Vols. 675-677

Abstract: Cubic boron nitride (cBN) grains treated with hydrochloric acid, nitric acid and sulphuric acid were investigated, respectively. The results indicated that the surface treatment technique with hydrochloric acid was an excellent method for improving the properties of cBN grains. After hydrochloric acid treating, cBN grains with clean surface and satisfied strength were obtained. Moreover, the bonding strength between cBN grains and vitrified bond was increased significantly for application in vitrified bond grinding tools.

Abstract: We prepared La2O3 doped SnO2 powder by a co-deposition method and used the doped material to manufacture tin dioxide electrodes by traditional ceramics methods.. The results showed that La2O3 led to an obvious decrease in electrical resistivity at room temperature but it had little influence at temperatures higher than 600 °C. The samples were characterized by X-ray diffraction and section scanning electron microscopy. We found that La2O3 increased the rate of tin dioxide crystal growth. At La2O3 concentrations as high as 1 wt%, a new La2Sn2O7 phase was present and this phase plays an important role in improving the electrical properties of the La2O3-doped SnO2.

Abstract: Porous silicon nitride ceramics with high-porosity were fabricated by carbothermal reduction reaction between silicon dioxide and carbon. The influences of carbon sources on microstructure and phase composition of the samples were studied. The result showed that the resultant porous silicon nitride ceramics using nano-meter carbon powder (C(n)) had finer microstructure, more uniform pore structure than those using activated carbon powder (C(a)). In order to illuminate the reaction and growth mechanism of b-Si3N4 grains, a series of intermediate sintering processes were investigated. X-ray diffractometry showed a difference in phase composition for the samples using the different carbon sources.

Abstract: Pr6O11 doped ZnO-based varistor powders were prepared by a method of pyrogenic decomposition nitrate, which were together with ZnO powders, made into rounded mass and sintered at different temperatures. The effects of sintering temperatures on the composition, microstructure and electrical properties of Pr6O11 doped ZnO-based varistors were investigated. The results show that the optimum sintering temperature is at 1150 °C with 6 μm ZnO average grain size and the samples possess the advantageous electrical properties: varistor voltage of about 480 V/mm, non linear coefficient reaching 44 and leakage current of 0.7 μA.

Abstract: Infiltration of a nano-sized alumina sol into the green bodies is proposed to facilitate the ceramic sintering. The green samples were prepared by a colloidal processing based on drying induced forming, and then were immersed in a nano-sized alumina sol prior to their sintering at 1350-1600 oC. The alumina sol was characterized by using transmission electron microscopy and the grain structure of sintered alumina ceramics were examined by scanning electron microscopy. Significant densification of sintered samples was evidenced, in a depth of ~ 0.5 mm, on the samples treated with the alumina sol infiltration. It is indicated that filling of nano-sol into the micro-pores between particles of green samples enhanced the sintering densification process. The infiltration treatment by nano-sol may also provide an effective way to produce structurally gradient ceramics with a gradual changing density and/or composition from the surface into bulk.

Abstract: Based on the thermodynamic and topological approach, Cu60Zr30Ti10 has been identified as the best bulk metallic glass forming composition in Cu-Zr-Ti system. Bulk metallic glass has been successfully produced using mechanical alloying of elemental blends and consolidation of the resulting glassy powders into pellets of 8 mm diameter. Dry sliding wear of glassy pellets at different annealed states showed that the relaxed metallic glass has excellent wear resistance.

Abstract: Oxidation kinetic of a Zr55Cu30Al10Ni5 bulk metallic glass (BMG) and its crystalline counterpart were studied under dry artificial air (20% of O2 and 80% of N2) at 673 K by thermogravimetry analysis (TGA) method. According to TGA profiles, the oxidation kinetic in both amorphous and crystalline states followed a protective parabolic law. However, the oxidation rates for the amorphous alloy were obviously higher than those for the crystalline alloy. Pseudo-grazing incident X-Ray diffraction (GIXRD) has been carried out to identify the oxides nature and their crystalline structure. Tetragonal-ZrO2 dominated the oxide scale formed on both alloys (BMG and crystalline) at T = 673 K; meanwhile, a slight amount of Cu was detected on the oxide surface of studied BMG alloy. The atomic diffusion mechanism was investigated using a two-stage oxidation treatment to study oxide scale growth kinetics. The studied specimens were oxidized firstly under dry artificial air and then under 18O2 isotopic tracer gas for 1.5 hours respectively at 673 K. The evident solute penetration zone and ion diffusion characteristic through the oxide scale were determined by Cs+ secondary ion mass spectrometry (SIMS) depth profile. The results showed the mechanism of the oxide layer formation of both alloys was not only due to Oxygen ions diffusion from oxide surface to interior scale, but also to an outward diffusion of Zirconium ions from substrate to oxide layer and the ZrO2 oxide growth seemed to occur at the oxide/gas interface in our studied case.

Abstract: Large-size Ni-based bulk metallic glass (BMG) composite samples exhibiting simultaneously high strength, enhanced plasticity and improved conductivity were produced by spark plasma sintering of mixed glassy powder blended with high-conductive Cu particulates. This opens new possibilities for the applications of the BMG composites as functional and structural materials.

Abstract: In this work, the Cu46Zr42-xAl7Y5Nbx alloys with different Nb contents were prepared by water-cooled copper mold casting. The effects of Nb addition on the structural and mechanical properties were studied. It was found that the structure remains in amorphous state at low Nb concentration (x =1, 2). However, some crystalline phases appear in the high Nb content alloy (x > 2), showing a composite structure, that is, the second phase particles dispersed in an amorphous matrix. The effects of Nb on the glass-forming ability of Cu46Zr42Al7Y5 were studied by differential scanning calorimetric (DSC). The compressive mechanical properties of Cu46Zr42-xAl7Y5Nbx alloys were studied. It was found that the compressive strength can be improved by introducing Nb. However, the ductile of the alloys were not improved by the introduction of Nb.

Abstract: The crystallization behavior of Cu45Zr48Al7 bulk metallic glass was studied by differential scanning calorimetry (DSC) and x-ray diffraction (XRD). Cu45Zr48Al7 bulk metallic glass exhibits two-stage crystallization in heating process. The crystallization peak temperature Tp1 and Tp2 shifted to higher temperature with increasing heating rate. It was found that the crystalline phases homogeneously nucleared at a constant rate and grew linearly at a constant rate in the supercooled liquid. Different bulk nanostructured alloys can be derived from Cu45Zr48Al7 bulk metallic glass by changing the heating rate and/or annealing temperature.