Fig. 5: Anomalous low-temperature irradiation creep.

a Irradiation creep is driven by the coarsening and coalescence of highly mobile self-interstitial clusters. At low dose, irradiated microstructure contains mostly interstitial-type (red) and rarely vacancy-type (blue) dislocation loops, as well as dispersed vacancies. With further exposure, the interstitial-type loops grow and coalesce, eventually forming a complex, system-spanning dislocation network of mixed character which enables plastic deformation of the crystal. Shown are defect clusters (N > 2) and dislocations (1/2\(\left\langle 111\right\rangle\): green, \(\left\langle 100\right\rangle\): red) as identified with the Wigner-Seitz and DXA methods, respectively. The reference crystal for Wigner-Seitz analysis is chosen to minimise the number of point defects, allowing for direct visualisation of plastic deformation. b Interstitial and vacancy-type loops orient themselves into opposing directions relative to the external uniaxial stress direction, which leads to a stress-bias in the eventual formation of the dislocation network. Shown are the mean alignments for a given dose (dots) and spline-interpolated values (line) with standard error (shaded). The dashed line indicates prismatic alignment. c The extent of volume swelling is independent of external stress and can be well estimated by 0.64 times the vacancy concentration, with the numerical factor arising from comparison of interstitial and vacancy defect relaxation volumes, see text. Shaded areas indicate the standard error over five repeated simulations.