Effects of such forging conditions as forging temperature, reduction ratio, forging numbers and metal flow of forging stocks on microstructure and mechanical properties of ZK 60A magnesium alloy were studied. The forging conditions have remarkable effects on the metal flow and mechanical properties in different manners in the longitudinal and long transverse directions. Forging at temperatures 350° to 400°C and 250°C ensure excellent mechanical properties in the longitudinal and long transverse directions respectively. Multistage forging, which includes the first stage forging at 350° to 400°C and the second stage forging at 250°C is recommended to ensure excellent mechanical properties both in longitudinal and long transverse directions.

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Recrystallized structures of 7xxx series aluminum alloys were examined paying attention to the difference of the effects between Zr and Cr additions. The Al-5.7%Zn-2.5%Mg-1.5%Cu alloys with 0.1%Zr or 0.2%Cr were mainly used as specimens. They were hot-rolled, solution-treated, cold-rolled and then subjected to recrystallization-heat-treatment (RHT) at temperatures ranging from 673 to 798 K. Growth rate of sub-grains at an early stage of RHT is greater in Zr-bearing alloy than in Cr-bearing alloy. Recrystallization occuring successively in the region having sub-grain structures is generally suppressed in Zr-bearing alloy, leading to the decreased fraction of recrystallized area. The fraction of recrystallized area and the recrystallized grain size become larger in Zr-bearing alloy, when RHT temperature increases. Because, coarsening of Al₃Zr particles is caused. The grain size in longitudinal direction in Cr-bearing alloy decreses at elevated RHT temperatures. These results of microstructural observations are almost the same in commercially available 7050 and 7075 alloys.

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The passivation behavior of high purity aluminum and the property of surface film in deaerated acidic distilled water were studied using a potentiostat. The anodic polarization curves show that the limiting current density is independent of pH and the concentration of Cl^-. The measurement of half-time of potential decay shows that the most stable film is formed at -600 mV. The time variation of current density shows that the film growth is resulted from migration and diffusion of Al³⁺ through the film. The activation energy of anodic current at -1250 mV is approximately 9 kcal/mol and is approximately 1.2 to 1.7 kcal/mol at potentials in a limiting current region. These low values of activation energy in the limiting current region are explained. Both the stabilization of film by polymerization and dehydration and the dissolution rate of film are accerelated at elevated temperatures. These two processes reversely contribute to the limiting current density.

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An aluminum alloy 2017 aged at 180°C for 4 h was tested. The relation between applied anodic current density and time to failure shows that the alloy to which anodic current more than 5 mA/cm² is applied fails in a shorter time than that anodic current less than 5 mA/cm² is applied. This is a characteristic phenomenon of the alloy having precipitates at grain boundaries and is caused by a difference in cracking form. The time to failure is not so susceptible to the magnitude of applied stress. Local anodic dissolution is a main controlling factor of stress corrosion cracking in the alloy.

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The growth rate of stress corrosion cracks, size of hydrogen embrittled zone (HEZ) and width of stretched zone (SZW) in the area ahead of SCC region through an overload test after SCC were measured for Al-8%Mg alloy aged at 200°C for 1 h. K_ISCC is 67 kg/mm^3/2. HEZ size is nearly constant at about 4 mm in the regions I and II of da/dt-K_I curve and rapidly reduces in the region III with an increase in stress intensity. SZW is also nearly constant at about 15 μm in the regions I and II, and rapidly expands in the region III with an increase in stress intensity. Dissolution of hydrogen in the alloy leads to a decrease in mean dimple size. The cause of these behaviors is discussed basing on the hydrogen effect on nucleation for microvoids.

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