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In the fusion environment, a complex stress field is generated in materials, which affects the evolution of radiation defects. In this study, the behaviors of radiation-induced defects under the effect of stress gradient field in tungsten are carefully simulated at the atomic scale with the molecular dynamics (MD) method. It was found that the stress gradient field affects the migration properties of interstitial defects, resulting in the energy barriers changing with the stress and stress gradient. In the axial stress gradient field, the movement of the 1/2 <111> interstitial dislocation loop is significantly accelerated, and it tends to move toward the region where the stress is concentrated. Within the time scale of the classical MD simulation, the stress gradient has little effect on the migration of vacancies. These results suggested that the stress gradient would cause interstitial defects to accumulate to the region where the stress is concentrated, thereby significantly changing the properties of the tungsten materials. [ABSTRACT FROM AUTHOR]

Tungsten (W) and W-based alloys are potential candidates for next-generation fusion reactors, which would withstand both irradiation damages and heavy heat load. In this work, we employed the molecular dynamic method to simulate the behaviors of different radiation defects under the effect of the temperature gradient field, which is induced by heat load. The rotation of the ⟨ 111 ⟩ dumbbell and habit plane of 1/2 ⟨ 111 ⟩ interstitial loops is analyzed in detail. The results show that these two behaviors are not significantly affected by the temperature gradient. Contrary to the thermal equilibrium state, temperature gradient results in the directional diffusion of ⟨ 111 ⟩ dumbbell and 1/2 ⟨ 111 ⟩ interstitial loops in tungsten from the cold to the hot region. The energy barrier is also reduced in the temperature gradient field, which accelerates the defect diffusion. These results indicate that the accumulation of radiation defects in the high-temperature region is expected in temperature gradient fields, which would lead to more severe radiation damages and degradation of materials. [ABSTRACT FROM AUTHOR]

In this work, we have carried out a series of displacement cascade simulations in tungsten (W), including seven different primary-knock-on atom (PKA) directions, and PKA energy covers 1 to 300 keV at 363 K; 10, 50, 100, and 200 keV cascades are considered at 600 K, and 10 and 50 keV cascades are simulated at 300 K. Then, a systematic database of W cascade simulations has been established, containing more than 2000 cascades. The number of Frenkel pairs, the type of defect, and its spatial distribution were all determined. The effects of PKA direction, PKA energy and temperature have been discussed in terms of defect generation, defect clustering, and dislocation loops. It is found that, the PKA direction may have an effect on the configuration of defects, such as the formation probability of 〈100〉 interstitial loops, C15 clusters and the sub-cascades, and also has an effect on the number of Frankel pairs when PKA energy larger than 50 keV. Furthermore, from 363 K to 600 K, the number of Frankel pairs is reduced by ∼10%, and high temperature can promote the clustering of interstitial. We further found that temperature may affect the distribution of dislocation loops, and the number of 〈100〉 interstitial dislocation loops is more at 363 K than at 600 K. These results can provide a database of key input parameters for the subsequent larger-scale simulations, and give an important reference for the prediction of defect behaviour, mechanical and thermal properties of W-based materials under fusion neutron irradiation. [ABSTRACT FROM AUTHOR]

