The structures and energies of small clusters of vacancies and substitutional solutes in body-centred-cubic metals

Abstract

The possible crystallographic configurations of small clusters of either vacancies or substitutional solutes in body-centred-cubic crystals, in which each point defect has at least one other at either first-or second-nearest-neighbour position, are enumerated, classified and illustrated. The case of mixed clusters of vacancies and solutes is then analysed. There are, for example, 35 configurations for tetravac-ancies, and 155 for mixed clusters of two vacancies and two substitutional solutes. The binding energies of these clusters are then determined using computer simulation techniques based on an empirical potential for iron. For the host-impurity interactions this potential is modified using a single convenient parameter. It is shown that the stability of the clusters is dominated by second-nearest-neighbour interactions. The binding energies increase as clusters become more compact, become linear and when they get larger. Unlike clusters in f.c.c. metals, those in b.c.c. structures do not collapse to take on alternative configurations. The need for additional experimental work on clusters in b.c.c. metals is stressed.