Microstructural changes during superplastic deformation at 573K have been investigated of a Mg-8 mass%Li eutectic alloy under both cast and thermomechanically processed conditions. The elongation of about 300% is obtained for ascast specimen under the superplastic condition, which is increased by finer grain-structure developed by more rapid solidification. Needle like α phase is observed to change to an equiaxed shape during superplastic deformation, and the mvalue of 0.2 at the initial period correspondingly increases during the deformation. ln the specimen of the recrystallized structure, twinning is observed in the α phase at the initial stage of superplastic deformation. The α grains are found to grow during the deformation, while the refining of β grains takes place. As a result, the mean grain size is slightly increased during deformation. These microstructural changes will play an important role to exhibit superplasticity.

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Modification of Al-22mass%Si and Al-22mass%Si-2.5mass%Mg alloys was examined by combined use of the Duplex Casting process and addition of Na and P. In the Duplex Casting process, two kinds of alloy melts with different compositions were cast in the mold at a given interval. In this study, the combinations of Na-treated Al-12mass%Si alloy or Na-treated Al-12mass%Si-5mass%Mg alloy as the first melt and P-treated Al-32mass%Si alloy as the second melt were used. In the conventional casting, simultaneous addition of Na and P tends to coarsen the primary silicon crystal. On the other hand, refined primary silicon crystals and modified eutectic silicon crystals are observed in the ingots obtained by combined use of the Duplex Casting process and addition of Na and P. In the vicinity of the side surface of the ingots, markedly refined primary silicon crystals are observed, which show the synergistic effect of the Duplex Casting process and P addition to the second melt. Origin of "Primary Silicon Free Zone" in the ingots is the first melt incompletely mixed. Acceleration of circulation flow of the second melt in the mold alleviates the macrosegregation of primary silicon.

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A new technique for making particulate composites has been investigated. Unstable oxides such as CuO, SnO₂ ZnO and Cr₂O₃ whose absolute value of standard free energy of formation is smaller than that of Al₂O₃ were added to moleton aluminum and Al-3.0%Mg alloy. Al₂O₃ particles are formed in Al-Cu matrix by adding CuO to moleton aluminum at 1273K. Furthermore, by adding CuO the moleton Al-3.0%Mg alloy, MgAl₂O₄ (spinel) particles are formed. No particles are found in the pure aluminum matrix by adding either SnO₂ Or ZnO because of the poor wettability between these oxides and moleton aluminum. However, in case of Al-3.0%Mg alloy matrix, MgAl₂O₃ particles are formed by adding these oxides because the wettability is improved by presence of magnesium. On the other hand, by addition of Cr₂O₃, whose absolute value offree energy of formation is relatively large and close to that of Al₂O₃, in situ particles are formed in neither pure aluminum nor Al-3.0%Mg alloy matrix. The size and shape of particles formed by the in situ reaction depend on temperature of molten metal. The size of in situ particles increases and the shape becomes complicatedly with increase in the temperature of reaction. Although oxide particles formed in this experiment are small and some of them are under 1μm in diameter, macroscopic distribution is not uniform.

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A fabrication process employing hot pressing to produce hybrid composites was established. Both tensile strength and wear resistance of the fabricated hybrid composites are found to be superior. The obtained tensile strength of the hybrid composites agrees with that calculated using the rule of mixture, and the abrasive wear resistance of the hybrid composites is improved by dispersing SiC particles in the aluminum alloy and conducting a heat treatment (T6).

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The serration behavior of solution treated Al-Mg-Si alloys containing Cu up to 0.6% has been investigated in relation to precipitates. The strong serration is observed on each alloy at an early stage of room temperature aging and on fully annealed excess Mg alloys containing Cu. However, no serration can be observed on artificially aged specimens. It is thought that the serration is caused by dynamic segregation of solute atoms on moving dislocations and appearance of the serration seems to reduce with decrease in the amount of solute Mg and Cu atoms during aging. The results of TEM observation and EDX analysis indicate that the composition of precipitates of fully annealed specimens depends on Mg, Si and Cu contents. Q-phase precipitates are observed on excess Si alloys containing Cu. But in excess Mg alloys which show strong serration, Cu content of precipitates is less than Q-phase. Serration behavior of annealed specimens may be related to the solute Cu content.

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The wettability of SiC fibres to molten aluminum was investigated by a dip-coverage method, and was compared with previous results of that of SiC block. At higher temperatures around 1273K, the coverage ratio of SiC fibres by pure aluminum is less than that of SiC block since the rate constant of overall reaction k₀ is distinctiy lower and the incubation time τ is longer. However, alloying of Ti, Hf and V, increases k₀ and shortens τ for SiC fibres. In this case, those values are as much as the level of SiC block. These adverse results can be explained by considering Wenzel's equation which expresses the effect of roughness of solid/liquid interface on the wettability. At a lower temperature of 1023K, addition of Ca or Ca with Mg improves the wettability between SiC block and liquid aluminum. When SiC short fibres were incorporated into Al-Mg-Ca alloy at 1073K, the distribuition of SiC short fibres was more uniform than that in Al-Mg alloy. Hence, the simultaneous addition of Ca and Mg obviously contributes to the good wettability of SiC fibres to liquid aluminum, too.

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Wear resistance of an Al-Mg-Si alloy is effectively improved by adding approximately 4%Cu and 1.2%Mn. Wear resistance of high concentrated solid solution of Al-Cu-Mg-Si and Al-Cu-Mn-Mg-Si alloys is better than that of low concentration. Primary inclusion is preferable for wear resistance of thease alloys. An addition of approximately 4%Cu and 1.2%Mn to a hypereutectic Al-Si-Mg alloy is also effective to improve the wear resistance. Wear resistance of an Al-Si-Cu-Mn-Mg alloy is significant influenced rather by the grain size of primary silicon than by silicon content. Wear resistance of this alloy increases with decreasing grain size of primary silicon, but it begins to decrease beyond the size of 3.3μm.

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