In the previous paper, we reported the relationship between the contact potential difference and various anodic oxide films of aluminum with thickness up to 600Å. And in this report, we studied the contact potential difference of aluminum oxide films formed on anode in various concentrations of acidic solution. The following conclusions were obtained: (1) The contact potential difference of oxide films changed with the concentration of electrolytic solutions. (2) Changes in the film thickness dependency of the contact potential difference with the acid concentration were not too large and it did not change linearly with the acid concentration. It also changed correspondingly with the oxide film density.

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The contact potential differences were measured for anodic oxide films of aluminum formed in various acidic solutions; oxalic acid, sulfuric acid, chromic acid and phosphoric acid. The effect of the oxide film thickness on the contact potential difference was studied for the film thickness ranging from 1000 to 5000Å. The following conclusions were made:
(1) The contact potential differences of the oxide films formed in all the acidic solutions were stabilized when their thickness was more than 3000Å.
(2) It was also considered that the oxide film density per unit volume was stable for the film thickness more than 3000Å.
(3) The contact potential difference of the oxide film formed in a sulfuric acid solution was unstable when its thickness was between 1000 and 3000Å.
(4) Anodic oxide film with a highly stabilized contact potential difference was obtained by using 2% oxalic acid solution or 15% sulfuric acid solution.

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Hardening and precipitation during aging of subperitectic Al-Zr alloys containing about 0.1%Fe or Si were studied in comparison with a binary Al-0.22%Zr alloy. The specimens were homogenized at 650°Ç for 8hr and quenched in ice brine, and then aged at 350°Ç, 400°Ç and 450°Ç for periods ranging from 5hr to 10⁴hr. Changes in hardness and electrical resistivity during aging were measured by a microvickers tester and by the standard potentiometric method. Changes in microstructure were also observed by means of optical and transmission electron microscopy.
The maximum hardnesses were obtained after prolonged aging of 10³10⁴hr at 350°Ç. A small addition of Si accelerated the hardening of the alloy remarkably, but an addition of Fe did not effect the age hardening rate.
There were two-step reactions in the resistivity-aging time curves of Al-Zr and Al-Zr-Fe alloys. It is thought that the first reaction corresponds to the precipitation due to grain boundary reaction and the second to continuous precipitation within the grain. The area fraction of the former was about 3 percent at most in Al-Zr-Fe alloy and nearly zero in Al-Zr-Si alloy. Coherent spherical precipitates were uniformly distributed within the grains. It is concluded that the precipitates are a predominant factor for the increment of hardness during aging. Platelike precipitates were also found in the three kinds of specimens, especially in an Al-Zr-Fe alloy. The remarkable acceleration of age hardening by a small addition of Si seems to be the acceleration of nucleation of the spherical particles within the grains due to Si atoms.

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Crystal structures of intermetallic MnAl₆ and MnAl₄ phases, were determined by the powder X-ray diffraction method on the specimens extracted electrolytically from the binary alloy. The transformation reaction from MnAl₄ to MnAl₆ was also discussed. Results obtained were as follows.
(1) The MnAl₆ phase is a C-face centered orthorhombic structure, lattice parameters were measured as a = 7.564Å, b = 6.500Å, c = 8.883Å, the space group is D¹⁷_2h-Cmcm, and the density is 3.25g/cm³. The crystal composition is MnAl₆ and a unit cell contained 4 formula units.
(2) The MnAl₄ phase is a simple orthorhombic structure, lattice parameters are a = 6.795Å, b = 9.343Å, c = 13.897Å, the space group is D²_2h-Pnnn, and the density is 3.65g/cm³. The crystal composition is MnAl₄ and a unit cell contained 12 formula units.

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Al-CuAl₂, Al-NiAl₃ and Al-Co₂Al₉ eutectic alloys were solidified unidirectionally by the thermal-valve technique, and changes of growth morphology with changes of freezing conditions were studied.
The results obtained are as follows:
(1) Each eutectic alloy showed a transition from planar to colony structures with the occurrence of constitutional supercooling which depended on the freezing conditions and the impurity contents.
(2) Growth morphologies of intermetallic compounds in Al-CuAl₂, Al-NiAl₃ and Al-Co₂Al₉ eutectics were also changed from degenerate, blade and ribbon type to lamellar, rod and fibre forms respectively with an increase of freezing rate.
(3) Linear relationships were obtained between the square root of freezing rate, R^1/2 and the reciprocal of interphase spacing, λ^-1 or the interface undercooling, ΔT. Applying these relations to Jackson and Hunt's equation, interphase boundary energy, σ_αβ and diffusion coefficient, D were estimated.
(4) Measured values of ultimate tensile strength of these unidirectionally solidified eutectic alloys were not sufficiently high as a composite material.

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