Continued from the previous report on fatigue crack propagation in aluminum alloys, this paper discusses on the detail of ductile striation structure.
In such materials having large stacking fault energy as aluminum alloys, striations are found to be curved in the neighborhood of tear lines.
Substructure formed during the fatigue had effects on the propagation of fatigue crack. As the results of electron factography, the effects of sub-boundary structures were classified into the following 4 classes.
(1) Arresting effect by curving and blunting of crack front; it will result in the formation of tear lines.
(2) Arresting effect by confusion of the direction of local crack growth; it will result in the formation of "shallow tear lines."
(3) Formation of terraces to separate the plane with striations. Arresting effect is very little in this case.
(4) Accelerated effect by serving as the path of crack growth.

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Continued from the previous report on fatigue fracture topography in aluminum alloys, this paper discusses on the effects of constituent particles in ductile striation structure on the rate of fatigue crack propagation by the observation of electron fractography.
As the results, even if the width of plastic zone at the crack tip was larger than the spacings between the particles arresting effects were observed under various conditions as well as accelerated effects of the constituent particles on the crack propagation, which had been pointed out by Piper³⁾and Pelloux⁴⁾.
The effects of constituent particles on fatigue crack propagation was so complicated that further systematic investigations are necessary in future in connection with the effects of particles on the formation of substructure in the plastic zone.

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Effects of Mg (0.20.9 at.%) and Si (0.10.6 at.%) contents and trace of additional elements (0.010.2 at.%) on the two-step ageing phenomena of AI-Mg-Si alloys were studied by the measurements of hardness and electric resistivity and also by electron microscopy.
The phenomena of two-step ageing of AI-Mg-Si alloys highly depended upon the contents of Mg₂Si and additional elements as well as the conditions of pre-ageing. The pre-ageing led to either refining or coarsening of acicular precipitates in finally aged condition, according to the composition of alloys. Consequently, it resulted in either increase or decrease (negative effect) in strength of the alloys.
For AI-Mg-Si alloys containing more than about 1.1 at.% of Mg₂Si the pre-ageing at room temperature caused coarsening of finally aged structure but it caused refining of precipitates for the alloys containing less than about 0.9 at.% of Mg₂Si. The negative effect of pre-ageing is interpretated as follows. It is based on the deleterions effect of impoverishment of solute atoms in matrix resulting from the clustering at room temperature, forming small clusters which modestly contribute to the strength.
The negative effect of pre-ageing of Al-0.70 at.% Mg-0.35 at.% Si (1.05 at.% Mg₂Si) alloy was decreased by the addition of transition elements such as Mn, Cr, Zr, V, and Fe, but increased by the addition of Ag, Cu, Be, Cd, and Zn. This may be caused by fast that the apparent supersaturation of Mg₂Si in the alloys is increased by the latter elements, but decreased by the former elements which easily form insoluble compounds with Si and Al atoms.

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Change of clectrical rcsistivity during ageing was measured on Al-Cu alloys containing Cd or In, and the other, further containing Mn as an additional element. Some differences were found in the effects of additional elements on change in electrical resistivity during ageing.
It is well known that the addition of Cd, In, or Sn accelerates the rate of artificial ageing, but retards the rate of natural ageing. As expected from the above fact, the temperature at which abrupt decrease of resistivity took place in isochronal ageing was lower in ternary alloys than in binary. Electrical resistivity continued to fall up to about 310°C under the conditions of the present experiments and there was found no difference between the two kinds of ternary alloys (Al-Cu-In and Al-Cu-Cd alloys) in that range of temperature. Electrical resistivity change in Al-Cu-In alloys for isochronal ageing at above 310°C was similar to that in binary alloys, but was different from that in Al-Cu-Cd alloys. The increase in electrical resistivity occurred at above 310°C, but the degree of increase was larger in the latter than in the former. In the temperature range from 310 to 400 and tens of °C, electrical resistivity could be explained by the combined effect of an increase due to re-dissolution of intermediate phase and a decrease due to precipitation of equilibrium phase. Therefore, the results of the present experiments suggest that the formation of equilibrium phase would be more difficult in Al-Cu-Cd alloys than in Al-Cu-In alloys.
At the early stage of isothermal ageing, an increase in electrical resistivity occurred at temperatures lower than 150°C in Al-Cu-In alloys. The above both temperature were rather lower than those claimed by Holmes et al. These temperatures can be elevated to some extent by the addition of Mn to the ternary alloys, though the rate of artificial ageing was retarded.

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Aluminum sheet (Fe = 0.072%, Si = 0.037%, and Al = balance) was immersed in artificial water, LSW 1 (water for forming macro-pits) and macro-pits were formed on the aluminum surface.
The composition of LSW 1 was composed of Na₂SiO₃ (5ppm of SiO₃^2-) + NaCl (5ppm of Cl^-) + pure water, and its pH value was 9.39.5.
The formation of macro-pits consisted of 5 stages as follows.
First stage: Microstructure of α (FeSi) contained in aluminum could not be observed by microscopy.
Second stage:Microstructure of α (FeSi) was observed by microscopy, because of the dissolution of passive film on α (FeSi). It was proved by the weight decreases of the specimen after immersion and after the removal of the film.
Third stage: Formation of macro-pits was observed around α (FeSi).
Fourth stage: Etch pits, crystallographic or cubic corrosions was observed near α (FeSi).
Fifth stage: Macro-pits were observed in the centre of the aggregate of etch pits. The formation of etch pits on the surface was proved by etching of aluminum by the etching solution (the composition was LSW 1 + any concentration of AlCl₃) at pH 1.82.7(corresponding to 15.34.6% of AlCl₃ concentration).
As the results of these experiments, the formation of macro-pits on the aluminum sheet was presumed as follows:
The thiner passive film covering α (FeSi) was dissolved by LSW 1, while, remainder parts of specimen surface being coverd with the passive film.
Aluminium around naked α (FeSi) particles becomes anode and micro-pits, are generated. At selected points (somewhere among micro-pits) etch pits are generated as a result of etching the aluminum surface by the solution with spontaneous high concentration of AlCl₃ resulted from dissolving aluminum. Therefore, necessary conditions generating macro-pit are presumed: existence of Cl^-, and trace of inhibitor (SiO₃^2-), thiner passive film covering on α (FeSi), and high pH of the artificial water(LSW 1).

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