Precipitation of aluminum in the silicon phase contained in W319 and 356 aluminum alloys
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NTRODUCTION
THE formation of the aluminum-silicon eutectic in cast aluminum alloys has been extensively studied and is the basis for a number of commercial aluminum casting alloys.[1–4] However, whereas characterization has been performed on precipitation reactions in the aluminum responsible for age hardening,[5,6] little detailed characterization has been undertaken on the silicon phase. Solubilities in silicon of other alloying elements such as aluminum and copper are very low. Both aluminum and copper exhibit retrograde points with the silicon solidus at about 1200 ⬚C in their binary phase diagrams, where the maximum solubilities are 0.016 and 0.002 (at. pct), respectively, and these elements have almost no solubility in Si at room temperature.[7,8] Additionally, diffusion in silicon is also another important parameter in considering precipitation in silicon. It is well known by the semiconductor community[9] that aluminum is the fastest p-type dopant in silicon and rapidly interdiffuses into silicon above 400 ⬚C, causing changes to the electrical conductivity of the silicon. Two published reports exist where precipitation of aluminum has been observed in silicon. Hogg et al.[10] identified 5to 10-nm aluminum precipitates by high-resolution electron microscopy with an orientation relation [111]Si 㥋 [001]Al, (110)Si 㥋 (110)Al in as-spray-formed hypereutectic aluminum-silicon alloys. These authors propose that the precipitates form because the solid solubility of aluminum in silicon is exceeded during atomization and that the semisolid slurry at the surface of the solidifying perform recalescences. Liquid aluminum droplets form in the primary silicon grains as the aluminum comes out of the solid solution to produce the observed precipitates. Sadana et al.[11] have also identified similar aluminum precipitates in aluminum ion-implanted silicon following annealing at 600 ⬚C and 1000 ⬚C. The two orientation relationships exhibited by these aluminum precipitates
are identical to those observed for silicon precipitates in an aluminum matrix by Saulnier[12] but are inconsistent with the orientation relation identified by Hogg et al.
II. MATERIALS AND METHODS The materials examined in this investigation were commercial 319-type (W319) and 356 aluminum alloys cast into wedge-shaped sand molds (with a copper chill located at the narrow end of the wedge) to produce a continuous range of solidification rates within one casting. Compositions of the castings were measured by optical emission spectroscopy using a spark emission source and are shown in Table I. Samples approximately 1.5 x 1.5 x 0.5 cm were cut from various positions of the wedges for heat treatment and for transmission electron microscopy (TEM) investigations. The TEM samples were prepared by sectioning and mechanically grinding 3.0-mm disks down to 75 m followed by dimpling to 25 m. A cyanoacrylate adhesive was used to attach the samples to the holders in order to avoid heating the samples using thermal setting resins. Samples were removed from the polishin
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