• The displacement cascade simulations in pure W and W-Re alloy containing GB and/or σ-phase have been systematically investigated. • Interstitials in W-Re alloy are preferentially absorbed by GB region. • Interstitials are preferentially absorbed by σ-phase, and Re atoms tend to aggregate near the σ-phase region. • The absorption of σ phases and the GB region leads to an increase in the aggregation of interstitials near the σ phase/GB region and a reduction of vacancies in the bulk region Transmutation rhenium (Re) produced in tungsten (W) neutron irradiation not only segregates and precipitates at grain boundaries (GBs) but also forms the σ-phase (WRe) in the bulk. In this study, using a molecular dynamics (MD) method, we conducted collision cascade simulations in W and W–Re alloys with 5 or 10 at% Re atoms containing a GB/σ-phase and systematically examined the effect of GB and the evolution of σ-phase. We find the interstitials in W-Re alloy are preferentially absorbed by the GB region such that the number of vacancies in the bulk region is higher in W–Re alloys than in W; the defect distribution is sensitive to the distance of primary knock-on atoms from the GB plane. Furthermore, similar to the tendency in the GB region, interstitials are preferentially absorbed by the σ-phase, and Re atoms tend to aggregate near the σ-phase region. The absorption of σ-phases and the GB region leads to a considerable defect aggregation, which provides suitable conditions for the regrowth of σ-phase structure during long-term evolution. The results provide a reference for understanding the GB structure and precipitations in the displacement cascades of W and W-based alloy materials. [ABSTRACT FROM AUTHOR]

• The interactions between Re and Σ5(130) grain boundary in W have been systematically investigated. • GBs are sinks for Re atoms through interstitial or vacancy assisted mechanisms. • Re atoms are more likely to locate at substitutional sites nearby the GB in kinetic segregation process. • Re clusters have strong effect on impeding the plastic deformation of the GBs, and may cause the embrittlement of materials. High energy neutron irradiation not only causes the transmutation of tungsten (W), but also induces transmutation elements to segregate and precipitate at grain boundaries. In this work, the segregation of transmutation-produced rhenium (Re) and their clusters at a Σ5(130) grain boundary (GB) in bulk W has been investigated by using both molecular dynamics and molecular statics methods. It is found that, for single interstitial, Re atom diffuses from the bulk to the GB region via a three-dimensional rotation and migration in the form of an interstitial 〈1 1 1〉 Re-W mixed dumbbell, because the rotation and migration barriers of a Re-W dumbbell are both small. When Re atoms are absorbed by the GB, it is noteworthy that Re atoms tend to occupy the substitutional sites near the GB. The segregation energy of interstitial Re is ranged from −7.67 to −6.12 eV, and the effective interaction distance between the GB and Re interstitials is about 7 Å, which increases with increasing the cluster size. By modeling the uniaxial tensile test of W-GB structure containing Re substitution clusters, the fracture strength and elongation of the GB are significantly reduced. The present results provide important insights into the detailed mechanisms of Re segregation at GBs and its possible effects on the mechanical properties of W-based materials. [ABSTRACT FROM AUTHOR]

Tungsten (W) and W-based alloys are the leading candidates for plasma-facing materials (PFMs) in future fusion reactors. However, the high energy neutrons generated in fusion reactions not only result in cascade damages but also cause W transmutation. Both the irradiation defects and transmutation products, mainly rhenium (Re), have serious effects on the service behaviors of W PFMs. In this work, we have systematically investigated the interaction between Re and the self-interstitial atoms, self-interstitial clusters and 1/2<111> interstitial dislocation loops in bulk W using molecular dynamics and statics simulations. It is found that there is a strong attractive interaction between an interstitial W atom and a substitutional Re atom, forming a Re–W dumbbell that migrates 3-dimentionally due to the low migration and rotation energies. The small SIA clusters strongly bind with both the substitutional Re atoms and an interstitial Re atom (Re–W mixed dumbbell), thus decreasing the mobility of these clusters. The strong attractive interaction between a Re atom and a 1/2<111> interstitial dislocation loop occurs when the Re atom is located at the core of the loop, and also, their interaction distance along <111> direction is large. The mobility of the 1/2<111> interstitial dislocation loop decreases progressively with increasing Re concentration. • An optimized W–Re empirical potential has been developed. • The interactions between Re and interstitial defects and dislocation loops in W have been systematically investigated. • There is a strong attractive interaction between Re atoms and interstitial-type defects. • The presence of Re atoms reduces the mobility of the self-interstitial clusters and 1/2<111> interstitial dislocation loops. [ABSTRACT FROM AUTHOR]