Abstract: Hybrid AZ91 Mg matrix composites reinforced with ceramic short fiber (Al2O3) and particle (Si or SiCp) were fabricated by squeeze casting method. Microstructure of hybrid Mg matrix was examined and mechanical and fracture properties were characterized. It was found that squeeze casting is effective method for the fabrication of hybrid Mg matrix composites with fairly well distributed reinforcement and near defect-free solidification microstructure. Strength and wear resistance were enhanced with hybridization of reinforcements. To investigate fracture properties of hybrid Mg matrix composites, in-situ micro-fracture experiment was performed in SEM. Crack was initiated and propagated mainly by decohesion of interfaces between Mg matrix and reinforcements. Large ceramic particles tend to fracture and thus seem to be undesirable for mechanical and fracture behavior of hybrid Mg matrix composites.
Abstract: Mg alloys have potential to use automotive parts because of their weight and castability. High temperature strength and damping capacity is important to the automotive power train parts. Mg alloy has lower creep and thermal fatique strength but has better damping capacity than Al alloy. It is known that short fiber reinforced Mg metal matrix composites(MMC) exhibits superior high temperature strength and graphite reinforced Mg MMC shows excellent damping capacity. Therefore, in this study, the effect of graphite particles(15-25%) and alborex (9Al2O3ּ2B2O3) whiskers(5-15%) on the damping behavior and mechanical properties of Mg MMC was studied. Graphite particles and alborex whiskers were chosen to increase damping capacity and high temperature strength, respectively. The Mg MMC was fabricated by squeeze casting and the total quantity of reinforcements(graphite + alborex) was maintained to 30 volume percent. The damping capacity of the metal matrix composites was increased and the flexural strength and hardness were decreased with increasing the volume fraction of graphite particles, that is, reducing the volume fraction of alborex whiskers.
Abstract: Lotus-type porous magnesium whose long cylindrical pores were aligned in one direction was fabricated by unidirectional solidification of the melt dissolving hydrogen in a pressurized hydrogen atmosphere. The sound absorption coefficient of porous magnesium whose specimen face has many open pores was measured by standing-wave method in the range up to the frequency of sound of 4 kHz. The relationship between absorption coefficient and pore structure of porous magnesium was studied. The absorption coefficient increases with decrease of the pore size, while it increases with increase of the porosity. Moreover, the peak value with high absorption coefficient is shifted toward higher frequency of sound when the thickness of the porous magnesium specimen decreases.
Abstract: Deformation mechanisms of Mg-Al-Zn (AZ31) alloys were investigated by performing tensile test at room temperature. In fine grain Mg alloys deformed at room temperature, nonbasal slip systems were found to be active as well as basal slip systems because of grain-boundary compatibility effect. Slip-induced grain-boundary sliding occurred as a complementary deformation mechanism to give rise to c-axis component of strain. With increasing grain size, the activation of the nonbasal slip systems was limited near grain boundaries. Instead of grain-boundary sliding, twinning occurred as a complementary deformation mechanism in large grained samples. Orientation analysis of twins indicated that twinning is induced by stress concentration due to the pile up of basal dislocations. The grain-size dependence on deformation mechanism was found to affect yielding behavior both microscopically and macroscopically which can influence various mechanical properties such as fatigue and creep.
Abstract: Grain refinement in AZ31 magnesium alloy has been attempted by hot-rolling and
annealing process. Specimens were solution heat treated at 673 K for 36 ks, then hot-rolled at 423 - 773 K with total reduction of 20 - 80 % by multi pass process. The rolled specimens were annealed at 473 - 673 K for 3.6 ks. Grain sizes after the solution heat treatment were about 20 to 150 µm. After hot-rolling at 573 K and annealing at 473 K, grain sizes decreased into about 5 to 10 µm. Suppression of grain growth by pinning due to precipitates was observed by transmission electron microscopic observations.
