Slow relaxation of cascade-induced defects in Fe

TL;DR

This study uses kinetic Monte Carlo simulations to analyze the slow relaxation of cascade-induced point defects in alpha-iron, revealing multiple atomistic effects and suggesting both Eyring and Gibbs models contribute to the relaxation process.

Contribution

It provides an atomistically-based assessment of models explaining slow defect relaxation, focusing on vacancy and SIA aggregation in alpha-Fe.

Findings

Defect concentration heterogeneities influence relaxation times

Mobility enhancement affects defect evolution

Cluster size and pressure impact defect stability

Abstract

On-the-fly kinetic Monte Carlo (KMC) simulations are performed to investigate slow relaxation of non-equilibrium systems. Point defects induced by 25 keV cascades in $\alpha$-Fe are shown to lead to a characteristic time-evolution, described by the \emph{replenish and relax} mechanism. Then, we produce an atomistically-based assessment of models proposed to explain the slow structural relaxation by focusing on the aggregation of 50 vacancies and 25 self-interstital atoms (SIA) in 10-lattice-parameter $\alpha$-Fe boxes, two processes that are closely related to cascade annealing and exhibit similar time signature. Four atomistic effects explain the timescales involved in the evolution: defect concentration heterogeneities, concentration-enhanced mobility, cluster-size dependent bond energies and defect-induced pressure. These findings suggest that the two main classes of models to explain slow structural relaxation, the Eyring model and the Gibbs model, both play a role to limit the rate of relaxation of these simple point-defect systems.