The resistivity recovery of high purity and carbon doped iron following low temperature electron irradiation

Abstract

High purity iron (residual resistance ratio ∼5000) and similar iron doped with carbon (15 at. ppm and 67 at.ppm) have been irradiated with 3 MeV electrons at low temperatures. The resistivity recovery of these specimens has been investigated in some detail. The results allow the following interpretations: Two recovery stages, Stage I _E (120–140 K) and Stage III (220–278 K), are related to free migration of elementary atomic defects. Other, secondary recovery stages related to long-range defect migration, appeared at 164–185 K, and at 340–360 K, the latter is due to the motion of interstitial carbon atoms. The Stage I _E defect and the Stage III defect, both react with carbon. The binding enthalpy for the I _E defect-carbon complex is 0.11 eV, for the Stage III defect-carbon ∼1.1 eV. The specific defect resistivity of the Stage II defect-carbon complex is distinctly lower than that of the Stage I _E defect-carbon complex. We ascribe the Stage I _E defect as an interstitial atom and the Stage III defect as a monovacancy. The secondary long-range migration at 164–185 K can be explained by the migration of di-interstitials. No support for long-range migration of an elementary atomic defect at 520 K could be obtained. These interpretations are in conflict with predictions, derived from high temperature equilibrium observations. Other informations and possible reasons for these discrepancies are discussed. The clue for the understanding of the defect reactions between 220 K and 560 K in carbon doped iron in terms of vacancy migration at 220 K can be seen in the fact that a monovacancy can attract more than one carbon atom, but will be saturated with a distinct small number (e.g. n=6). This has been also emphasized by other authors. The 3 dominant defect reactions in this temperature range are then: at 220 K the formation of monovacancy-carbon complexes by vacancy migration, at 350 K the formation of multiple carbon-vacancy complexes by carbon migration and at 560 K the break-up of these clusters. If this interpretation is true, this sequence of defect reactions should have a wider range of application for example, interstitial solutes in vanadium, niobium and tantalum.