Condition of Boron in Alpha Iron

Abstract

THE question as to whether boron forms an interstitial or a substitutional solid solution in α-iron is of considerable theoretical and practical interest. Recently measurements have been made, by orthodox methods, of the solubility and diffusivity of boron in both α- and γ-iron^1–3. These gave the maximum solubility of boron in α-iron as about 0.008 weight per cent at 900° C. The data on the diffusion in α-iron⁴ were not precise but indicated an activation energy of 62 kcal. per mole and a D ₀ of 10⁶ cm.² sec.⁻¹. This suggests that boron behaves substitutionally. Some calculations based on estimates of the atomic radius of boron³ also suggest that boron may be substitutional in α-iron. However, we felt that this evidence was somewhat speculative and not sufficiently conclusive. Obviously, if boron were interstitial, it should be possible to observe an internal friction peak similar to that caused by carbon and nitrogen in α-iron^5,6. The purpose of the present communication is to describe some exploratory measurements on internal friction which suggest that boron is in interstitial solution in α-iron.