Degree of microsegregation in Al-0.21%Ti, Al-0.25%V and Al-0.68%Cr alloys cast into a permanent mold is extremly high, and Ti, V and Cr-enriched cored structures form in Al matrix. Ti concentration inside of the cored structure is 3.6 times as large as the initial Ti content, V concentration is 2.2 times the initial V content and Cr concentration is 1.2 times the initial Cr content, and their concentrations outside of the cored structure are 0%Ti, 0%V and 0.43%Cr respectively. The degree of microsegregation unchanges by heavy rolling and the cored structure remains distinctly. These cored structures are homogenized very slowly at high heat treatment temperature 650°C, and require more than 3 weeks for complete homogenization in Al-Ti and Al-V alloys. The interdiffusion coefficients for Al-Ti, Al-V and Al-Cr alloys were measured in concentration profiles of Ti, V and Cr during homogenization by means of Matano's method.

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Unidirectionally solidified Al-Al₃Ni eutectic alloy was diffusion welded using various insert-metals. The weldments made with commercially pure copper foil (5μm in thickness) at 630°C for 30min under 6.5kg/cm² in welding pressure show re-solidified Al₃Ni phase in the microstructure at the joint interface, and have the tensile strength equivalent to that of the base metal. The weldments made with 2024 alloy foil (100μm in thickness) at 600°C for 30min under 6.5kg/cm² in welding pressure have 72% of tensile strength and 81% of Charpy impact value to those of the base metal, and show little Al₃Ni phase at the joint interface, but coasening of the phase occurs with an increase in welding temperature. In both cases, partially melted insert-metals play an important role in diffusion welding processes.

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Al-4.5%Mg alloys containing Mn 0-0.5% and Cr 0-0.3% were unidirectionally solidified. Test specimens were cut out from the solidified alloys in transverse (T) and longitudinal direction (L) to the solidified direction and were fatigue cracked. They were tested by the instrumented Charpy impact testing machine at -196°C to room temperature. The fracture toughness of T test speciments lowers with temperature but does not in alloys free from Mn and Cr. That of L test specimens unchanged with temperature. Fine precipitates formed by heating for 24h at 550°C decrease slightly the fracture toughness.

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The stress corrosion resistance of commercial 7075, 7175 and 7475 alloys plates 60mm in thickness was investigated. The resistance of these specimens strongly depends on the orientation of specimen, T-L specimens have the resistance highly superior to that of S-L specimens. The overaging treatment (T7) is extremely effective for improvement of the resistance of these alloys. The residual stress removing treatment (TX51) has a considerable effect to improve the resistance of S-L specimens, but has little effect to do the resistance of T-L specimens. S-L specimens show intergranular stress corrosion cracking, while T-L specimens show ductile fracture with many laminated cracks on intergranular stress corrosion cracks. Stress corrosion cracks are attended by a hydrogen embrittled zone pre-existing ahead of the crack. Nucleation and propagation of the crack take place under an assistance of hydrogen embrittlement.

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Effect of alloy purity and aging on the fracture toughness of three type alloys has been examined to clear the relationship between the fracture toughness and tensile properties. The fracture toughness were evaluated by the critical crack tip opening displacement (CTOD_c) derived from stretched zone analysis. The general magnitud of CTOD_c increases with reduction in iron and silicon contents. The value of CTOD_c depends on the aging condition. It lowers as the yield strength rises, or the value of the strain-hardening exponent decreases. All underaged alloys show a superior to toughness to the overaged of ones having the same yield strength. The total impurity content is, accordingly, a dominant fractor governing the fracture toughness of these alloys. The final values of fracture toughness were also determined by the tensile parameters, yield strength and strain-hardening exponent.

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The toughness of Al-Zn-Mg was a function of dendrite arm spacing and concentration of nonequilibrium secondphase. The primary and secondary dendrite arm spacing becomes smaller gradually according to an increase in cooling rate, and the toughness value becomes higher. When these materials are solution treated the toughness value increases by prolonging solution time. It depends on the solid solution of the nonequilibrium second-phase. In this case, longer solution time is necessary to obtain the maximum toughness when the initial concentration is higher. The highest toughness is achieved within short solution time when the secondary arm spacing is smaller. If the grain boundary lies at preference site of crack initiation and is propagation, the fracture toughness depends on the secondary arm spacing and is proportional to the minus one-half power of its spacing. The fracture toughness is indicated as a function of nonequilibirium second-phase particles volume fraction, and is proportional to the minus one-seventh power.

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