DIRECT ELECTRIC-FIELD ELECTROTRANSPORT OF CARBON AND NITROGEN IN IRON AND IRON ALLOYS

Abstract

The interstitial mobility of carbon and nitrogen in austenite iron is determined from diffusion experiments carried out in a direct electric field. Diffusion-penetration curves are obtained, using a microhardness technique, and related to the "apparent charge" of the solutes by the Einstein equation. The apparent charge of carbon is positive, increasing from +8.6 ± 1.0 at 950 °C to +13.7 ± 1.7 at 842 °C. The apparent charge of nitrogen is negative, increasing from −8.3 ± 1.0 at 1 000 °C to −14.1 ± 1.7 at 922 °C. Positive Hall coefficients are found for both systems at 925 °C. The effect of 4% manganese in iron is to increase the apparent charge of carbon to ~ +9.3, while 2% silicon in iron decreases the apparent charge of carbon to ~ +4.7 at 950 °C. The results obtained are discussed in relation to the band structure of the alloys and the effect of electron or "hole" momentum transfer to the lattice. A possible alternative mechanism is also proposed which could contribute to the high apparent charge obtained for the solutes.