In this paper, the improvement of casting properties of the high-strength AlZnMgCu alloys is described. This improvement was achieved by additions of Ni, Mg, and Si, which formed eutectic phases, thus reducing the crystallization interval of the alloy. The comparison of the castability and hot-tearing tendency was made for three alloys: nearly single-phase AlZn6Mg2 alloy, quasi-ternary eutectic AlZn7Mg7Cu1Ni3Si3 alloy, and common casting AlSi10 alloy. It was shown that the castability and hot-tearing tendency of the quasi-ternary eutectic alloy are similar to those of the AlSi alloy.

The casting properties of high strength Al -7Zn -7Mg -1Cu -3Ni -3Si (wt-%) alloy are described. Compared with common Al -Zn -Mg -Cu alloys, an improvement of casting properties has been achieved by adding elements (Ni, Mg, Si) that form eutectic phases, thus reducing the solidi cation interval of the alloy. A comparison of thermal cooling curves, castability and hot tearing tendency has been carried out for three alloys: Al -7Zn -2Mg -1Cu (structure consists mainly of solid solution), quasi-ternary eutectic alloy Al -7Zn -7Mg -1Cu -3Ni -3Si and the common casting alloy Al -10Si. In addition, the effect of melt protection against oxidation on castability has been evaluated. It is shown that the casting properties of the protected quasi-ternary eutectic alloy are signi cantly better than those of the common Al -7Zn -2Mg -1Cu alloy and that they achieve a level close to that of Al -10Si alloy.

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The influence of buoyancy-driven convection on the lamellar spacing of an Al-Zn cutectic was studied under conditions of oriented crystallization. The convection due to huoyancy was affected by the mold position respective to thc direction of action of the force of gravity. It has been found that the buoyancy-driven convection does not oxcrt, any direct effect on the lamellar spacing. Rather, it only affects the rate of advance of tho crystallization front. The lamellar spacing 1 of the Al-Zn eutectic can be described as function of this rate of advance V by the relation 2 = I.'-0.46. 9.0

The results of a study on eutectic crystallisation in the hypoeutectic alloy under different convection conditions are presented. Buoyancy driven convection was suppressed by processing the samples under microgravity conditions. The effect of convection level on the eutectic crystallisation is shown to be significant: In the absence of convection, the non-equilibrium level of the process is enhanced. The characteristic dependence of the particle size on the distance from the surface of a sample, observed on the minor phase of the eutectic, is discussed.Dam cet article nous prkntons les rbultats d'une etude sur la crystallisation eutectique d'un alliage hypoeutectique sous diffhrentes conditions de convection du flux. Nous avons supprim6 la convection dirigees par la poussee en realisant les echantillons sous microgravite. L'effet du niveau de convection sur la crystallisation de l'eutectique est remarquable. En supprimant la convection nous nous eloignons davantage de l'etat d'equilibre. Nous discutons les raisons possibles de la dependance de la taille des particules en fonction de leur distance par rapport a la surface. IntroductionThe presented work is part of a systematic study on the solidification process of multiple-component melts under different convection conditions. Both the typical anomalous behaviour of the Ag-Ge eutectic system and the significant sensibility of eutectic solidification to the external conditions were the reasons for the Ag-Ge system to be chosen as an appropriate model material.A number of authors has studied the crystallisation process in the absence of buoy-