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2020 Vol. 41, No. 6
Published: 2020-12-28
485
Research progress of irradiated helium effect on mechanical properties of metallic materials
Investigation on the mechanical behaviors of irradiated metallic materials with helium effect have much significance for the design of anti-irradiation materials and engineering application. During the high energy neutron irradiation process, helium (He) bubbles can be produced in metallic materials through nuclear transmutation reactions. The plastic deformation behavior of irradiated materials can be significantly affected by the presence of helium bubbles. Hence, we summarized the research on the mechanical behaviors of metallic materials with helium effect from the atomic scale to the macro scale, including the generation of helium atoms, the formation of helium clusters/helium bubbles, the microscopic behavior of helium bubbles in materials and the influence of helium on the macroscopic mechanical properties of materials. Finally, some scientific problem for future study are also presented.
2020 Vol. 41 (6): 485-497 [
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498
Progress in molecular dynamics simulations of irradiation-induced defects in metallic materials
Under irradiation, high-energy particles introduce dense defects inside metallic materials, leading to severe degradation of mechanical properties and reducing the service life of irradiated materials. Since irradiation-induced defects are mostly at the nanoscale, molecular dynamics method is a powerful tool for simulating the defects, and has been widely used in recent years to study the evolution of irradiation-induced defects. In this paper, we introduced the progress of molecular dynamics research on irradiation-induced defects in metallic materials, including collision cascade, point defects, voids, helium bubbles, Frank loops, stacking fault tetrahedrons, as well as their interactions with dislocations and grain boundaries. The mechanisms and models revealed by molecular dynamics method deepen the understanding of irradiation effects, and help to improve the mechanical properties of irradiated materials and design irradiation-resistant materials.
2020 Vol. 41 (6): 498-512 [
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513
Application of Dislocation Dynamics Method in Irradiation Damage Mechanics
Nuclear energy is one of the most ideal energy for humans and plays an important role in the world's power supply. Irradiation, induced by nuclear fission or nuclear fusion, causes damage in materials. This significantly affects the mechanical properties of materials, leading to irradiation hardening, embrittlement, creep, swelling and other phenomena. It is urgent to establish the theory of plasticity mechanics and damage mechanics under extreme irradiation conditions, in order to predict the service life of irradiated materials and design new irradiation resistant materials. Molecular dynamics provides numerous valuable information for understanding atomic-level interaction mechanisms in irradiated materials. However, it is difficult to establish mechanical theoretical models directly, due to the limitation of the considered temporal and spatial scale. Crystalline plastic finite element method can be used to predict the mechanical response of irradiated materials, but this kind of method is based on given physical models, and needs to obtain parameters through fitting experimental data. On the other hand, dislocation dynamics method connects nanomechanics with continuum mechanics. For large quantities of microstructures, it is a powerful method to reveal their cumulative interaction mechanisms, making it possible to establish the theory of plasticity mechanics and damage mechanics based on physical mechanisms. Dislocation dynamics method originated in the 1980s. At first, it was mainly used to study the short-range and long-range interactions between dislocations, and calculate the plastic deformation, hardening, softening, and plastic flow localization caused by dislocation movement. This article will summarize three methods of coupling dislocation dynamics and irradiation damage field, then gives a systematic overview about the recent studies of irradiation hardening, plastic flow localization, grain boundary effects, temperature effects through this method.
