Spatial ordering of nano-dislocation loops in ion-irradiated materials
  Defect microstructures formed in ion-irradiated metals, for example iron or tungsten, often exhibit patterns of spatially ordered nano-scale dislocation loops. We show that such ordered dislocation loop structures may form spontaneously as a result of Brownian motion of loops, biased by the angular-dependent elastic interaction between the loops. Patterns of spatially ordered loops form once the local density of loops produced by ion irradiation exceeds a critical threshold value. 
S.L. Dudarev, K. Arakawa, X. Yi, Z. Yao, M.L. Jenkins, M.R. Gilbert,   P.M. Derlet
cond-mat.mtrl-sci