Dislocation-enhanced experimental-scale vacancy loop formation in hcp Zirconium in one single collision cascade.

Wei Zhou,Jian Zheng,Jianming Xue,Shuming Peng,Jiting Tian

doi:10.1038/srep21034

Wei Zhou, Jian Zheng + Show 3 more

Open Access

https://doi.org/10.1038/srep21034

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Abstract

Large defects are the main factor leading to the degradation of material properties under irradiation environments. It is commonly assumed that the large defects are mainly formed through cluster growth under continuous irradiations. Besides this mechanism, recent experiments and simulations show that sometimes an individual ion can also directly create a large defect. Here we report a novel mechanism for the formation of the large defects, as discovered by our Molecular Dynamics (MD) simulations of the collision cascades in hcp Zirconium (Zr): a pre-existing edge dislocation (ED) can significantly promote the nucleation of the vacancy clusters, and even facilitate the direct formation of an experimental-scale large vacancy loop (about 3 nm) in only one single displacement cascade. This dislocation-related mechanism may be the key for understanding the experimental results in the low-dose irradiated Zr where the high-density large dislocation loops are observed but difficult to be explained by the two mechanisms mentioned above. Considering that intrinsic dislocations exist in nearly all crystalline materials, our results provide a significant concept: pre-existing dislocations have a strong influence on the primary damage production, and taking them into account is indispensable for assessing and improving the material’s irradiation-resistance.

Highlights

Experiments where aggregation of point defects is difficult, significant rare cascade events are the key for understanding the damage production[6]
MD simulations show that defect clusters formed in a 20 keV cascade in a single crystal of hcp Zr always contain less than 50 point defects[10], and no simulations for higher-energy PKAs have been performed, implying that whether the rare events could occur in hcp Zr is still unclear
This method for counting defects has previously been used in the research on cascades near a grain boundary[11]

Summary

Introduction

Experiments where aggregation of point defects is difficult, significant rare cascade events are the key for understanding the damage production[6]. A typical example of this kind of the rare events is that Soneda and coworkers[8] have found that in one of 100 MD simulation runs of 50 keV cascades in defect-free alpha-Fe, a large vacancy loop containing 153 vacancies is directly created by a single PKA, while the defect clusters in the other 99 simulations are much smaller (containing less than 50 point defects) Very recently, this kind of rare events are observed in self-ion irradiation experiments of W at cryogenic temperature, and the occurring frequency is in perfect agreement with MD simulations (both less than 1% per ion)[9]. Our results, combined with the traditional “diffusion and aggregation” mechanism, offer a reliable explanation for the formation of high-density large defects in the low-dose irradiation experiments of hcp Zr7

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Conclusion