Tungsten is a candidate for plasma-facing components in fusion reactors, where it must withstand extreme heat and neutron irradiation. Nanocrystalline tungsten (NC-W) shows improved radiation tolerance due to grain boundaries acting as defect sinks, but its plasticity degrades after irradiation. Although the mechanical response depends on the interplay between grain size and defect microstructure, the underlying mechanisms remain unclear. Using molecular dynamics, we investigate the combined effects of grain size and vacancy defects on the plastic deformation of NC-W. In defect-free samples, decreasing grain size shifts deformation from dislocation glide to twinning, leading to strengthening followed by softening, with a Hall-Petch to inverse Hall-Petch transition at approximately 25.0 nm. Vacancy defects alter this behavior: randomly distributed single vacancies cause only minor softening, whereas vacancy clusters suppress twinning and promote dislocation-mediated and grain-boundary-mediated deformation. Consequently, vacancy clustering weakens the grain-size dependence of flow stress and promotes vacancy-assisted softening, accompanied by pronounced strain localization. These results indicate that grain refinement alone cannot ensure mechanical stability in irradiated W. Instead, optimal performance requires coordinated control of grain size and vacancy defects, particularly through microstructures that suppress vacancy-cluster accumulation.

Interstitial hydrogen (H) clusters with rock salt structure exhibit energy stability in tungsten (W) and play a crucial role in enhancing its hardness. However, the underlying physical mechanisms and the specific hardening behavior remain unclear. To this end, we systematically investigate the effect of the H cluster on the slip behavior of an 1/2 [111] (110) edge dislocation in W by using the molecular dynamics method. We first study the slip of the edge dislocation in the absence of the H cluster, which reveals typical phonon drag control characteristics. Based on this, a slip drag coefficient B(T) is obtained, enabling accurate prediction of the dislocation mobility under various temperatures and stresses. In the presence of H clusters, the hardening effect in W is significantly enhanced. Notably, the geometric parameters of the H cluster, i.e., height and diameter, exert significant regulatory influence on the slip behavior of the edge dislocation through a quantitative correlation. Furthermore, the critical resolved shear stress (CRSS) displays a slight dependence on temperature within the range of 100 K-800 K, indicating that the dislocation motion is primarily governed by the geometry of the H cluster. These results provide new insights into the mechanisms of H-induced irradiation hardening in W, offering valuable data to support the development of high-performance W-based materials with enhanced irradiation resistance and long-term service stability.

Capture efficiency is a key metric for evaluating the ability of grain boundaries (GBs) to absorb irradiation defects. However, the accurate description of capture efficiency and clarifying the influence of GB characteristics remains a formidable challenge. In this study, we systematically investigate the interaction of point defects (PDs) with different GBs in tungsten using molecular statistic/dynamic, elastic dipole tensor, and kinetic Monte Carlo methods. It is found that the formation energy and absorption range of PDs in high-angle GBs (HAGBs) is much higher than that in other GBs, implying the strong interaction strength of PDs with HAGBs. More importantly, we emphasize the significance of net strain field (total strain field minus absorption range), which shows a strong positive correlation with capture efficiency of GBs. Therefore, different from the interaction strength, the capture efficiency and sink strength of low-angle GBs (LAGBs) are much higher than that of HAGBs, which is in good agreement with the experiments. Besides, temperature and grain size show significant sensitivity in the capture efficiency for different GBs. The capture efficiency and sink strength of LAGBs are 3-4 times higher than that of HAGBs at 600 K, while this difference decreases to below 50 % at 1200 K. Additionally, the capture efficiency for a 10 nm grain size increases by approximately 0.3-1.2 times compared to 40 nm. These results not only explicitly clarify the influence of GB characteristics on the capture efficiency of GBs for PDs, and provide an important reference to the development of radiation-resistant materials.

The pursuit for long-term uranium storage, handling and disposal of spent nuclear fuels has driven extensive explorations on uranium oxidation over the past decades. Experimentally, the oxidative kinetics of different stages were proposed, but an atomic insight into the oxidation process remains elusive. In this work, by combining particle swarm optimization algorithm with first-principles calculations, we systematically investigate the initial oxidation behaviors of gamma-U(001) surface and the physicochemical properties of the grown oxide layer. It is found that the oxidation initiates with the chemisorption of O on the surface, and with increased O coverage, O atoms penetrate into the surface that ultimately result in the formation of a well-ordered, compact UO2(111) film. Calculations on the adsorption of O2 reveal that compared to pure gamma-U(001) surface, the growth of a UO2(111) film greatly weakens the molecule-surface interaction. Remarkably, a distinct chemisorption-tophysisorption transition is revealed with increasing thickness of the oxide layer, highlighting its protective role against oxidation. Effects of the defect in oxide are also discussed, and the underlying electronic properties are analyzed. Our work not only provides fundamental insights into the initial oxidation of uranium in the context of nuclear industry, but also sheds light on future design and fabrication of corrosion-resistant metals/ alloys for diverse applications.

