Electrotransport of cobalt, iron and chromium in BCC zirconium

Abstract

The electrotransport behaviour of Co, Fe and Cr in beta -zirconium from 1557 K to 1950 K was investigated. The radioactive tracer technique was used in this study which included 60Co, 59Fe and 51Cr. All three solutes were found to possess negative effective charge (Z*) in the temperature range investigated. This means that all these solutes migrate towards the anode, indicating that the electron wind force dominates in the electrotransport process in BCC zirconium. The Z* values were also found to increase with increasing temperature as expected. The diffusion coefficients of these three solutes, Co, Fe and Cr, were found to obey the simple Arrhenius behaviour with activation energies of 1.00, 1.15 and 2.27 eV, respectively. These values suggest that Co and Fe diffuse interstitially in beta zirconium. Comparison with available data in the literature, especially at lower temperatures, shows that Co and Fe diffuse with their respective single activation energy values given above at all temperatures in the beta phase. However, the diffusion of Cr exhibited the anomalous behaviour commonly observed in the self diffusion of BCC metals (curvature in the Arrhenius plot). This anomalous behaviour can be attributed to the soft-phonon contribution of the omega -phase, in which case Cr is assumed to diffuse via a vacancy mechanism. The implication of the findings in this study on electrotransport purification are also discussed.