The ductility of Al-5wt%Mg alloy at elevated temperatures which was affected by a trace addition of Y, Ce, Sm and Be or by melting atmospheres was examined. Tensile tests in vacuum were made in a temperature range from 200°C to 400°C at a strain rate 8.3×10^-4s^-1, where the grain size of test pieces was controlled to be 0.35mm. Hightemperature embrittlement was clearly observed at about 300°C in the Al-Mg binary alloy prepared by melting in air in accord with the previous papers. However, the embrittlement almost disappeared by the addition of Y, Ce, Sm and Be when the alloy was prepared by melting in air. The phenomenon was attributed to the fact that formation and growth of cavities on grain boundaries were effectively checked, because strain-induced boundary migration easily occurred during tensile tests in those alloys. It was suggested that the high-temperature embrittlement so far reported was related to certain gas impurity atoms introduced in the course of melting in air. The impurity atoms would prevent the boundary migration. The additives would have a strong interaction with the impurity atoms above described.

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Dynamic superplasticity is attributed not only to the transformation but also to the recovery process. The latter, recovery dynamic superplasticity, behaves in the same way as the transformation superplasticity. A large strain increasing up to several percent is induced in one cycle. The applied stress and induced strain are related linearly with each other. A straight line showing the above relation passes the original point, and the strain rate sensibility index m is congruent to 1. The dynamic plasticity is observed in the transformation process and also generated by applying a stress during the state change of the material.

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RI-ITMT method was applied to improve the toughness of Al-Zn-Mg-Cu alloys. This method is applicable most effectively to the Zr bearing Al-Zn-Mg-Cu alloys, but insufficiently to the Cr bearing alloys. Reduction of impurities Fe and Si is not so effective for toughening the alloys. The size of recrystallized grains in the alloys to which RI-ITMT method has been applied is extremely finer than that of the alloys to which a conventional method has been applied. A relationship lies between transgranular dimple fractured surface ratio and toughness, where transgranular dimple fractured surface expands with toughness. The ratio of this fracture surface type increases with fine transgranular precipitates, because they prevent the formation of rough slip bands in the grain.

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A relationship between fracture toughness or critical crack tip opening displacement (CTODc) and Fe- and Si-rich insoluble inclusions in 7000 type alloys was studied. The magnitude of crack tip plastic blunting or fracture toughness is determined by forming characteristic dimples just in advance of the stretched zone. The dimples on fracture surfaces are classified into two types of populations by scanning electron micrography and energy dispersion spectroscopy analysis. One population includes coarse dimples 10μm in average size associated with coarse in clusions containing Fe or Si. The other includes smaller dimples less than 1μm in diameter having Cr-rich dispersoid particles. The average size of coarse dimples is in good agreement with the average spacing of inclusions 1μm or more in size. The growing stretched zone never exceed the average spacing of these inclusions. The characteristic dimples effective to determine CTODc are formed by the inclusions containing Fe or Si.

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Application of Matthiessen's rule to resistivity measurement was examined. An induced empirical relation between resistivity and resistance ratio is accurate enough for measurement of the resistivity of A1050 rolled sheets. The resistivity change with various heat treatments leads to the following facts. Some parts of Si in the alloy precipitate during cold rolling as elemental Si and the amount of the precipitates increases by annealing below 250°C. Dissolution of precipitated Si completes at 400°C. Most parts of Fe in the alloy have precipitated. Fe further precipitates by annealing below 400°C after cold rolling. The precipitates dissolve by annealing above 400°C up to the equilibrium solubility in Al-Fe binary diagram. Precipitation of Fe and Si is accelerated by cold rolling, The equilibrium solubility of Fe lowers with increasing Si content. Activation energy for a resistivity decrease with isothermal aging above 400°C agrees with that of diffusion of Fe in Al. Impurities other than Fe and Si are always in solution comparing chemical compositions and solid solubilities of the impurities.

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