Effects of controlled potential on fracture time, t_f, crack-initiation time, t_i and crack-propagation time, t_c in the stress corrosion cracking (SCC) of an Al-6.13 wt%Zn-2.10wt%Mg alloy were investigated by the Weibull's distribution method. Specimens were solutlon treated at 460°C for 1h, quenched into ice-water and aged at 120°C for 24h. The SCC test was performed at 33°C in 3.3%NaCl+0.2%H₂O₂ aqueous solution of pH 5.2 under controlled potential. The constant stress of 60% of the yield stress was applied. All the shape parameters (m) were larger than 1 indicating the wearing-out failure. The shape parameters for t_f and t_i decreased under the nobler potential than the rest one, while the shape parameters for t_f, t_i and t_c increased under the lower potential than the rest. The life time, η for t_f and t_i decreased under the nobler potential than the rest and increased under the lower potential than the rest. The life time for t_c was shorter than the half for t_i. The major part of the life time for t_f was that for t_i. This implies that the life time for t_f depends on that for t_i. The probability density function f(t) which corresponded to the actual failure was determined from the values of m and μ.

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Effect of heat treatment on the Charpy impact and tensile properties in a 5083 aluminum alloy was investigated at 77K. Specimens were aged at 473K for 0.5 to 100h for the Charpy impact test and for 0.5 to 500h for the tensile test after solid solution treatment at 723K for 3h. Both the initiation and propagation energies absorbed during the Charpy impact test decreased with the increase in aging time after 50h. The decrease in the absorbed energy was due to the decrease in both of the maximum impact fracture load and the displacement. The rupture energy absorbed during the tensile test decreased after aging for 100h. This was because that the fracture elongation decreased without decrease in the ultimate tensile load which was associated with the β-phase precipitation.

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The effects of Si particle size, strength of the matrix and addition of Mn and Fe on the fatigue crack propagation were investigated for high-purity Al-Si casting alloys. The fine distribution of Si particles resulted in the retardation of the crack propagation rate (in the da/dN-ΔK curves) in an Al-12%Si alloy. In Al-7%Si and Al-Si-Mg alloys the retardation of crack propagation rate was pronounced in a high stress intensity range when the strength of the matrix was high. At the threshold region, however, the crack propagation rate was retarded in the decreased matrix strength due to the crack closure effect. The fatigue crack propagation rate (da/dN) in a high stress intensity range was retarded by the addition of Mn and Fe in an Al-Si-Mg alloy.

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Fracture toughness, S-N curve and fatigue crack propagation behavior in melt-forged aluminum casting alloys such as AC4C-T6 and AC8A-T6 were investigated at room temperature and 423K, and were compared with these in a commercial 6061-T6 wrought alloy. The static or dynamic fracture toughness, J_IC or J_d for AC4C-T6 and AC8A-T6 alloys was lower than that for a 6061-T6 alloy. The S-N curves for AC4C-T6 and AC8A-T6 alloys were situated lower than that for a 6061-T6 alloy. This was because of a pre-existed surface flaw effect by coarse Si particles in the casting alloys. The fatigue crack propagation in AC4C-T6 and AC8A-T6 was unstable in a high stress intensity range, because a large number of fracture-initiation sites such as Si and compounds were distributed in these alloys comparing with a 6061-T6 alloy. On the contrary, the value of ΔK_th increased in a lower stress intensity range because Si particles and compounds acted as obstacles against crack propagation.

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In order to develop a method for predicting the variation in material properties during warm and hot forging of a 5083 aluminum alloy, the hardness distribution and microstructure were investigated. The distribution of equivalent plastic strain was calculated using a forming simulation of an elastoplastic FEM analysis. The relationships between hardness, microstructure and equivalent plastic strain were obtained for different forming temperatures. The variation in material properties was predicted through the analysis of equivalent plastic strain using computer simulation.

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The purification behavior in liquid aluminum alloys containing impurities of Mn, Fe and Si was investigated by the segregation process of impurity elements. The distribution of impurity was examined as a function of the solidified length after liquid aluminum alloys were unidirectionally solidified. For an aluminum containing Mn the concentration was the same as the initial one and no segregation of Mn was observed. Thus, the purification of an aluminum containing Mn was difficult by this method. For an aluminum containing Fe the distribution of Fe depended on the initial concentration. This behavior was explained by the shape change in the solid-liquid interface. For an aluminum containing Si the concentration was lower than the initial one. This indicated that the segregation process was effective to remove Si from the liquid aluminum. The purification degree for the aluminum alloys depended on the solidification condition. This was explained in terms of the equilibrium distribution coefficient and solid-liquid interface shape. The purification of an aluminum containing 0.014 wt%Fe or 0.107 wt%Si proceeded in accordance with the theoretical solidification curve when unidirectionally solidified with mechanical stirring.

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Properties and polarization behavior in a 0.5N NaCl solution were investigated for HIP (hot isostatic pressing) treated Al-Zn-Mg P/M (powder metallurgy) alloys as well as an I/M (ingot metallurgy) alloy. The density of HIP-treated P/M alloys was close to the ideal value. The pitting potential of HIP-treated P/M alloys was nobler than that of an I/M alloy of the same composition. The passivation current density of HIP-treated P/M alloys was smaller than that of an I/M alloy. The pitting potential of P/M alloys became nobler and the passivation current density was reduced by the HIP treatment.

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Lubricated wear of extruded 5052 and 6061 aluminum alloys to Al-Si alloy castings containing Si up to the eutectic point increased with the increase in Si in Al-Si alloys. The wear resistance of a 6061 alloy which contained large amounts of inclusion particles was superior to that of a 5052 alloy. This was because that the inclusion particles effectively acted to prevent the adhesive wear. Hyper-eutectic Al-Si alloy castings resulted in the increased wear to both of the extruded alloys due to the abrasion wear. However, the wear loss in the hyper-eutectic Al-Si alloy castings was small. In this case, the wear resistance of a 5052 alloy was superior to that of a 6061 alloy. This was because of an enhancement of the abrasion wear in a 6061 alloy by the inclusion particles. An addition of Cu to the hypo-eutectic silumin resulted in the unfavorable effect on the wear of both extruded alloys at high sliding loads. An addition of about 0.4%Mg gave a preferable effect. More addition of Mg, however, resulted in the increased wear for both extruded alloys.

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