Growth mechanisms of silicon in Al-Si alloys

Eutectic silicon in Al-Si alloy has been examined by optical, SEM and TEM methods. Normal flake silicon grows anisotropically and has a low twin density; quench modified fibrous silicon is essentially twin free and grows isotropically (non-faceted); sodium modified silicon contains a very high twin density and the fibers are somewhat faceted. It is concluded that in the absence of sodium, molecular attachment by the TPRE mechanism is incidental but in the presence of sodium this is the dominant growth mechanism. Various aspects of the modification mechanism are discussed. © 1985