Atomistic and experimental study of microstructural evolution in nanocrystalline iron subjected to irradiation

TL;DR

This study combines atomistic simulations and experiments to investigate how nanocrystalline iron's microstructure evolves under irradiation, revealing grain coarsening, defect reduction, and irradiation-induced annealing effects.

Contribution

It provides new insights into irradiation effects on nanocrystalline iron through combined molecular dynamics simulations and experimental X-ray diffraction analysis.

Findings

Nanocrystalline grains coarsen and become single crystalline above 2 dpa.

Lower dislocation density and lattice swelling in nanocrystalline compared to pristine iron.

Experimental data supports simulation trends, showing irradiation-induced micro-strain reduction.

Abstract

Nanocrystalline materials have been proposed for use in future fusion reactors due to their high grain boundary density that may act as a sink for irradiation-induced defects. We use molecular dynamics to model collision cascades in nanocrystalline iron and compare the damage evolution to that observed in initially perfect, single crystalline iron. The nanocrystalline material is generated either by Voronoi tessellation or severe plastic shearing. Upon irradiation, the grains in nanocrystalline simulations coarsen, with all ultimately becoming single crystalline above 2 dpa. Above a damage dose of 1 dpa, nanocrystalline cells show a lower dislocation density and lower lattice swelling than their initially pristine counterparts. Experimental X-ray diffraction data is collected on nanocrystalline iron samples subjected to self-ion irradiation. Line profile analysis data agrees with the trends observed in the atomistic simulations, revealing the presence of an irradiation induced annealing process, with a clear reduction in micro-strain with increasing dose. We attempt to determine why some grains in our atomistic simulations grow, while others shrink, by creating a Toy Model that simulates volume exchange between grains based on different hypothesised exchange mechanisms. This suggests that irradiation-induced grain growth is consistent with random growth.