Diffusion of Nb in Fe and Fe alloys

Abstract

The diffusion coefficients of Nb have been studied in unalloyed γ Fe over the temperature range 948–1201°C and in Fe–1·5Mn, Fe–0·6Si, and Fe–1·5Mn–0·6Si alloys over the range 1081–1201°C by the direct sectioning method using the isotopes ⁹⁴Nb and ⁹⁵Nb. The Arrhenius curves for diffusion into pure γ Fe and γ Fe alloyed with Mn and/or Si are linear in the temperature range 1081–1201°C, and give approximately the same activation energy Q = 267·4 kJ mol⁻¹. Below these temperatures lnD v. 1/T, where D is the volume diffusion coefficient and T the temperature, is non-linear for diffusion into γ Fe. The values of Q and D ₀ (the frequency factor) obtained in pure γ Fe are more representative than the values Q = 344·5 kJ mol⁻¹ and D ₀ = 5·30 × 10⁻² m² s⁻¹ found in the literature. It was possible to examine the influence of the substitutional alloying elements Mn and Si on the behaviour of Nb diffusion in γ Fe and the following effects were observed: the diffusivity of Nb decreases with Mn content but increases with Si content and when both Mn and Si are added the two effects appear to compensate each other.