Simulations of the effects of 2-D interstitial diffusion on void lattice formation during irradiation

This paper describes the use of simulation techniques to examine some of the processes involved in the alignment of voids under the influence of 2-dimensional self-interstitial atom (2-d SIA) transport. It follows an earlier paper in which the effects of 1-d SIA transport were investigated and uses similar methodology. In contrast to the 1-d SIA results, with 2-d SIA transport there is no difficulty in simulating the formation of simple cubic void lattices. In addition the work has been extended to demonstrate the formation of perfect bcc and fcc void lattices. One important feature was that lattice formation took place some 100 times faster than found experimentally. The possibility that this might be due to a diluting effect of out of plane jumps was investigated but seemed very unlikely. This led to the alternative conclusion that the 2-d diffusing defect could not be the basic single interstitial but instead was a larger interstitial defect, such as a di-interstitial, present in far lower concentrations. One consequence of the new approach is that the phenomenon of void swelling would be essentially unaffected by the defect responsible for void lattice formation. This crucially avoids the fast swelling which, as suggested recently, might otherwise occur as void lattices form, and which would conflict with the often quoted association of void swelling saturation and void lattice formation. This association is now more likely to be controlled by a common factor such as high void density than by any direct mechanism. A possible explanation for the influence of impurities on void lattice formation in ion irradiated Nb, Nb-1% Zr, Ta, and Mo is discussed.