2020 Vol. 41 (6): 513-531 [
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532
Effects of collision cascades on the mechanical properties of pearlites
Minsheng Huang Li Zhenhuan
Lamellar pearlites consisting of ferritic and cementite layer is a kind of common metallographic structure in ferritic-alloyed steels, the most widely used structural materials in nuclear energy engineering. Understanding the irradiation effects on the mechanical behaviors of lamellar pearlites is of importance for designing and life-prediction of such materials under high-irradiation conditions. By implementing molecular dynamics simulations, the microstructure variation of cementite/ferritic interface in lamellar pearlites caused by successive low-energy collision cascades are investigated. Besides, the initial yielding behaviors for lamellar pearlites experienced different dose of irradiation are discussed for both uniaxial tensile and compressive loadings. The main conclusions can be summarized as follows: A) The irradiation can destroy the structure of misfit dislocation arrays on the interface, and further promote the diffusion of carbon atoms from the interface to the ferritic phase. B) Under uniaxial tensile loading, collision cascades change the yielding initiation from the activation of {112}<111> slip system to the nucleation and expansion of dislocation loop from the interstitial clusters. C) Under uniaxial compressive loading, collision cascades change the yielding initiation from the activation of {110}<111> slip system to the activation of {112}<111> slip system. D) For both tensile and compressive loading types, the dislocation nucleation stress (or yielding stress) can be increased by the irradiation effects. These results may provide new nano-scale explanations on the irradiation hardening and embrittlement and shed light on the optimal design for ferritic-alloyed steels working under the irradiation environments
2020 Vol. 41 (6): 532-544 [
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545
Effects of material texture on the macro-scale mechanical constitutive relations of FeCrAl alloys
Due to good corrosion resistance and oxidation resistance to high temperatures，Iron chromium aluminum (FeCrAl) alloys have been considered as a promising candidate for accident-tolerant fuel (ATF) cladding materials. Some important texture fibers including α (<110>//RD) and γ (<111>//ND) fiber are observed in FeCrAl alloys after rolling or annealing, which could affect the macro-scale mechanical performances and deep processing abilities. In this study, the representative volume elements (RVE) are developed for the polycrystalline materials of FeCrAl alloys with different textures, and the crystal plasticity model is used to describe the anisotropic behavior of single crystal. By applying periodic boundary conditions and using homogenization theory, the crystal plasticity finite element method is adopted with ABAQUS/Explicit to simulate the macro-scale stress/strain curves under the uniaxial loading. The effects of different textures on the macro-scale mechanical constitutive relations are analyzed. The research results indicate that the stress-strain curves along the rolling direction show small deviations for the materials with random orientations, α and γ fiber. However, γ fiber will result in intensive anisotropy. It is found that the yield strength along the normal direction of rolling plane is much larger than those along the rolling direction and transverse direction for the materials with γ fiber. This is induced by the fact that <111> directions of the grains are parallel to the normal direction of rolling plane, which makes the dislocation slip difficult to be activated. An increase of the texture fraction of γ fiber will enhance the yield strength along the normal direction of rolling plane. For the RVE models with a γ fiber texture, the cumulative shear strains of slip systems follow a normal distribution, and the Standard Deviation with loading along the normal direction is smaller than that with loading along the rolling direction. The simulation results could provide a reference for the optimization of material texture, processing conditions and mechanical performances for FeCrAl alloys.
2020 Vol. 41 (6): 545-554 [
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555
Constitutive Model for Silicone Rubber Foam Over A Wide Range of γ Radiation
Silicone rubber foams are widely used in industrial and biomedical fields. When exposed to γ radiation, silicone rubber foams undergo chemical changes and show degraded mechanical performances. Hence, it is essential to establishment a radiation dose-related mechanical model to describe the mechanical behavior of silicone rubber foams under γ radiation field. In this work, the uniaxial compression behaviors of silica reinforced silicone rubber foams under γ-radiation exposure with a wide range of 0~1000 kGy were investigated by mechanical tests and theoretical modeling. Experimental results show that γ irradiation results in a significant hardening effect on the silicone rubber foam. The initial compression modulus and the stresses for a particular strain both increase linearly with γ radiation dose. The radiation-induced hardening behavior is ascribed to the predominated chemical crosslinking reactions of silicone rubber matrix, which is confirmed by solvent swelling measurement. The crosslink density of the silicone rubber matrix increases linearly with radiation dose. Scanning electron microscope shows that the void structures of silicone rubber foam do not change under the irradiated environment. Based on experimental observations, the Ogden Hyperfoam model is generalized to describe the hyperelastic response of silicone rubber foams over the wide range of γ radiation, by correlating model parameters with radiation dose. Initial shear modulus related parameters are redefined as a linear function about radiation dose, and strain hardening exponents and Poisson ratio related parameters are independent of radiation dose. In the end, model parameters for the second order of the generalized Hyperfoam model are estimated based on uniaxial compression test data, and the prediction ability of the radiation-related model is verified. Results indicating that our model is suitable to characterize the large deformation behavior of silicone rubber foam for the investigated dose range. The model can help to prepare silicon rubber foam materials with high-performance under γ irradiation condition, and design optimized silicone rubber foam structures used in γ irradiation environments.
