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〈100〉

Dislocation Loop Formation and Characterization in Ferritic Materials: Comparison between Experiments and Modeling

Jaime Marian1, Brian D. Wirth1, Robin Schäublin2 and J. Manuel Perlado3 1 Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 2 CRPP, Fusion Technology and Materials, EPFL, Villigen PSI, SWITZERLAND 3 Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, Madrid, SPAIN ABSTRACT Transmission electron microscopy (TEM) observation of irradiated ferritic materials 1 reveals the existence of large, interstitial dislocation loops with Burgers vectors 2 〈111〉 and 〈100〉. These loops cause hardening of the material by pinning dislocations and impeding their glide during deformation. However, numerous molecular dynamics simulations of collision 1 cascades in α-Fe have evidenced the exclusive formation of small, highly mobile, 2 〈111〉 clusters. Additionally, continuum dislocation theory and atomistic simulations have shown that 1 〈111〉 loops are energetically favored. This introduces the need to explain the mechanisms of 2 formation and growth of 〈100〉 loops from small, cascade-produced clusters. The understanding of the physics underlying these phenomena is important for the development of solid damage accumulation models in ferritic materials that are being considered for fusion applications. In this work we propose a comprehensive set of dislocation reactions that explain the nucleation of 1 〈100〉 loops from 2 〈111〉 clusters. The growth up to TEM visible sizes of 〈100〉 loops through 1

absorption of one-dimensionally migrating 2 〈111〉 clusters is also assessed. Finally, a direct comparison of TEM experimental micrographs with atomistic simulation-derived images is presented to show an example of how to help close the gap that exists between modeling and experiments. INTRODUCTION It is well established that examination of ferritic alloys by transmission electron microscopy (TEM) following low dose irradiation (2 consist of aggregates of 〈111〉-oriented split dumbbells [4,5] that exhibit very high mobility for one-dimensional (1D) motion along 〈111〉 directions. As well, MD simulations of displacement cascade evolution have consistently revealed the formation of SIA clusters with 〈111〉 orientations, which again exhibit high mobility [6,7]. Here we provide a mechanism for the formation and growth of 〈100〉 dislocation loops that reconciles the long-standing experimental 1 observation of these defects in irradiated ferritic materials with recent MD studies of 2 〈111〉 cluster stability, their production in displacement cascades and high mobility in one dimension. Finally, we qualitatively compare the MD-simulated loops with actual TEM experimental micrographs through an image-simulation interface. Thus, by using this virtual electron microscope, we partially bridge the gap that exists between multiscale modeling and experiments within the study of radiation damage effects in metals. RESULTS Loop energetics Clearly, an energetics analysis based exclusively