Stress corrosion cracking of Al-Mg alloys, particularly the relation between the size of the grain boundary precipitates and the time to failure was investigated by the tensile test, electron micrographic observations and electrochemical methods. Stress corrosion of specimens containing 7 to 9wt%Mg were tested in 1 mol NaCl+0.9%H₂O₂ solution, after aged at a temperature below 200°C for less than 100min. The results obtained are summarized as follows:
(1) In the electron micrographs of the specimen aged at a temperature below 200°C for less than 100min., only grain boundary precipitates were observed. As aging proceeded, their growth was observed.
(2) For an Al-9.0wt%Mg alloy aged at 200°C less than 100min., the larger the grain boundary precipitates the shorter was the time to the stress corrosion failure.
(3) The corrosion potentials of Al-Mg alloys became less-noble with the growth of the grain boundary precipitates, e. g. the corrosion potential of an Al-9.0wt%Mg alloy quenched from 430°C into iced water was -0.55V (vs. N. H. E.) and that of a specimen aged at 200°C for 100min. was-0.76V.
(4) The corrosion potential of the stressed speciman of an Al-9.0wt%Mg alloy was independent of the size of the grain boundary precipitates. Its value was between -0.51V and -0.54V (vs. N. H. E.).
(5) The anodic and cathodic current densities of an Al-7.0wt%Mg alloy aged at 200°C for 10min. increased in proportion to the applied stress at a constant over voltage.

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It is well known that non-metallic compounds in aluminum and its alloys have important effects on the properties of cast or wrought products. Accordingly, it is very important for industry to investigate the non-metallic compounds. Nevertheless, the observation methods have been rarely studied and it has been very difficult until now to examine the non-metallic compounds.
We made experiments on the observation methods of non-metallic compounds. Results are as follows:
1) Large oxide films revealed on the observation plane by fracture or by etching with Br₂-CH₃OH were dyed and then distinctly observed.
2) Oxide inclusions on the polished plane which were dyed after ultrasonic washing were distinctly observed through a microscope equipped with an oblique xenon lamp.
3) Various non-metallic compounds were distinctly observed and classified by the secondary electron image and characteristic X-ray image obtained by a scanning electron microscope equipped with X-ray micro-analyzer.

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The morphological transition from planar to cellular interface in Al-Ti alloys occurs at a constant value of C₀•R/G and the cell-dendrite transition occurs at a constant value of C₀•R^1/2/G, where C₀ is the initial solute concentration, R growth rate and G temperature gradient. As the planar interface breaks down to form depressions, a rod-like cored substructure is formed in each cell when the interfacial morphology changes successively from depressions to irregular cells, elongated cells and hexagonal cells. The rod-like substructure is transformed into the dendrite as the cellular interface changes into dendritic. The length of rods and dendrites becomes shorter with an increase in C₀•R/G. The cored region is enriched with Ti, and its content is about 2.5-4.0 times of the initial solute concentration. The matrix of cellular and dendritu interface is extremely depleted in the solute, and its concentration is about 1/3-1/5 of C₀.
The diffusion coefficient for the solute Ti in the liquid Al at the liquidus temperature is calculated to be 0.87×10^-5cm²/sec.

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Aging phenomena of Al-(3, 4)wt%Cu-Mg alloys with various Mg contents up to about 2 wt%were investigated in the temperature range from 0°C to 150°C mainly by the electrical resistivity measurement. The specific heat measurement was also employed. The results obtained are as follows:
(1) In the case of aging at 0°C, the initial rate of resistivity increase (Δρ) in ternary alloys was suppressed compared with the binary alloy as Mg content increased. But the total amount of Δρ after prolonged aging was increased in ternary alloys with an increase of Mg content. In the case of aging at 50150°C, the initial rate in ternary alloys was, in contrast to that obtained at 0°C, enhanced substantially with increasing Mg content, so each of aging curves almost corresponded with that obtained at the later stage of 0°C aging. These interesting aging behaviors in ternary alloys were considered to be due to the nature of two kinds of G. P. zones (G. P._θ, G. P._B) and the ratio oftheir amounts which naturally varied with Mg contents. This means that the estimation of precipitation rate based on the rate of Δρ will be unreliable at least in such alloys in which two kinds of G. P. zones are formed.
(2) The amount of heat evolution associated with formation of G. P._B zone was found to be considerably large, while that of G. P._θ zones was too small to be detected. The various results of specific heat measurements suggested that Δρ in ternary alloys was closely related to the formation and growth of G. P._B zones.

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