AZ91C magnesium alloys were unidirectionally solidified at various liquid cooling rates (V _LC), average cooling rates in solidification range (V _SC), temperature gradients (G), moving velocities of solid-liquid interface (R) and G/R ratios. Equiaxed dendrite structure was obtained by the superheating of molten metal. Dendrite cell size decreases exponentially with increasing liquid cooling rate. With increasing average cooling rate and with decreasing temperature gradient, the amount of eutectic compounds increases and it extends at triple junctions of cell boundaries and individual cell boundaies. In AZ91C-T6 alloy, the eutectic compounds dissolve by solution heat treatment. Discontinuous precipitates along cell boundaries were observed by optical microscopy and their width decreases with increasing liquid cooling rate.

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To clarify the influence of solidification conditions on mechanical properties of unidirectionally solidified AZ91C magnesium alloy, tensile strength, 0.2% proof stress and elongation in tensile test, Brinell hardness, and maximum load, crack initiation energy, crack propagation energy and total absorbed energy in instrumented Sharpy impact test were examined. In as-cast condition, the elongation, proof stress and maximum load are strongly affected by the temperature gradient at solid liquid freezing interface. The moving rate of the freezing interface has significant effects on hardness, crack propagation and total absorbed energy. The average cooling rate during solidification markedly influences crack initiation energy. In T6 condition, the elongation is affected by the temperature gradient and moving rate of the interface. The liquid cooling rate before solidification has effects on proof stress and tensile strength, the moving rate of the interface on hardness and average cooling rate on impact properties.

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The hydration of amorphous and crystalline aluminas (γ and η) has been investigated by the dipping treatment in water at the temperatures of room to 95°C. Amorphous aluminum hydroxide, pseudoboehmlte, bayerite, hydrargillite and nordstrandite are formed by the hydration of amorphous alhmina. By the dipping at low temperatures, although the formation of amorphous aluminum hydroxide or pseudoboehmite is observed in the beginning of hydration, alumina trihydrates such as bayerite, and hydrargillite and/or nordstrandite appear in the dipping for about ten days and their formed amounts increase with increasing the period of dipping. With rising the dipping temperature, however, the rate of hydration is promoted, and pseudoboemite is formed at frst and then bayerite and nordstrandite are formed, afterwards the amount of nordstrandite formed increases when the period of dipping is extended more longer. Accordingly it is inferred that the hydration of amorphous alumina proceeds in the following sequence:
amorphous alumina→amorphous aluminum hydroxide pseudoboehmite→alumina trihydrate
On the other hand, the hydration of crystalline aluminas is little caused by the dipping treatment in water at room temperature, but η- and γ-aluminas are hydrated to alumina trihydrates and pseudoboehmite, respectively, when the dipping temperature is rised.

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Hardness and electrochemical polarization characteristics of Al-Zn-Mg alloys containing up to 0.1 at% of Cu, Cr, Si and Ti were investigated. The Vicker's hardness of the alloys aged at 180°C did not change remarkably by the addition of these contents up to 0.1at%. Pitting potential of the alloys in 0.5N NaCl solution changes into noble value with increase in Cu and/or Cr contents. Passivation current density of the alloys reduces with increase in Cu, Cr and Ti contents. Pitting potential changes into noble value and passivation current density reduces with increasing aging time. These phenomena correspond to the change of solute quantity of Zn and/or Mg in the surface layer of the alloy materials.

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The increase in free energy accompanying the formation of G. P. zone in an Al-2at% Cu alloy was discussed in terms of the changes in binding enthalpy, configurational entropy and strain energy. As far as the driving force for precipitation is derived from the decrease in binding enthalpy of the system, segregation of copper atoms to (100) planes in the aluminum lattice against the decrease in configurational entropy, can not take place at temperatures higher than about-100°C. Since the change in free energy shows a monotonous increase according to the increase in the radius of G. P. zone, the preferential formation of platelet-type precipitates does not occur at a normal aging temperature. The amounts of strain energy around G. P. zones and solute copper atoms were evaluated by a finite element method, and it is shown that the strain energy of copper atom is largest in a solid solution. While the strain energy around G. P. zones increases approximately in proportion to their radii, a strain energy of copper atom or a total strain energy of the system decreases progressively with respect to the growth of platelets. Therefore, the G. P. zone in an Al-Cu alloy is formed mainly through the relaxation of strain energy around solute copper atoms.

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Attempts have been made to clarify the occurrence of the maximum current on anodic polarization curves for Al-Fe alloys after cathodic treatment in deaerated 0.1 M Na₂SO₄ solution. The origin of the maximum anodic current is the oxidation of the undissolved intermetallic iron compound FeAl₃ into ferrous hydroxide.

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