In irradiated tungsten (W), the formation of numerous helium (He) bubbles has a significant impact on the diffusion behaviors of hydrogen (H) isotopes. To investigate the influence of the microstructure of He bubbles on the diffusion behaviors of deuterium (D), we utilize the reconstructed experimental data, the phase-field method, and the steady-state diffusion equation to calculate the effective diffusion coefficients of D based on four possible diffusion paths, i.e., D atoms diffuse in the bulk W, inside the He bubbles, and along the outer and inner surface of He bubbles. Simulation results based on the reconstructed depth-dependent distribution of He bubbles reveal that the diffusion paths remarkably affect the effective diffusion coefficient of D. By varying the radius and number density of He bubbles, the effective diffusion coefficient of D undergoes significant changes. Subsequently, we fit an empirical formula of the effective diffusion coefficient of D as a function of the radius and number density of He bubbles for different diffusion paths of D based on simulation data. Besides, the growth behaviors of He bubbles are simulated by the phase-field model, and the effective diffusion coefficient of D as a function of the evolutionary time for different diffusion paths is also discussed. At last, we investigate the influence of the D concentration and temperature on the effective diffusion coefficient of D along different diffusion paths. These results indicate that the effective diffusion coefficient of D increases with decreasing the D concentration and increasing the temperature for four diffusion paths. The current study provides a reference for investigating the diffusion behaviors of D in W in the presence of He bubbles, and may benefit the efforts of developing low D retention W-based materials.

In this work, we develop a phase-field model to simulate the growth behaviors of the dislocation loop in irradiated tungsten. The model considers the fact that plenty of defect clusters such as dislocation loops and voids are formed by the diffusion and aggregation of self-interstitial atoms (SIAs) and vacancies created by cascade collision damages. The stable void phase and plate-like morphology of the dislocation loop can be reproduced. We then study the effects of the concentration of SIAs, kinetic coefficient, lattice misfit strain energy, applied shear stress, and the type of the dislocation loop on the growth behaviors of the dislocation loops. It is found that the growth rate of the dislocation loop increases with increasing the concentration of SIAs, kinetic coefficient, and applied stress. The current results can help to understand the growth behaviors of dislocation loops in irradiated W and other irradiated materials.

Employing the smooth overlap of atomic position (SOAP) descriptors, we established an artificial neural network (ANN) model with the ability to effectively and accurately predict the segregation energy Eseg distributions of hydrogen (H) atoms in various strained regions of tungsten (W). The model is verified to have comparable accuracy with the molecular statics (MS) relaxations, as quantitively evidenced by the root mean square error (RMSE) values of 0.018 eV for the homogeneously strained regions, averagely 0.025 eV for regions near different dislocations, and 0.032 eV for regions surrounding an interstitial H cluster. Besides, the predictions yield no systematic bias at both high and low Eseg extremes, which is a common issue in the elastic theory (ET) calculations. Moreover, Eseg values are precisely predicted with the RMSE value of 0.041 eV surrounding an interstitial H cluster that is not encountered during the learning phase of the model, demonstrating the model’s robust generalization capability. Considering its markedly enhanced computation speed, this accurate ANN model holds promising prospects in circumstances that require massive Eseg calculations.

Irradiation-induced cascade collisions produce numerous point defects within materials, which can severely deteriorate their thermo-mechanical properties and overall performance. We propose a computational scheme that combines molecular dynamic (MD) simulations with a denoising diffusion probabilistic model (DDPM) to rapidly and accurately predict the spatial coordinates of point defects at any given primary knock atom (PKA) energy, ranging from 0 to 100.0 key. Importantly, this capability extends to PKA energies that are exclusive from the training data set, demonstrating the robustness and generalizability of the model. The proposed scheme has been thoroughly validated by several designed indicators, including the Frechet inception distance, the number of point defects, the distance from vacancies and self-interstitial atoms (SIAs) to their respective centroids, the inter-centroid distance between the vacancies and SIAs, the probability density of clustered defect sizes, and the sub-cascade number. Compared to MD simulations, the DDPM can generate point defects at a specific PKA energy at least ten thousand times faster. By offering a rapid and reliable means to model defect distributions across various energy levels, the proposed scheme benefits the comprehension of the cascade process and provides a valuable database for both experimental investigations and large-scale simulations.

