Strontium modification of AI-12.7wt-%Si alloys

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Abstract
Aluminium silicon alloys are cast usually in the modified condition. Thin sections can be quenched modified and thicker sections impurity modified by adding small quantities of sodium or strontium. The benefits and features of modification (namely, an improvement in mechanical proPerties, a refinement of the eutectic structure, a lower eutectic growth temperature, and an increase in the amount of primary aluminium) are well known but the modification mechanism is not completely understood. Earlier interpretations of impurity modification considered the impurity to affect the nucleation of the silicon phase and the reduced growth temperature was considered to be the temperature at which the silicon phase nucleated repeatedly during solidification. More recent discussions have attributed modification to a restriction of the growth of the silicon phase. The authors are not aware of undercooling-velocity data for the modified structure and the purpose of this communication is to present such data and associated structural observations made during the controlled directional solidification of AI-12.7wt-%Si alloys containing 0.01 or 0.04 wt-%Sr and to compare them with similar data reported previously for unmodified AI-12.7Si alloys.

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