| It has been a matter of concern in as-cast hypereutectic Al-Si to control the morphology of silicon phases. In order to obtain both refinement and modification, the effect of Ti on solidification progress and morphological change of silicon phases was studied, including the interaction effects of P and Na. The morphology and crystallography of both star-shaped silicon crystals and feathery silicon crystals were examined to relate these structures to the general growth mechanism.; Cooling curves were evaluated to detect the precipitation of phases and the amounts of undercooling for six different combinations of controlled additions (P, Na, and Ti). Selected area electron channeling pattern and microdiffraction pattern analyses were carried out on the primary silicon crystals. Microstructures were analyzed using the optical microscope, SEM, EDXS and microprobe.; Even though the Ti addition neutralized the refining effect of P and suppressed primary silicon precipitation, the microstructure revealed relatively small idiomorphic primary silicon crystals when Na was added compared to those obtained from a P + Na addition. The melt treated with Na, either with or without P and Ti, showed 8-12{dollar}spcircrm C{dollar} undercooling from the eutectic temperature with P, 580{dollar}spcircrm C{dollar}, while the melt without Na showed 4-6{dollar}spcircrm C{dollar} undercooling. Electron channeling patterns on a star-shaped silicon crystal showed a diamond cubic structure with twin relationships between neighboring arms. SEM microstructures revealed the progress of primary silicon crystal formation, showed multiple parallel twin plane re-entrant edges at the edge parallel to the cozonal five-fold axis, i.e. (110), and also showed hopper crystals. Microdiffraction patterns on the stem of a feathery silicon crystal showed a consistent crystallographic orientation across the section thickness.; The addition of 0.04% P + 0.17% Ti + 0.20% Na produced a refined and modified microstructure. Morphologic and crystallographic study of the primary silicon crystals indicated that different growth mechanism are operative; a twin plane re-entrant edge and tip effect mechanism for the star-shaped primary silicon crystals, and a tip effect mechanism for the feathery primary silicon crystals. |
