The mechanism of silicon modification in aluminum-silicon alloys: Impurity induced twinning

Abstract

Modification of silicon by sodium in aluminum silicon eutectic alloy has been examined in detail by optical, SEM, and TEM methods. The aluminum phase is not significantly affected but the silicon becomes very heavily twinned. Modification by quenching does not involve an increase in twin density. Consideration of the atomic positions which attend the formation of growth twins on {111} planes suggests that adsorbed impurity atoms of suitable size, on the solid-liquid interface, could be responsible for changing the {111} stacking sequence, so promoting ‘impurity induced twinning’; the optimum hard sphere radius ratio would be ≈ 1.65. It is proposed that this condition could be the first and principal requirement for a modifying agent to be effective in this system. It is shown further, that other reputed modifiers do also induce a higher twin density. Variations in the efficiency of individual elements to promote such an effect are discussed in terms of other relevant factors which include melting points and vapor pressures, the free energies of formation of compounds — notably of oxides, and the forms of alloy phase diagrams.