Modelling metastable configurations of vacancy and interstitial defects in Fe and W

Iron-based ferritic-martensitic steels are major structural components of fusion power plant design, and tungsten alloys and composites are among the most promising plasma facing materials. It is therefore important to understand the differences and similarities between the structure and behaviour of the atomic defects produced in these metals by irradiation. Using molecular dynamics (MD) simulations we investigate how the relative stability of vacancy loops, planar voids (open vacancy loops), and spherical voids varies as a function of their size. We find that conventional collapsed vacancy loops become energetically favourable relative to planar voids only once they exceed a critical size (∼2 nm in Fe, 3.5 nm in W), whereas a spherical void represents the most stable configuration for a cluster of vacancies of any size. Simulations under constant temperature conditions show the mechanism by which vacancy-evaporation and reattachment can allow metastable planar void configurations to transform into these energetically stable spherical voids. These results explain why the vacancy loops observed experimentally in irradiated Fe have the size that is systematically larger than 2 nm. At the same time the disagreement between the predicted critical stable vacancy loop size of 3.5 nm and experimental data for irradiated W shows that vacancy clusters produced by collision cascades in W are metastable. This is confirmed by the fact that the characteristic times t ∼ L2/Dv of formation of vacancy clusters in a collision cascade under normal conditions range from approximately one second in iron to 1016 seconds in tungsten. Comparison of formation energies of interstitial and collapsed vacancy loops, which shows that interstitial loops are of lower energy, indicates an elastic 'asymmetry' between the two structures. Similar asymmetry is observed in the tension and compression strain fields of the two types of defects. Long timescale MD simulations of equivalently-sized defects carried out over a broad temperature range show that vacancy loops diffuse on similar timescales as interstitial loops, but at a systematically slower rate. (authors)