Al-7%Si-Mg casting alloys containing 0.2 to 0.8%Mg and 0.04 to 0.8%Fe were preaged at room temperature prior to artificial aging. Hardness, tensile and yield strength decrease by preaging before artificial aging at 140° or 155°C for 5hrs. Ductility or toughness is also improved. Artificial aging at 200°C for 5hrs is unsusceptible to preaging. These facts are discussed on the bases of change in supersaturation caused by cluster formation during preaging and stability of clusters during artificial aging.

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Hardness measurements, X-ray small angle scattering methods and transmission electron microscopy were applied in this work. The second bead has little effect on the aging behavior of the heat-affected zone in the two-pass weld, when it is made under the same heat input as the first bead at low inter-pass temperature or under the heat input as half as the first at high inter-pass temperature. The second bead, however, has remarkable effects on the aging behavior of the heat-affected zone in the two-pass weld, when it is made under the same heat input at high inter-pass temperature.

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The fatigue life and the fatigue crack propagating rate were investigated of precipitation hardening 7075 alloy heat treated under various conditions. Extremely underaged and extremely overaged alloys have excellent fatigue strength in spite of their low yield strength. The alloys aged at 80, 120 and 160 C for 24hr having practically static strength exhibit better fatigue strength in this order. The fracture toughnesses of them are, on the other hand, in the reverse order. These facts are reasonably exlained from a cencept of zone dissolution by dislocation cutting. Fractographs show the stable crack propagating in the grains independently of the precipitation free zone.

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The improvement in age-hardenability due to excess silicon was investigated in Al-Mg₂Si alloys by means of hardness, electrical resistivity and specific heat measurements and high-resolution electron microscopy.
The improvement is related to the increase in the amount of Al-Mg-Si ternary clusters leading to the finer and denser distribution of needle-shaped G. P. zones. If excess silicon more than 0.2% is contained, however all of ternary clusters does not always transform to G. P. zones because of their compositions considerably different from those of G. P. zones, leading to the slight improvement in age-hardenability. In silicon excess alloys, plate-shaped precipitates of hexagonal structure are found as an intermediate phase.

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The phase change of metastable Al₆Fe or Al_mFe into stable Al₃Fe was investigated in Al-0.5%Fe alloys having either Si up to 0.11% or Mg up to 1.2%. Specimens containing Al₆Fe or Al_mFe were prepared by unidirectional solidification and isothermally annealed at 600°C. Progressive changes in the eutectic structures were examined by means of optical and transmission electron microscopies, and X-ray diffraction method.In Si containing alloys, the phase change is accelerated with increasing Si contents, where stable Al₃Fe is found in the eutectic region to form in the cast state and to grow at the expense of metastable phases. In contrast to this, in Mg containing alloys, the phase change is markedly suppressed with increasing Mg contents, where the diffusion of Featoms in the matrix is suggested to be largely restrained due to interactions with Mg atoms. In all cases, the time required for the phase change is shorter in Al_mFe than in Al₆Fe.

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