THE MODIFICATION OF ALUMINIUM-SILICON ALLOYS BY SODIUM

Measurements by autoradiography and activation analysis have revealed the distribution of sodium in the microstructures of aluminum --7.5% silicon alloys modified by the addition of various amounts of sodium. Electronmicroscopic studies indicate that there is no fundamental difference between modication by chill casting and modification by sodium addition. A previously unobserved submicroscopic phase, the amount of which depends on the degree of modification, has been found. New thermal measuremente, employing an inert atmosphere to prevent sodium loss, were made. The several types of experimental results have been used to establish the mechanism of modification. A theory of modification similar to an hypothesis advanced by Edwards and Archer is proposed. According to it, the addition of a modifying agent retards nucleation of eutectic silicon at the unmodified'' eutectic temperature and causes solidification to occur at a steadystate temperature considerably below the freezing point, where diffusion is slower and the individual silicon crystals cannot attain so large a size. (auth)