We systematically study the interaction behaviors between a point defect (PD) and two types of 1/2 <111> {110} and 1/2 <111> {112} edge dislocations using molecular dynamics (MD), elastic dipole tensor, and kinetic Monte Carlo (KMC) methods in tungsten (W). We first determine the capture region of a vacancy and four types of self-interstitial atoms (SIAs) by calculating their binding energies to both edge dislocations using MD. We then find anisotropic distributions of the diffusion energy barriers of PDs in dislocation systems employing the MD and elastic dipole tensor methods. Subsequently, we utilize the KMC method to obtain the lifetime of PDs and calculate the capture efficiencies of both edge dislocations to PDs at various temperatures and dislocation densities. Results show the capture efficiency of a vacancy decreases with increasing temperature, while it first increases and then decreases with increasing temperature for SIAs. As dislocation density increases, the capture efficiency of PDs increases. Ultimately, the swelling rate is estimated based on the capture efficiency and falls within the range of experimental data. The swelling rate increases first and then decreases with increasing temperature, and the maximum swelling rate occurs around 1200 K, while it increases with increasing the dislocation density. The present results are critical for understanding the interaction behaviors between PDs and 1/2 <111> edge dislocations in W, which provide an essential database for large-scale simulation methods such as rate theory and phase-field simulations.

We systematically study the interaction behaviors between a point defect (PD) and two types of 1/2 (111) {110} and 1/2 (111) {112} edge dislocations using molecular dynamics (MD), elastic dipole tensor, and kinetic Monte Carlo (KMC) methods in tungsten (W). We first determine the capture region of a vacancy and four types of selfinterstitial atoms (SIAs) by calculating their binding energies to both edge dislocations using MD. We then find anisotropic distributions of the diffusion energy barriers of PDs in dislocation systems employing the MD and elastic dipole tensor methods. Subsequently, we utilize the KMC method to obtain the lifetime of PDs and calculate the capture efficiencies of both edge dislocations to PDs at various temperatures and dislocation densities. Results show the capture efficiency of a vacancy decreases with increasing temperature, while it first increases and then decreases with increasing temperature for SIAs. As dislocation density increases, the capture efficiency of PDs increases. Ultimately, the swelling rate is estimated based on the capture efficiency and falls within the range of experimental data. The swelling rate increases first and then decreases with increasing temperature, and the maximum swelling rate occurs around 1200 K, while it increases with increasing the dislocation density. The present results are critical for understanding the interaction behaviors between PDs and 1/2 (111) edge dislocations in W, which provide an essential database for large-scale simulation methods such as rate theory and phase-field simulations.

Loop-punching is the manner a helium (He) bubble enlarges itself through emitting a dislocation loop during its growth in metals. It has been demonstrated that there are two types of loop-punching: the two-stage loop-punching for small bubbles and the direct loop-punching for large bubbles. In this work, via the molecular dynamics (MD) simulations of helium bubble growth in tungsten (W), we revealed the crucial process from the bubble-attached self-interstitial atoms (SIAs) to the escapable dislocation loop in the two-stage loop-punching. Different from the conventional theory that the loop is merely the collection of the sequentially kicked-out SIAs, it is formed by the synergistic action of the atomic plane on the bubble surface being pushed into interstitial sites and the attached SIA cluster slipping. Building on this, two plastic deformation (PD) modes, the tangential PD and the normal PD, are extracted due to the generated SIAs’ directions tangent or normal to the bubble surface. The SIA-accumulation stage and the loop-emission stage of the two-stage loop-punching correspond to the tangential PD and the normal PD, respectively, while the direct loop-punching merely consists of the normal PDs. Therefore, the direct loop-punching is only a special form of the two-stage loop-punching that contains zero previous tangential PDs. Further, it is found the temperature plays an important role in the PD mode switch, suggesting the modes compete on the energy barriers, which accounts for the detachment of the attracted SIA cluster from the bubble.

