On the nature of boron solid solution in α-iron

X-ray lattice parameter and internal friction measurements were carried out on a B-αFe high-purity alloy in order to define the nature of boron solid solution in α-iron. B-αFe alloy specimens, practically free from carbon, were obtained by exploiting the very different diffusion rates of B and C in the iron b.c.c. matrix. By comparing the lattice parameter values between pure iron and carbon-depleted B-αFe alloy, a decrease is experienced for the latter. Two internal friction maxima, strictly connected with the carbon picked up during the boronizing process, were detected in the nondecarburized B-αFe alloy. The main peak corresponds to interstitial carbon diffusion in the αFe-B-C alloy (ΔH=21.9 kcal/mol); the « secondary » one (ΔH=16 kcal/mol) could be associated with a diffusion process of interstitial-substitutional C-B couples. No peak whatsoever has been detected in a carbon-depleted B-αFe alloy. The results obtained make it possible to establish that boron occupies substitutional positions in the α-iron lattice; solubility at 880 °C may be evaluated in about 0.015 wt%. The uncertainties existing on the subject up to now are probably the consequence of experiments carried out on B-αFe alloys not completely carbon free.