Elucidating the mechanism of strain-driven <100> dislocation loop formation in bcc iron

Dislocation loop is a primary indicator of irradiation, and has a profound impact on the microstructural evolution and thermo-mechanical properties of materials. Although both 1/2<111> and <100> loops are clearly identified in bcc metals after irradiation, the underlying mechanism of <100> loop formation remains elusive especially considering its low stability. Here, we explicitly demonstrate the formation of <100> loops through the interaction of two gliding 1/2<111> loops under uniaxial strain via molecular dynamics simulations, while there is no occurrence of <100> loops under strain-free conditions. Such strain-enhanced formation of <100> loops is different from conventional dislocation reaction mechanisms, which contain rigorous prerequisite conditions on the topology and size of the reactant loops, and thus provide a potential explanation for the frequent occurrence of <100> loops. The microscopic analysis suggests that the generation of <100> dislocation loops via bi-loop reaction is a complicated atomistic process involving the coordinated movement and/or rearrangement of multiple interstitials. The activation energy barriers of each reaction step are determined, and generally decrease with increasing uniaxial strain. Specifically, we develop a predictive model to describe the formation probability of <100> loops under different conditions, which is in good agreement with molecular dynamics simulations. These results shed new light on understanding the <100> loop formation, provide a direct link between simulations and experiments, and enable the accurate assessment of irradiation damage evolution in bcc metals.