Point defects in body-centered cubic transition metals

Abstract

Body-centered cubic transition metals are sensitive to gas and carbon impurities which can form impurity interstitials. As a consequence of the lattice geometry, impurity interstitials are “elastic dipoles” giving rise to anelastic effects and to interactions with other lattice defects. Research on intrinsic lattice defects such as vacancies and interstitial lattice atoms has taken into account the behaviour of impurity interstitials and the reactions between impurity interstitials and intrinsic defects. Progress in the understanding of gas-metal reactions and advanced techniques such as electron beam melting and ultra-high vacuum technique enable us to carry out successful research work on intrinsic point defects in the impurity-sensitive b.c.c. metals. In the case of tantalum and niobium exceptional conditions exist for purification, doping and control of impurity interstitials. The present state of knowledge on intrinsic point defects in b.c.c. metals is reviewed and available results are collected in tables. Self-diffusion in b.c.c. transition metals is probably related to vacancy formation and migration. Anomalies in some metals are under discussion. Reliable results on formation and migration enthalpies of vacancies are only available on tungsten and to a less extent on molybdenum. The properties of Frenkel-pairs and intrinsic interstitials are reflected in annealing steps of irradiated or cold-worked material. There is no general rule which relates annealing steps to the melting temperature. It appears that one has to distinguish between two groups of metals of different behaviour. First, tungsten and molybdenum possess a large stage III-recovery above room temperature, probably being intrinsic and in the case of tungsten related to the migration of a 〈110〉 interstitial. There exists evidence for another freely migrating interstitial of unknown symmetry at low temperatures (tungsten). Second, the metals V, Nb, Ta and Fe exhibit no intrinsic stage III-recovery above room temperature. These metals may possess only a low-temperature interstitial that in Fe appears to have 〈100〉 symmetry. A small intrinsic stage III in Fe and Ta may occur below room temperature; its interpretation is uncertain. The clustering of intrinsic and impurity defects should be investigated in more detail. Progress is possible, if the purity problem is carefully considered. We mention as the main problems for future work: the vacancy properties in niobium, tantalum and iron and the question whether or not it can be established that in some b.c.c. metals several interstitial type defects may occur.