발간논문

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Variation of damping capacity with cooling rate and aging condition was investigated in Al-40% Zn, Al-60% Zn and Al-78% Zn alloys. Additional investigation for structural changes due to different cooling rate was made in those alloys. The Al-Zn alloys were found to show constant values in damping capacity regardless of maximum surface strain, The damping capacities of the Al-40% Zn and Al-60% Zn alloys were increased with decreasing cooling rate. This may be due to increase in βphase precipitation in accordance with decrease in cooling rate. However, the Al -78% Zn alloy showed high damping capacity with increasing cooling rate. This was because at the rapid cooling rate the two phases(α+β) were decomposed into homogeneous and fine grain structure by spinodal decomposition, while at the slow cooling rate the coarse lamella structure was formed by discontinuous precipitation. The damping capacities of the water-quenched Al-40% Zn, Al-60% Zn and Al-78% Zn alloys were increased with aging time at i50℃ and 250℃. However, the damping capacity of the Al-60% Zn alloy was decreased on further aging above critical times at 150℃ and 250℃, because coarsening in the lamellar structure of the alloy occurred on aging above the critical time at each temperature.

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Metal matrix composites(MMC) reinforced on Al₂O₃ fibre are gaining increasing attention for structural application. However, the data base concerning its mechanical properties and microstructural characterization remains limited. Al₂O₃ fibre(FP-fibre from Dupont) reinforced Al -2.5Li composite has been investigated in the present study. Continuous fibre Al₂O₃/Al-2.5Li composites with the 0°and 90° direction were used for the investigation. Processing of the material was performed by vacuum melt infiltration. Tensile and compressive tests were performed for mechanical characterization. In order to get sufficiently good results, a special MMC related test equipment was designed. The best results were achieved when cylindrical hourglass shaped specimens were used. Fatigue testing of the metal matrix composites showed that a pronounced improvement of the fatigue behaviour could be achieved with the fibre reinforcement. Transmission electron microscopy was performed in order to study the fibre/matrix interface. Li addition increased the wettability of the fibre in the matrix, because of the formation of two compounds in the interfaces, namely, α-LiAlO₂ and LiAl_5O_8 spinel. Precipitation free areas around the fibres containing any δ`particles, which in turn are present in the matrix, were found.

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