Mechanism of one-dimensional glide of self-interstitial atom clusters in alpha-iron

Recent molecular dynamics (MD) computer simulations of pure copper and iron have shown that clusters consisting of up to a few tens of self-interstiti al atoms (SIAs) are highly mobile along close-packed crystallographic direc tions. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms o f cluster motion. In the present paper, results of MD modelling of the ther mally-activated motion of clusters of three, nine and 17 SIAs in alpha -iro n in the temperature range from 90 to 1400 K are analysed. The correlation between the motion of the centre of mass of a cluster and the individual ju mps of its constituent SIAs is revealed. It is found that the SIAs in a clu ster jump almost independently and their jump frequency depends on the numb er of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The reason for th e deviation of the cluster jump frequency from a simple Arrhenius relations hip is discussed. It is shown that such clusters only exhibit an effectivel y random walk, that is a correlation factor of one, when the jump length de fining diffusion is taken to be 3b to 4b, where b is the magnitude of the v ector 1/2 (111).