Energy evolution in nanocrystalline iron driven by collision cascades

TL;DR

The paper investigates how collision cascades affect energy evolution in nanocrystalline iron and how grain boundaries act as defect sinks. Using MD simulations, it compares nanocrystalline samples generated by Voronoi tessellation and severe plastic deformation with defect-free crystals, tracking excess potential energy as a defect proxy under irradiation up to several dpa. A key finding is that irradiation induces grain growth, and all nanocrystalline configurations converge to a similar energetic state at high dose, describable by the model $E = Ad^{-1} + E_{\mathrm{inf}}$ with $A \approx 0.307$ eV·nm and $E_{\mathrm{inf}} \approx 2.2\times 10^{-2}$ eV/atom. This provides a unified energetic endpoint for irradiated iron nanostructures and a simple predictive framework for energy evolution relevant to fusion-relevant materials.

Abstract

Nanocrystalline materials are promising candidates for future fusion reactor applications, due to their high density of grain boundaries which may serve as sinks for irradiation induced defects. We use molecular dynamics to simulate collision cascades in nanocrystalline iron and compare these to collision cascades in initially defect free single crystals. We create nanocrystalline samples via Voronoi tessellation of initially randomly placed grain seeds and via severe plastic shearing. An irradiation induced annealing is observed whereby after ~ 2 displacements per atom (dpa), irradiation drives all simulation cells to a single crystalline state. Irradiation-induced defects that distort the lattice generate elastic strain, so we use excess potential energy as a measure of defect content. At low doses, the Voronoi samples feature a few large, low energy grains, whereas the sheared samples show many small, high energy grains due to the high defect and grain boundary content caused by severe deformation. As dose increases beyond 1 dpa however, all nanocrystalline samples converge to a similar behaviour. Excess potential energy mirrors this trend, plateauing above ~ 4 dpa. We hypothesise that the initially pristine cells will also reach a similar plateau after 5 dpa, which is seemingly confirmed by running a single instance of each cell type to 10 dpa. A model is developed to explain the energy evolution.

Figures (6)

• Figure 1: Grain number, obtained using grain segmentation in Ovito, as a function of dose for all initially nanocrystalline cells. A grain was taken to be a minimum of 100 atoms.
• Figure 2: Excess potential energy versus dose for initially nanocrystalline and pristine cells in eV/atom; (a) Plotted between 10$^{-3}$ dpa and 5 dpa, (b) Plotted between 2 dpa and 5 dpa.
• Figure 3: Damage against excess potential energy for a single instance of each simulation type between 2 and 10 dpa.
• Figure 4: Excess potential energy histograms for individual grains for each starting nanocrystalline configuration; (a) d$_{c}$ = 16.6 nm, (b) d$_{c}$ = 5.2 nm, and (c) sheared cells.
• Figure 5: Predicted evolution of excess potential energy with increasing grain size fitting.