Effects of impurities on one-dimensional migration of interstitial clusters in iron under electron irradiation

One- dimensional ( 1D ) migration of small interstitial- type dislocation loops was studied for Fe specimens of different purities at room temperature under electron irradiation using a high- voltage electron microscope. Most 1D migration appeared as discrete jumps ( stepwise positional changes ) at irregular intervals, and sometimes involved back and forth motion between certain points. The distribution of jump distances extended to over 100 nm in high- purity specimens; it was less than 30 nm in low- purity specimens. Jump frequency was almost proportional to electron beam intensity and was on the same order as the rate of atomic displacement by electron irradiation. Molecular dynamics simulation suggested the suppression of 1D migration of an interstitial cluster ( 7i ) by an oversized solute Cu atom located in the dilatational strain field of the cluster. We proposed that the 1D jump process occurs in the following sequence: ( 1 ) interstitial clusters are in a stationary state due to trapping effect by impurity atoms, ( 2 ) incident electrons hit and displace impurity atom to cause detrapping, ( 3 ) liberated clusters cause fast 1D migration at low activation energy, and ( 4 ) the cluster is trapped again by another impurity. Experimental results were analyzed and discussed in terms of the proposed model.