Mechanism of one-dimensional glide of self-interstitial atom clusters in α-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-interstitial atoms (SIAs) are highly mobile along close-packed crystallographic directions. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of cluster motion. In the present paper, results of MD modelling of the thermally-activated motion of clusters of three, nine and 17 SIAs in α-iron 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 jumps of its constituent SIAs is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number 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 the deviation of the cluster jump frequency from a simple Arrhenius relationship is discussed. It is shown that such clusters only exhibit an effectively random walk, that is a correlation factor of one, when the jump length defining diffusion is taken to be 3b to 4b, where b is the magnitude of the vector ½(111).