Ordering processes and atomic defects in FeCo : Dedicated to Professor Dr. Knut Urban on the occasion of his 65th birthday

Abstract

The present knowledge on self-diffusion and lattice vacancies in near-stoichiometric body-centred cubic FeCo alloys is critically reviewed and related to the establishment of structural order in the alloy after quenching from elevated temperatures. The available data may be consistently explained in terms of the so-called six-jump cycle mechanism of Huntington, Elcock and McCormick, plus the additional concept of ‘pseudo-selfdiffusivity’. It is shown that this leads to an understanding of the observed combinations of high activation enthalpy and large pre-exponential factor in the Fe self-diffusivity on the one hand, and of a lower activation enthalpy and ‘normal’ pre-exponential factor in the Co self-diffusion on the other hand. For some vacancy properties numerical estimates are given. The potential of muon spin rotation measurements for filling existing gaps in our knowledge is emphasized.

A critical discussion of the “500°C anomaly” shows that the explanations attempted in the literature are unsatisfactory. It is proposed that the tetragonality of the compounds FeCo ₃ and Fe ₃Co predicted by ab-initio calculations of Díaz-Ortiz et al. may lead to a metastable pattern of anti-phase domains on a very small scale, the dissolution of which may cause the anomaly.