Advanced Materials Research Vols. 97-101

Abstract: Both ZnO nanocomb and nanotetrapod structures have been synthesized via a simple chemical vapor deposition process in a horizontal tube furnace. The morphology of the products is affected by the distance between the source material and the deposition substrate. The formation of nanocombs has been attributed to the self-catalysis effect of polar surface. The octahedral multiple-twin model is responsible for the formation of tetrapods and tripod-like arms at the end of nanowires. The evolutive process of them has been given out in detail and the results provided valuable models in understanding crystal growth mechanisms in nanometer building-blocks and to further grow net nanostructures. And their photoluminescence properties also have been discussed.

Abstract: The rotating bending fatigue tests have been carried out to investigate the fatigue limit of specimen with double-notch which is constructed by the step and the blind hole, and the effect of stress concentrations at the double-notched bottoms on the fatigue limits are analyzed using three-dimensional elastic finite element method (3D-EFEM). There are fatigue limits of 17 groups on the single-notched specimen and double-notched specimen obtained in the fatigue tests. The stress field interactions are discussed between the stress field by the blind hole notch and the stress field by step using 3D-EFEM. The results obtained in this study are as follows: The fatigue limit of the double-notch specimen is insensitive to the distance between the blind hole and step for the low carbon structure steel with better ductility; while, for the high-strength steel, superposition and intensification of the stress concentration by the blind hole and step mutually may be avoided, and their adverse effects on the fatigue strength may be become to minimal, when taken appropriate distance between the blind hole and step. The results are significant for engineering design of the multi-notched parts, and studying fatigue strength theory.

Abstract: Directional solidification experiments on Pb-Bi peritectic alloys have been conducted at very low velocity (V=0.5 μm/s) and high thermal gradient (G=25 K/mm). Incomplete banded and oscillatory structures have been observed in both of hypoperitectic and hyperperitectic compositions over several millimeters of growth. These structures resulted from the repeated nucleation and competition between properitectic α- and peritectic β-phases. The banded or oscillatory structures are found to be transient and the final steady-state phase was only the peritectic β-phases. With an increase in composition, β phase formed and α phase disappeared at a lower solidified distance. Composition variations in the banded structure are measured to determine the solute distribution along the growth direction.

Abstract: The effect of increasing the amount of added grain refiner and combination electromagnetic filed on grain size and morphology has been investigated by die casting of commercial aluminum. The results show the grain size and the grain morphology both are transited with the increasing of the addition of Al-5Ti-B. The grain size continuously decreased with increasing the addition of grain refiner. Without grain refiner additions, an obvious transition in grain size and morphology is observed with the application of low frequency electromagnetic field (LFES). The efficiency in grain refinement of LFES decreases with increasing the amount of grain refiner. However, even when the addition of grain refiner is 2 wt. % Al-5Ti-B, 63 % grain size reduction still is observed by the affect of LFES. This result shows that the LFES can enhance the efficiency in grain refinement of grain refiner.

Abstract: Influences of additives on the physicochemical characteristics and technologies of binder are researched, and a new compound phosphate heat-cured foundry binder coded B8M15 is developed. The binder is made from environmental friendly inorganic materials such as phosphoric acid, aluminum hydroxide, B-ion additive and Mg-ion additive. Laboratory results indicate that combined effect of B-ion additive and Mg-ion additive can improve the storage stability of the binder and its sand mold or core, as well as they can increase the dry strength of its sand mold or core from 0.77MPa to 2.51MPa. This binder has low gas evolution and good collapsibility.

Abstract: A wear corrosion resistance high-Cr cast iron was introduced in this paper, and the cast forming technique of 150ZGB impeller was also presented. The produced impellers, according to the technique, met the quality demands, such as no crack, surface smoothness, and high hardness. The service life of the impellers, in phosphoric acid slurry condition, were 4 to 6 times as long as that made from UB6 (0Cr20Ni25Mo5).

Abstract: A new technique of introducing gas bubbles during isothermal period of AZ31 magnesium to prepare semi-solid slurry has been reported. The effect of resting time after stirring on the microstructure of the slurry is discussed. Results show that the dendrites and liquid phase reduced, the non-dendritic grains grew up and became spheroidized. A fully grain-refined structure can be obtained with 60s resting time.

Abstract: 7050 aluminum alloy ingots were produced by low frequency electromagnetic casting (LFEC) and direct chill casting (DC) respectively. As-cast microstructures and homogenization behaviors of LFEC and DC ingots were investigated experimentally. The optical microscope and DSC analyses shown that the grain size of LFEC ingot was finer and the content of constituents and eutectic structure was less than that of DC ingot. Accordingly, the homogenization behaviors of the LFEC and DC ingots were significantly different. The remnant constituents of LFEC ingot were less in content and smaller in size than that of DC ingot after homogenization at 480°C for various lengths of time. Similar to the dissolving of constituents, the LFEC ingot exhibited faster diffusion kinetics of alloying elements from grain boundary to inner. The concentrations of Cu, Mg and Zn inside grain of LFEC ingot after homogenization for 12 h were 2.4%, 2.2% and 6.5% respectively, but the DC ingots had not reach the level even for 48 h.

Abstract: Assuming phenomenological thermal flow balance and solute conservation, a numerical model was proposed for recalescence in bulk undercooled alloy. During the calculation, the evolution of residual melt temperature and concentration is solved as function of the transformed solid fraction. After quantitative thermodynamic calculations, a transition from non-equilibrium to equilibrium process can be described by the change of liquid/solid Gibbs energy difference. The proposed model has been applied to Cu-Ni solid solution and Fe-B hypereutectic alloy. The predicted post-recalescence temperature TR agrees well with the experimental results.

Abstract: The feasibility of especial technology which complex cavity aluminum alloy castings are formed by corroding pre-burying copper pipe core is discussed by researching on the size and microstructure of cavity formed by corroding pre-burying copper pipe core. The results indicate the size of cavity formed by corroding pre-burying copper pipe core is basically consistent with the original size of the castings, with a dimensional tolerance in permission scope which is about 0.01mm. Meanwhile, the microstructure of cavity wall formed by corroding pre-burying copper pipe core is identical with that of the castings without corrosion. Moreover, because the pre-burying copper pipe core is equal to the chilled iron, bringing cold effect, the crystal grain of cavity wall formed by corroding pre-burying copper pipe core are tinier. Therefore, the technology of using pre-burying copper core to form complex cavity of aluminum alloy casting is entirely feasible.