Annihilating vacancies via dynamic reflection and emission of interstitials in nano-crystal tungsten

Abstract

Radiation damage not only seriously degrades the mechanical properties of tungsten (W) but also enhances hydrogen retention in the material. Introducing a large amount of defect sinks, e.g. grain boundaries (GBs) is an effective method for improving radiation-resistance of W. However, the mechanism by which the vacancies are dynamically annihilated at long timescale in nano-crystal W is still not clear. The dynamic picture for eliminating vacancies with single interstitials and small interstitial-clusters has been investigated by combining molecular dynamics, molecular statics and object Kinetic Monte Carlo methods. On one hand, the annihilation of bulk vacancies was enhanced due to the reflection of an interstitial-cluster of parallel crowdions by the GB. The interstitial-cluster was observed to be reflected back into the grain interior when approaching a locally dense GB region. Near this region, the energy landscape for the interstitial was featured by a shoulder, different to the decreasing energy landscape of the interstitial near a locally loose region as indicative of the sink role of the GB. The bulk vacancy on the reflection path was annihilated. On the other hand, the dynamic interstitial emission efficiently anneals bulk vacancies. The single interstitial trapped at the GB firstly moved along the GB quickly and clustered to be the di-interstitial therein, reducing its mobility to a value comparable to that that for bulk vacancy diffusion. Then, the bulk vacancy was recombined via the coupled motion of the di-interstitial along the GB, the diffusion of the vacancy towards the GB and the accompanying interstitial emission. These results suggest that GBs play an efficient role in improving radiation-tolerance of nano-crystal W via reflecting highly-mobile interstitials and interstitial-clusters into the bulk and annihilating bulk vacancies, and via complex coupling of in-boundary interstitial diffusion, clustering of the interstitial and vacancy diffusion in the bulk.

Interaction of radiation-induced defects with tungsten grain boundaries at across scales: a short review

“…To show whether the defects near the GB have a thermodynamic tendency to segregate to the GB (processes 4/4′ and 5/5′ in Fig. 1), researchers conducted molecular statics (MS) calculations of energetic properties of the V/SIA in a certain range of the GB at 0 K [20,[22][23][24][25][26][27][28]. To do this, one removes or inserts an atom near the GB, and then relaxes the GB structure using an energy-minimization method, e.g.…”

“…Extensive theoretical efforts were made to understand the radiation performance of NC W in terms of the defect production [14,[17][18][19][20][21] and energetic and kinetic interactions with GBs of the defects [14,17,20,[22][23][24][25][26][27][28][29]. In this paper, the theoretical advances in the across-scale selfhealing mechanisms for the radiation damage in NC W are reviewed.…”

“…Figure 1 illustrates the possible three interaction types of radiation with GBs in W (types I, II, III). Although a complete interplay picture involves the defect behavior not only near the GB [14,[17][18][19][20][21][22][23][24][25][26][27][28][29] but also in the bulk [30][31][32][33][34] or near the surface [35][36][37][38], the focus in the present paper is on the former one. Type I is relevant to defect production near the GB [14,[17][18][19][20][21].…”

“…The cascade could also directly affect the GB's structure, energy and mobility. Type II is related to the defect behavior near the GB [14,20,[22][23][24][25][26][27][28]. The stable defects formed after the annealing of the cascade interact with the GB via complex processes of segregation, annihilation, clustering, diffusion, dissociation, and emission from the GB.…”

“…Researchers proposed several self-healing mechanisms involving individual or multiple parts of these processes in W [14,20,[22][23][24][25][26][27][28]. These mechanisms include GB-accelerated V/ SIA migration and SIA-V recombination [14,20,[22][23][24][25][26][27][28], interstitial-emission (IE) induced annihilation [24,27,28], coupling of the V approaching to the GB with the interstitial motion within the GB (CVI) [27], and interstitial reflection (IR) by the locally dense GB region [27]. In addition, the defects trapped at the GB also modify the GB's kinetics (type III), leading to other self-healing mechanisms, such as the GB-motion induced trapping of bulk V-clusters [29].…”