2020 Vol. 41 (6): 555-566 [
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567
Stability of stacking-fault-tetrahedron-like structures in single copper crystal
Stacking fault tetrahedron (SFT) is a kind of 3D defect commonly seen in the irradiated metals, which could significantly impact the plasticity of metals. Using molecular statics (MS) and molecular dynamics (MD) methods, different sizes and shapes of stacking-fault-tetrahedron-like structures (SFTLS) are created in a single copper crystal by considering different sizes and shapes of vacancy clusters on the (111) plane. We analyze the dislocation evolution mechanisms and variations of formation energies of SFTLS with different shapes during their formation processes. Then, we discuss the vacancy formation energy distribution in the vicinity of a SFTLS as well as the variation of the lowest vacancy formation energy vs the size of SFTLSs. Finally, we analyze the deformation mechanism and yield stress variation of single copper crystal embedded with a SFTLS of different sizes. It is found that SFTLS forms through the process of vacancy cluster collapse, Frank loop dissociation and formation of SFTLS edge due to the intersection of Shockley partial dislocations. The lowest vacancy formation energy site in a SFTLS changes as the size of SFTLS increases, and lowest vacancy formation magnitudes are strongly connected to the transition between stable, metastable and unstable states of SFTLS. Moreover, marked size effect in lowest vacancy formation variation is seen in the stable state of no-apex SFTLS. Furthermore, there are two dislocation nucleation mechanisms corresponding to the incipient plasticity of single crystal with a SFTLS under shear, i.e. glide of Shockley partials on the tilt (111) planes and dissociation of stair-rod dislocations on the basal plane of a SFTLS. The yield stress of single crystal with a SFTLS basically decreases as increasing the size of SFTLS.
2020 Vol. 41 (6): 567-584 [
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585
Atomistic study of equilibrium pressure of helium bubble in irradiated FCC copper and its abnormal size effect
Helium density inside helium bubble and its corresponding internal pressure have a significant influence on the mechanical properties of materials with radiation-induced helium bubble. The equilibrium internal pressure, size effect, temperature effect and stress field of FCC copper containing helium bubble are systematically explored by means of molecular simulations in this paper. Based on energy principle and stress criterion, the method to determine the equilibrium internal pressure is proposed. It is found: 1) The Equilibrium pressures of the bubble obtained by means of the criterion of the minimum energy principle and stress criterion are self-consistent. 2) An abnormal size effect of helium bubble on the equilibrium pressure is found, i.e. when the size of the He bubble is less than a critical one, the equilibrium pressure does not increase but drops slightly with the decrease of bubble size, which is different from the prediction of Young-Laplace equation. The critical size for He bubble is about 3 nm. 3) Compared with molecular simulation results, the traditional Young-Laplace equation overestimates the equilibrium internal pressure of helium bubbles, and the error increases significantly with the decrease of the size of helium bubbles. For instance, when the aperture is 3nm, the relative error exceeds 63%.4) Due to the polyhedron characteristics of the nanoscale helium bubble and the anisotropy of the material, there exists an additional stress field in the surrounding copper matrix of the bubble under the action of the equilibrium internal pressure, which implies that helium bubble-induced stress cannot be eliminated by the equilibrium pressure completely. However, this residual stress decreases rapidly as the increase of the distance from the center of the bubble. In addition, a method to reduce the effect of thermal disturbance or fluctuation on the stress field is proposed by means of the superposition of time average and spatial average. The method adopted in this paper can be extended to investigate the effects of helium bubbles on the mechanical properties of irradiated materials and the interaction between irradiated defects.
2020 Vol. 41 (6): 585-599 [
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600
Irradiation hardening and mechanical properties of high entropy alloy
The multi principal element alloy forms a simple phase structure due to high entropy effect, resulting in excellent properties, such as, high strength, high wear resistance, corrosion resistance, thermal stability, and excellent radiation resistance. However, the prediction of mechanical properties of high entropy alloy induced by irradiation is still lack, which seriously limits the evaluation of its long-term service performance. Based on the crystal plasticity theory and experiment, the cavity-shape dependent hardening behavior, dislocation-ring induced hardening behavior and oxide dispersion enhanced mechanical properties in high entropy alloy are studied. The results show that the spatial interaction of polyhedral voids and dislocations in FCC metal is considered, and the yield stress of irradiated FCC metal is accurately predicted; lattice distortion has an important contribution to the yield strength; oxide dispersion plays a strong role in pinning the dislocation movement, thus affecting the strength and directly determining the radiation resistance. As a new type of structural material with comprehensive excellent mechanical properties, high entropy alloy is expected to be widely used in advanced nuclear power systems, such as nuclear fuel cladding tubes of nuclear reactors.
2020 Vol. 41 (6): 600-613 [
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