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F. Dubrous Laboratoire d'Electrothermie

de Chedde, Pechiney Electrometallurgie, 74190 Le Fayet, France

(Received 25 January 1990; accepted 7 June 1990)

Optical metallography, scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy were used to characterize metallurgical grade silicon, produced in an electric arc furnace. Coincidence fraction determinations were assumed to be 27 and 29 when grain boundaries are underlined by precipitated phases and 23 when they are not. The study of intergranular compounds was emphasized; ten compounds were found, the main ones being Si2Ca, Si8Al6Fe4Ca, Si2Al2Ca, Si2FeTi, and Si24Fe (a leboite). The precipitation of these compounds was discussed according to the principal impurity concentrations in silicon. The crystalline structure of Si8Al6Fe4Ca was determined to be triclinic with a = 1.3923 nm, b = 1.3896 nm, c = 1.3900 nm and a = 92.4°, p = 110.3°, y = 119.9°.

I. INTRODUCTION

Metallurgical grade silicon is widely used in industry for aluminum alloys, copper alloys, steels, silicones, ceramics, and as a starting material for solar grade and semiconductor grade silicon. Some of the above applications require improvements in silicon quality which cannot be obtained only by considering the chemical analysis of the material; in order to categorize the main impurities in silicon (solid solution, phases) it is necessary to study its microstructure. Indeed, in the industry of silicones and aluminum alloys, the nature and volume fraction of the different phases are important parameters. The present paper reports results obtained on industrial alloys. Synthetic alloys with various compositions were also investigated to define the specific or cumulative influence of different impurities on the observed microstructure. II. EXPERIMENTAL The metallurgical grade Si samples were produced in an electric arc furnace at Montricher factory (Pechiney Electrometallurgie, Savoie, France). An oxidizing refinement was performed during casting. The thickness of the cast ingot was about 10 cm and its cooling to room temperature was natural. Table I shows the overall analysis of the main impurities in the studied

industrial silicon, corresponding to a classical nuance in silicones and light alloys industries (Fe < 0.5 wt. %, Al < 0.5 wt.%, Ca < 0.3 wt.%, and Ti < 0.1 wt.%). The technique of atomic absorption was used in the analysis of every element. Main impurities are Fe, Al, Ca, and to a lesser extent Ti. The synthetic alloys were prepared at the Laboratoire d'Electrothermie de Chedde in high frequency furnaces with graphite crucibles and using high purity Si, Ti, Fe, Al, and Ca as melting materials. In order to characterize the metallographic structure, the following analysis methods and techniques were used: (1) porosity measurements with Archimedes pushing in a carbon tetrachloride bath; (2) optical metallography on etched or nonetched planes; (3) scanning electron microscopy (SEM) with a SUPER IIISI, operating at 15 kV and equipped with an x-ray energy