Details

Autor(en) / Beteiligte

• Sun, Y-B

Titel

Microstructure Control and Investigation of Nucleation and Growth Mechanism of Silicon Crystal Grown From the Hypereutectic Al--Si Melt

Erscheinungsjahr

1990

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• It has been a matter of concern in as-cast hypereutectic Al--Si to control the morphology of Si phases. To obtain both refinement and modification, the effect of Ti on solidification progress and morphological change of Si phases was studied, including the interaction effects of phosphorus and Na. The morphology and crystallography of both star-shaped Si crystals and feathery Si 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 Si 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 Si precipitation, the microstructure revealed relatively small idiomorphic primary Si 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 deg C undercooling from the eutectic temperature with P, 580 deg C, while the melt without Na showed 4-6 deg C undercooling. Electron channeling patterns on a star-shaped Si crystal showed a diamond cubic structure with twin relationships between neighboring arms. SEM microstructures revealed the progress of primary Si 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 Si 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 Si crystals indicated that different growth mechanisms are operative; a twin plane re-entrant edge and tip effect mechanism for the star-shaped primary Si crystals, and a tip effect mechanism for the feathery primary Si crystals. (DA9006906).--AA