Abstract: An attempt has been made to measure the temperature dependence of dynamic Young's modulus together with the related variation of internal friction in polycrystalline copper. A mechanical spectroscopy study was used a standard servo hydraulic fatigue testing machine equipped with a scanning laser extensometer. Dynamic Young’s modulus and internal friction are measured over a temperature range of 298 to 873K at very low frequencies of 0.1, 0.05 and 0.01Hz. One internal friction peak was observed over the ranges 450K to 700K, together with marked decreases in the dynamic Young.s modulus in the same temperature ranges. From a quantitative analysis of the experimental data with the relaxation strength, relaxation time and activation energy, it is concluded that the peak phenomenon is due to grain-boundary sliding relaxation.
Abstract: The process of Direct Bonding Copper (DBC) is performed by a spinel reaction between CuO and Al2O3. In order to develop DBC on alumina substrate with high bonding strength, alumina substrate was preformed as follows: Cu was sputter-deposited on alumina substrate. Sputter-Deposited Cu (SDC) on alumina substrate was oxidized at 673K for 30min in air atmosphere and then stabilized at 1273K for 30min in N2 gas atmosphere to improve bonding strtrength between preformed alumina substrate and SDC layer. Subsequently, the Cu-foil (300µm) was bonded on preformed-alumina substrate in N2 gas atmosphere at 1342~1345K. It was found that optimum condition of DBC on preformed-alumina substrate could be successfully obtained at 1345K for 30min. Consequently, bonding strength of DBC on alumina substrate was the high value of 80N/cm. Observation and analysis of microstructure for Cu sputtered DBC showed that reaction compounds such as CuAlO2 and CuAl2O4 approved to be formed in the vicinity of interface between Cu and alumina substrate.
Abstract: In the present study, thermal properties of the electroless-deposited Cu thin film were investigated. The Cu thin film of good adhesion was successfully deposited on the TaN barrier layer by a electroless deposition method. The multilayered structure of Cu/TaN/Si was prepared by electroless-depositing the Cu thin layer on the TaN diffusion barrier which was deposited by MOCVD on the Si substrate. In order to investigate the effect of post-heat treatment the specimen was annealed in H2 reduction atmosphere. Crystallization and agglomeration of the electroless-deposited Cu film occurred through annealing at H2 atmosphere and resulted in the decrease of film resistance. Thermal stability of Cu/TaN/Si system was maintained up to the annealing temperature of 600°C in H2 atmosphere above which the intermediate compound of Cu-Si was formed through diffusion into the TaN layer
Abstract: The microstructures and electrical conductivity of newly developed Cu-Ca alloys for semi-solid forming have been investigated. High purity calcium was added to molten copper up to 1.4% by weight and mold-cast into a rod. Thermomechanical treatment (TMT) has been carried out to evaluate the variation in electrical conductivity and microstructures of Cu-Ca alloys. The electrical conductivity of copper was reduced linearly with the concentration of calcium by , where k is a constant having the values ranging from 16.7 to 20, depending on the processing condition. The introduction of prestrain significantly reduced the grain size during subsequent heating by recrystallization, influencing the electrical conductivity of Cu-Ca alloys.
Abstract: A new high strength titanium alloy system with low cost alloying elements, such as Al,
Fe, has been recently developed. In present study the expensive V was replaced with Fe, and Si was added from 0 to 7.5wt.%. The effect of Fe and Si on the microstructure and tensile properties of Ti-6Al-4Fe-xSi (x=0, 0.1, 0.25, 0.5, 0.75wt.%) alloys was investigated. The room and high temperature mechanical properties of Ti-6Al-4Fe alloys were better than those of the Ti-6Al-4V. It was mainly due to the phase boundary strengthening at ambient and high temperature. The strength and elongation of the developed alloys depended upon the Si contents. The Si elements made the grain boundary and colony size fine, and increased the strength of the developed alloys by solid solution and precipitation hardening. The tensile strength variation with the Si contents at room temperature and 400°C, and at 450°C and 500°C showed a similar behavior, respectively.