Materials Science Forum Vols. 510-511

Abstract: SiC particulates reinforced metal matrix composites (MMCs) were fabricated by two different processes, pressureless infiltration and thermal spray. 10, 20, 30, 40 and 50 vol% SiC reinforced Al matrix composites (AMCs) were fabricated by these two processes. For these AMCs, dry sliding wear tests were performed under a normal load of 3 N, a constant sliding speed of 0.2 m/s and sliding distance of 1000 m against an AISI 52100 ball. Microstructures and wear behavior were studied by means of scanning electron microscope (SEM) and electron probe micro-analysis (EPMA).

Abstract: The stability of nanosized Ni dispersions with varying concentrations of poly (1-vinyl-2- pyrrolidone) (PVP) was investigated by means of a visual inspection, transmission profile measured by Turbiscan, zeta potential, and transmission electron microscopy. For dispersions with PVP, the ζ potential values varied from positive charge to negative charge. For dispersion with a 0.04 weight percentage of PVP, the ζ potential value showed 31.71 mV, which was the highest value, and the flocculation of Ni nanoparticles was not observed. Stable dispersions of Ni nanoparticles were produced by electrostatic, steric interactions, and the formation of bridging aggregation.

Abstract: Microstructures, tensile properties, and damping capacity of the hot-rolled and annealed K1A (Mg-0.7%Zr) wrought alloy were investigated in comparison with those of a commercial AZ31 (Mg-3%Al-1%Zn-0.2%Mn) wrought alloy. The K1A alloy in hot-rolled state shows 116 and 183MPa of tensile yield strength (σYS) and ultimate tensile strength (σUTS), respectively, which are significantly higher than those (40 and 130MPa) of the K1A casting alloy. The hot-rolled K1A alloy exhibits 10.4% of specific damping index (SDI) and subsequent annealing treatment remarkably enhances its SDI up to 19.7% without sacrificing the tensile strength. Although the annealed AZ31 alloy has 3.9% of low SDI, its tensile properties are still higher than those of the K1A alloy.

Abstract: Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures up to 300oC were investigated for ZE41 (Mg-4%Zn-1%Zn)-(0~1.0)%Ca permanent mould casting alloys, based on the results from X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and tensile tests. The microstructure of ZE41 alloy is characterized by dendritic α-(Mg) grains surrounded by Mg7Zn3-based eutectic network phase. The average size of α grains decreases continuously with an increase in Ca content. The Ca-containing ZE41 alloys have Mg7Zn3-(RE)-(Ca) eutectic phase, in which Ca is distributed inhomogeneously owing to its strong segregation power. It is noteworthy that tensile yield strength (YS) for the ZE41 alloy was enhanced with increasing Ca content at all temperatures up to 300oC, which demonstrates that Ca can play a beneficial role in improving its tensile strength at room and elevated temperatures. The ZE41 alloy with 0.5%Ca showed the highest ultimate tensile strength (UTS) at room temperature, but in the range of 175 to 300oC, the higher the Ca content, the greater the UTS. The improved tensile strength of the Ca-containing ZE41 alloy would be caused by the refined α grains and higher thermal stability of the Mg7Zn3-(RE)-(Ca) eutectic phase.

Abstract: Organic-inorganic hybrid blends were prepared in variation with the ratio of sol solution to urethaneacrylic resin. Such sol solutions were synthesized from colloidal silica (CS)/ methyltrimethoxysilane(MTMS)/vinyltrimethoxysilane(VTMS) by sol-gel reaction through two step reactions. Thin films of organic-inorganic hybrid blends were prepared using spin coater on the glass and aluminum sheets. In order to understand surface properties of thin films, contact angle and roughness were measured. Surface contact angle and roughness of thin films increased as the amount of sol solution increased. Thermal dissociation temperature of thin films was observed using TGA. Thin films were stable until 270 oC. Thermal dissociation temperature of thin films improved with increasing sol solution. In order to observe insulation property of thin films, electrical resistance measurements were performed. Surface and volume resistance of thin films increased as the amount of sol solution increased.

Abstract: Colloidal silica/silane sol solutions were prepared in variation with the ratio of silane to colloidal silica. Such sol solutions were synthesized from colloidal silica/tetramethoxysilane (TMOS)/methyltrimethoxysilane(MTMS). Sol solutions were prepared by sol-gel reaction through two step reactions. To understand their physical and chemical properties, dip coating of sol solutions was performed on the glass substrates. Contact angle and thickness of coating films increased with increasing the amount of MTMS. The surface free energy of coating films decreased with increasing amount of MTMS. Coating films were stable until 550°C. Thermal degradation temperature of coating films decreased with increasing amount of MTMS. Plastic hardness decreased with increasing amount of MTMS. Elastic portion increased with increasing amount of MTMS.

Abstract: The effects of processing parameters such as addition type and Mg content on the fabrication of Al2O3p/Al composites by in-situ reaction process of molten Al were investigated in terms of infiltration behavior of molten Al and the variation of the microstructure and hardness of the composites. It was ascertained that additional Mg content is the most important parameter for the infiltration of molten Al. Furthermore, increase in additional Mg content and processing temperature resulted in a high infiltration ratio of molten Al and a high hardness value of the composites. However, these caused a greater scatter of the hardness value of the composites due to the nonuniform dispersion of the reinforcement phase, which arose from an excessive reaction of Mg.

Abstract: The effects of additional Mg content and the size of the reinforcement phase on the mechanical properties of 20 vol.% SiCp/AC8A composites fabricated by pressureless metal infiltration process were investigated. The hardness of SiCp/AC8A composites increased gradually with an increase in the additive Mg content, while the bending strength of SiCp/AC8A composites increased with an increase in additive Mg content up to 5%. However, this decreased when the level of additive Mg content was greater than 5%. The increase in hardness by additive Mg content is due to the formation of hard reaction products such as Mg2Si and AlN during the process, whereas the decrease in bending strength when Mg content was above 5% is due to the formation of coarse precipitates by excessive Mg reaction and an increase in the porosity level. The hardness and strength of the composites increased with a decrease in the size of SiC particles. It was found that the composites with smaller particles enhanced the interfacial bonding more than those with bigger particles from fractography of the composites.

Abstract: The effect of size and volume fraction of ceramic particles with sliding speed on the wear properties were investigated for metal matrix composites fabricated by a pressureless metal infiltration process. The particulate metal matrix composites exhibited about 5.5 - 6 times greater wear resistance compared with AC8A alloys at high sliding speed, and by increasing the particle size and decreasing the volume fraction the wear resistance improved. The wear resistance of the metal matrix composites and AC8A alloy represented different aspects: the wear loss of the AC8A alloy increased with sliding speed linearly, whereas, the metal matrix composites displayed more wear loss than the AC8A alloy in the slow-speed region. However, a transition point of wear loss was found in the middle-speed region, which shows the minimum wear loss. Furthermore, wear loss in the high-speed region exhibited almost the same value as the slow-speed region. In terms of wear mechanism, the metal matrix composites showed abrasive wear at a slow to high sliding speed generally. However, the AC8A alloy showed abrasive wear at low sliding speed and adhesive and melt wear at a high sliding speed.

Abstract: The effects of antimony (Sb) and strontium (Sr) additions on the microstructure of Mg-Al-Si alloys were studied. The results show that the additions of Sb and Sr can modify the Mg2Si particles from Chinese script shape to refined polygonal shape. Tensile strength and creep resistance are improved, and tensile elongation is also increased in the modified alloy. Sr modification is more effective than Sb modification of AS52 alloy for refining the microstructure and thus improving its properties.