Collision cascade effects near an edge dislocation dipole in alpha-Fe: Induced dislocation mobility and enhanced defect clustering

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Abstract

Collision cascades near a 1/2⟨111⟩{110} edge dipole in alpha-iron have been studied using molecular dynamics simulations for a recoil energy of 20 keV and two temperatures, 20 K and 300 K. These simulations show that the collision cascade induces the migration of the dislocations through glide along its slip plane. The motion of the dislocations starts at the peak of the collision cascade and expands a time scale much longer than the cascade duration, until restoring the equilibrium distance of the dipole, regardless of the damage produced by the cascade. At the initial stages, kinks are formed at the dislocation that enhance glide. When defects reach the dislocations, jogs are produced. We show that the initial dislocation motion is triggered by the shock wave of the collision cascade. The cascade morphology is also strongly influenced by the presence of the dislocations, having an elongated form at the peak of the displacement, which demonstrates the strong interaction of the dislocations with the cascade even at the early stages. Finally, we show that larger vacancy clusters are formed in the presence of dislocations compared to isolated cascades and that these clusters are larger for 300 K compared to 20 K.

Introduction

The mechanical behavior of metals is governed by the characteristics and mobility of their dislocations. In the presence of defects produced by irradiation, this behavior can be altered modifying the plastic response of the material. Effects such as hardening or loss of plasticity are common in metals exposed to irradiation [1], [2]. The development of radiation resistant materials for fusion applications can benefit from a fundamental understanding of dislocation-defect interactions [3].

Molecular dynamics with empirical potentials has been an important tool to provide atomic level information about these phenomena. Most of the calculations performed consider the interaction of a single dislocation with a defect under an applied stress [4], [5], [6]. The interaction of collision cascades with pre-existing dislocations has been studied only recently [7], [8], [9], [10], [11], with most of the calculations performed in f.c.c. crystals [7], [8], [11] and, to a lesser extent, in b.c.c. metals [9], [10]. These simulations have shown how damage produced by the cascade can induce significant changes in the dislocation structure leading to dislocation climb or cross-slip [11] and, in some cases, glide [9]. Calculations in b.c.c. iron next to a single 1/2⟨111⟩{110} edge dislocation have been performed at low recoil energies (5 keV) [10] showing a dependence of the cascade damage on the distance between the dislocation and the initial recoil, with enhanced vacancy clustering at certain distances and reduced defect production for both vacancies and self-interstitials at closer distances from the dislocation.

In this work, we have studied the effect of collision cascades produced by 20 keV recoils on a 1/2⟨111⟩{110} edge dipole and two different temperatures, 20 K and 300 K. After the two dislocations reach their equilibrium distance, an initial energetic recoil is started in the center between the two dislocations. These calculations show that the collision cascade induces glide of the two dislocations, something also observed previously in simulations done in tungsten [9], although not studied in detail. The importance of interstitials on dislocation climb has also been addressed recently through dislocation dynamics [12]. Here, we measured the displacement produced by the two dislocations and study the effect of the defects produced by the cascade on this motion. Even more interesting is the strong influence of the dislocations on the morphology of the collision cascade, even at the very early stages. Finally, we have analyzed the damage produced by the collision cascade in terms of vacancies and self-interstitials and compare the results with those in a pristine lattice, without the presence of the dislocations.

Section snippets

Methodology

We have used in our simulations a b.c.c. Fe crystal constituted by 1.7 million atoms with two perfect edge dislocations, as shown in Fig. 1(a). The x, y and z axes correspond to the [21¯1¯], [011¯] and [111] directions respectively. The dislocation lines are in the x direction which corresponds to the [21¯1¯] direction of the b.c.c. crystal and the y axis is perpendicular to the glide plane of the dislocation. Our cell has dimensions of approximately 283 Å  ×  280 Å  ×  245 Å  in the x, y, and z

Cascade induced dislocation glide

The first noticeable effect observed in these calculations is that the two dislocations in the dipole are displaced from their equilibrium position when the collision cascade is initiated. In order to study the motion of both dislocations after the 20 keV collision cascade in the crystal thermalized at T=20 K (or 300 K), we have identified the atoms close to the dislocation core with CNA [17], [18], choosing only those atoms that do not have a b.c.c. environment. This allows us to discard the

Conclusions

We have studied the influence of dislocations on the evolution of collision cascades in b.c.c. Fe. An edge dislocation dipole is considered where the effects of the dislocations on the collision cascade is more clearly observed. These simulations show that the recoils induce glide of the dislocations, occurring in the early stages of the collision cascade and related to the shock wave produced by the energetic recoil. The dislocations also influence the morphology of the collision cascade,

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

CRediT authorship contribution statement

S. Heredia-Avalos: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. C.D. Denton: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. J.C. Moreno-Marín: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review &

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was partly supported by the Generalitat Valenciana through PROMETEO2017/139. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. EM gratefully acknowledges support from the U.S. DOE, Office of Science, Office of Fusion Energy

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