搭建了劳埃德式紫外激光干涉光刻系统,利用劳埃德镜产生双光束分波阵面干涉,通过精确调整、控制光束的入射角,设计了曝光图样.基础实验部分可以完成一维光栅制备,分析光栅周期与波长、入射角的关系;在拓展实验中,制备设计部分可以完成二维点阵或准晶结构制备,前沿拓展部分可以制备表面等离极化激元微腔.基于劳埃德式激光干涉光刻实验操作便捷、展示度高,同时涵盖光干涉原理的介绍和光路调整、样品制备和表征等实验内容,有助于学生掌握激光干涉光刻的实验技能.

Previous experimental studies have demonstrated that the recrystallization in nuclear materials is very sensitive to the annealing temperature, dislocation density, and original grain morphology. However, the synergistic effect of these intrinsic and extrinsic factors on recrystallization has been rarely studied due to the elevated temperatures of recrystallization and the costliness of experiments, especially in tungsten (W). In the present work, we have developed an approach that combines a phase-field model with the physics-based classical nucleation theory to study the synergistic impact of these factors on the recrystallization process. We systematically investigate the synergistic effect of annealing temperature, dislocation density, and original grain morphology on the recrystallization rate and the average recrystallized grain size. The simulation results show that increasing the dislocation density and the annealing temperature can effectively reduce the average grain size after full recrystallization. For an annealing temperature above 1523 K, the recrystallization rates have minor changes with increasing the dislocation density and annealing temperature. Furthermore, we employ an empirical model to quantitatively calculate the Vickers hardness of deformed W during the recrystallization process based on the phase-field microstructures. Notably, columnar grain crystals are found to be more effective in reducing irradiation hardness than isometric grain crystals. We believe that these simulations can provide a valuable reference for the preparation and design of radiation-resistant W materials.

Tungsten (W) is regarded as a viable choice for plasma-facing materials in nuclear fusion reactors. However, its mechanical properties are significantly degraded by hydrogen (H) atoms during irradiation, of which the mechanism is still elusive. In this study, we conduct molecular dynamics (MD) simulations to study the impact of H atoms on the propagation of a crack in single crystal W. The results show that the propagation rate of the crack slows down with increasing temperature. This is due to the enhanced plastic deformation, leading to blunting of the crack tip. A pre-existing crack in W is then considered at various temperatures and uniaxial applied tensile strain conditions. The propagation rate of the crack decreases with the increase of the applied tensile strain rate. This phenomenon occurs due to the relaxation of the stress around the crack tip following the emission of the dislocation at high strain rates. After introducing H atoms, it can be observed that at low temperatures, H im-pedes the propagation of the crack, while at high temperatures, H promotes it. This is primarily due to the formation of voids at the slip traces of dislocations and the reduction in surface energy. Additionally, the crack tip becomes blunted and its propagation rate decreases with increasing strain rate. These results indicate that providing sufficient time for H atoms to migrate is a key factor affecting the mechanical properties of W. The current results provide valuable insights into understanding the interaction mechanism of a crack and H atoms in W.

随着信息技术的快速发展和学科间不断交叉渗透,作为与理论物理和实验物理并列的第三种研究物理现象和规律的方法,计算物理变得越来越重要.然而,由于学科发展相对较晚以及学科自身的一些特点,本科生和研究生的计算物理课程教学尚未形成较成熟的体系.本文主要介绍北京航空航天大学本科生和研究生计算物理课程协同建设的探索和实践情况.本科生课程定位于掌握计算物理的基础理论知识和解决物理问题的案例实践,课程教学小班化;研究生课程定位于构建计算物理坚实的专业基础和宽广的知识结构,突出专业性和前沿性,重视计算实践能力,并开设计算物理二级学科平行课.本文主要介绍了从课程内容、教学方式和考核手段等方面进行的协同建设的探索与实践,这项探索使得本科生和研究生课程在内容、结构和方式上具有连续性和统一性.

Using a phase-field approach with the heat conduction equation, we predict the grain growth behaviors in tungsten (W) and their effects on effective thermal conductivity. Results show that the simulated grain growth kinetics is basically consistent with experimental observations. An empirical correlation is derived, describing the averaged grain area as a function of temperature and time. Further, we study the effect of grain growth, columnar crystal structure, and recrystallization on the effective thermal conductivity of W. It is found that the effective thermal conductivity increases nonlinearly with increasing grain size, and a simple correlation of converting two-dimension into three-dimension effective thermal conductivity is obtained. Interestingly, the effective thermal conductivity of the columnar crystal is relatively high along the elongated direction and higher than that of the isometric crystal. Nevertheless, the effective thermal conductivity decreases with the occurrence of the recrystallization due to the increased grain boundary density. Our results reveal that grain growth and grain structure can affect the capacity of heat transfer at high temperatures, which could be considered in the transient event of the long-time service of W materials in fusion devices.

