On studying the machinability of Al-Si alloys, a difference in the cutting mechanism was found between a commercial alloy and a high purity alloy. As the reason, it was considered that some impurities contained in the commercial alloy had an influence on the cutting mechanism. In this paper, Mg, Mn, Zn and Fe were chosen as additional elements and their effects on the machinability were examined.
When Mg was added, the cutting resistance decreased and surface roughness was improved in high speed cutting. Mg addition was thus found to be effective in improving the machinability of Al-Si alloys. Mn addition was not so beneficial as Mg addition, but had some effect on the machinability. Zn addition showed no effect. When Fe addition was less than 0.9%, it showed little effect.

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A study was made on the formation of 'fir-tree' structure during D. C. casting of Al-Fe alloys. Al-Fe alloys containing 0.58%Fe and less than 0.01%Si were unidirectionally solidified. The alloys were also D. C. cast. The cooling rate during unidirectional solidification was measured by thermal analysis, while the cooling rate during D. C. casting was estimated through measurements of the dendrite arm spacing of the ingots. Identification of constituent phases were carried out by optional microscopy, electron microscopy and X-ray diffraction, and some of their chemical properties were discussed.
The results obtained are summarized as follows:
1) Al₃Fe, Al₆Fe and Al_mFe (a body-centered tetragonal Al-Fe phase⁸⁾) were identified as secondary constituents. They were found to be formed in the following ranges of cooling rate:
Al₃Fe: less than 2°C/sec
Al₆Fe: from 2 to 20°C/sec
b.c.t.Al_mFe: more than 20°C/sec.
2) Al₃Fe and b.c.t. Al_mFe were oxidized during anodizing in 15%H₂SO₄ solution, while Al₆Fe did not undergo oxidation. Al₃Fe and b.c.t. Al_mFe were etched to appear dark-brown by 0.5%HF solution, while in the case of Al₆Fe, only a slightly grey tint was detected.
3) The microstructure of the D. C. cast ingots was divided into the four regions corresponding to different sizes of dendrite cells and to different constituent phases. The first, these cond and the third regions were outside the 'fir-tree' structure, while the fourth region was inside. Body-centered tetragonal Al_mFe and a small amount of Al₃Fe were secondary constituents in the first region which solidified very rapidly. Al₃Fe phase existed in the second and third regions which solidified more slowly. Al₆Fe phase was an interdendritic phase in the fourth region which solidified fairly fast.
4) The cause of formation of 'fir-tree' structure seems to be attributable to the fact that the cooling rate during solidification varies from the surface to the center of D. C. cast ingots. In other words, the cause is likely attributed to macroscopic segregation of different types of intermetallic compounds.

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Stress corrosion cracking of Al-Zn-Mg alloys was widely studied and many theories have been proposed.
The purpose of this paper was to grasp a more fundamental behavior of stress corrosion cracking, and so the experiments were carried out on the environment-sensitive mechanical behavior of Al-Zn-Mg alloy single crystals.
The results obtained are summarized as follows.:
1) The mechanical properties of Al-Zn-Mg alloy single crystals in the tensile test were affected sensitively by the environment. Significant embrittlement was found in the corrosive medium to cause stress corrosion cracking.
2) The degree of embrittlement was affected by the strain rate in the tensile test. The failure occurred in the tensile test with a slow strain rate was similar to that caused by stress corrosion cracking.
3) Al-Zn-Mg alloy single crystals containing coherent G. P. zones showed remarkable embrittlement.
4) Embrittlement of Al-Zn-Mg alloy single crystals might be caused by the reduction of surface energy. This reduction might be due to adsorption of a certain ion, e. g. Cl- ion, or hydrogen in the corrosive medium.

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The purpose of this paper was to investigate stress corrosion cracking of Al-4.2%Zn-1.4%Mg-0.32%Mn alloy single and bicrystals. The single and bicrystals investigated were prepared by strain annealing.
The results obtained are summarized as follows.
1) In single crystals, stress corrosion cracking occurred in the same way as polycrystals. In stress corrosion cracking tests, the relation between applied stress and time to failure for single crystals was similar to that for polycrystals.
2) In single crystals, stress corrosion cracking occurred along (111) slip plane, i.e. cracks initiated and propagated along the slip band.
3) Stress corrosion cracking in bicrystals did not always occur at the grain boundary. Only in the bicrystals in which the angle between the grain boundary and the tensile axis was 45°, stress corrosion cracking occurred at the grain boundary.
4) The resistance to stress corrosion cracking of bicrystals was not affected by the angle between the grain boundary and the tensile axis. It might be caused by the reason that the bicrystals used had small angle grain boundaries.

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