HREM studies on twinning behaviour of eutectic silicon in unmodified and Sr- modified Al-Si alloys

Abstract

The addition of trace amounts of sodium or strontium to commercial Al-Si alloys promotes a massive increase in twin density in the eutectic silicon which is associated with a change in morphology of the silicon phase from a flake to a fibrous form. In the flake form the internal twins are always parallel to external faces of an individual flake as in Fig. 1a, resulting in a consistent <112> growth direction by the twin-plane reentrant edge (TPRE) growth mechanism. In the fibrous form the high twin density promotes a zigzag alternation of <112> growth directions, as in Fig. 1b, with a net <001> growth axis,so that twins can be parallel to any of four <110> axes. In the flake form all twins lie in a single <110> zone. The essential morphological features of flake and fibrous growth are conveyed in two dimensions by Figs. 1a and 1b. HREM has been employed in the present study to investigate further the nature of the twinning in eutectic silicon of flake and fibrous morphologies.