Incident neutrons in irradiated W can cause the formation of solid transmutation elements, of which Rhenium (Re) is the most abundant. We apply the phase-field method to investigate the clustering and growth of the Re-rich precipitate in irradiated W based on the spinodal decomposition mechanism, and their effects on the mechanical and thermal properties of W. Needle-like precipitates are reproduced and comparable to experimental observations. We then vary the irradiation dose and temperature to study their influences on the microstructure evolution of the Re-rich precipitate. Simulation results show that the average diameter, the number density, and the coverage rate of the precipitates significantly increase with the increase of the irradiation doses but slightly increase with the increase of the temperature. The effects of the Re-rich precipitate on the mechanical and thermal properties of W are also investigated. Results show that the Vickers hardness increase and the thermal conductivity degrade due to the formation of the Re-rich precipitate, especially at high irradiation doses. Conventional simulation methods can hardly handle the effect of the needle-like precipitates on the mechanical and thermal properties. In this work, we compare these two properties by using needle-like precipitates and circular precipitates. Our results clearly show that the needle-like precipitates give a better consistency with experimental results of the Vickers hardness increase, and reveal the anisotropic ability of the heat transfer in neutron-irradiated W. The current results can provide a systematic understanding of the Re clustering behavior from the microstructure evolution to its influences on the mechanical and thermal properties of W materials.

In this work, we propose an efficient numerical method to study the effects of microstructures on the effective diffusion coefficient of the diffusion component in materials. We take the diffusion of hydrogen (H) atoms in porous polycrystalline tungsten (W) as an example. The grain structures and irradiated void microstructures are generated by using the phase-field model. The effective diffusion coefficients of H in these microstructures are obtained by solving the steady-state diffusion equation, using a spectral iterative algorithm. We first validate our simulation code for calculating the effective diffusion coefficient by using three simple examples. We then investigate the effects of the grain morphology and porosity on the effective diffusion coefficient of H in W. Regardless of whether the grain boundary is beneficial to the diffusion of H or not, it is found that the effective diffusion coefficient of H along the elongated grain direction in columnar crystals is always greater than that in isometric crystals. The increase of the porosity can significantly decrease the effective diffusion coefficient of H from the simulations of the porous W. A correlation of converting the two-dimensional (2D) effective diffusion coefficient into three-dimensional (3D) in the porous and polycrystalline W is fitted by using our simulation data, respectively. Two fitted correlations can be used to predict the synergistic effect of the porosity and grain boundary on the effective diffusion coefficient of H in W. Consequently, our simulation results provide a good reference for understanding the influence of the complex microstructures on H diffusion, and may help to design W-based materials for the fusion reactor.

正电子湮没技术能够快速、精确、无损地对材料内部微观缺陷进行探测.将慢正电子束多普勒展宽能谱仪引入实验教学,选取核聚变堆壁材料金属钨为测试对象,调节正电子束能量,采集正电子在钨中湮没产生的γ光子信号.通过对γ光子多普勒展宽能谱进行处理分析,获得辐照前后样品近表面空位型缺陷数量随深度的变化规律,使学生掌握基于正电子湮没技术研究材料缺陷行为的实验技能.

Tungsten (W) is considered to be the most promising plasma-facing material in fusion reactors. During their service, severe irradiation conditions create plenty of point defects in W, which can significantly degrade their performance. In this work, we first employ the molecular static simulations to investigate the interaction between a 1/2[111] dislocation loop and a vacancy-type defect including a vacancy, di-vacancy, and vacancy cluster in W. The distributions of the binding energies of a 1/2[111] interstitial and vacancy dislocation loop to a vacancy along different directions at 0 K are obtained, which are validated by using the elasticity theory. The calculated distributions of the binding energies of a 1/2[111] interstitial dislocation loop to a di-vacancy and a vacancy cluster, showing a similar behavior to the case of a vacancy. Furthermore, we use the molecular dynamics simulation to study the effect of a vacancy cluster on the mobility of the 1/2[111] interstitial dislocation loop. The interaction is closely related to the temperature and their relative positions. A vacancy cluster can attract the 1/2[111] interstitial dislocation loop and pin it at low temperatures. At high temperatures, the 1/2[111] interstitial dislocation loop can move randomly. These results will help us to understand the essence of the interaction behaviors between the dislocation loop and a vacancy-type defect and provide necessary parameters for mesoscopic scale simulations.