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The effects of irradiation on twin boundary migration in BCC Fe are studied by atomistic simulations. It is found that under the applied shear strain–stress, thermal spikes may create twinning dislocation loops (TDLs) at twin boundaries, so triggering twinning. Irradiation-generated clusters of point defect at twin boundaries may act as sources to nucleate TDLs. When a vacancy loop intersects with a twin boundary, the critical stress to activate a TDL is less than half of that required for a defect-free twin boundary. [ABSTRACT FROM AUTHOR]

Although ion beam technology has frequently been used for introducing defects in graphene, the associated key mechanism of the defect formation under ion irradiation is still largely unclear. We report a systematic study of the ion irradiation experiments on SiO2-supported graphene, and quantitatively compare the experimental results with molecular dynamic simulations. We find that the substrate is, in fact, of great importance in the defect formation process, as the defects in graphene are mostly generated through an indirect process by the sputtered atoms from the substrate. [ABSTRACT FROM AUTHOR]

• Diffusion anisotropy of self-interstitial atoms is considered in the model. • Evolutions of 〈a〉 interstitial, 〈a〉 vacancy and 〈c〉 vacancy loops are involved. • Irradiation growth of annealed and pre-deformed Zr single crystals are obtained. • Deformation mechanisms in the irradiation growth are revealed. • Effects of dislocation density and anisotropy factor are investigated. To predict the irradiation deformation of hexagonal materials, a cluster dynamics model is proposed in this study describing the evolutions of point defects and defect clusters with the diffusion anisotropy of self-interstitial atoms involved. Nucleation and growth models for 〈c〉 vacancy, 〈a〉 interstitial, and 〈a〉 vacancy dislocation loops are developed. Irradiation growth strains are obtained considering the defect formation, growth of dislocation loops, and climb of dislocations. Simulated growth strains along a - and c -axes agree well with the experimental data of annealed and pre-deformed Zr single crystals irradiated at different temperatures. It is found that the dominant contributions to the growth strains by different deformation mechanisms are changed by varied dislocation densities. The irradiation growth could be greatly affected by diffusion anisotropy due to the variation in the absorption efficiency of point defects by sinks. This study reveals the deformation mechanisms in the irradiation growth of Zr single crystals and could provide an effective calculation model for the irradiation deformation of hexagonal materials. [ABSTRACT FROM AUTHOR]

Abstract In this work, a strain-gradient crystal plasticity theory with irradiation effect is proposed for the surface nano-indentation of ion-irradiated FCC metals. In order to characterize the indentation size effect and irradiation hardening of ion-irradiated materials indented by the Berkovich indenter, the hardening contributions of geometrically necessary dislocations and non-uniformly distributed defects are incorporated into the classical crystal plasticity theory. The constitutive equations are implemented into ABAQUS through the user material subroutine VUMAT to numerically simulate the surface nano-indentation of ion-irradiated single crystal copper. The theoretical model is calibrated by comparing the numerical results with experimental data. Both the simulated force-depth relationship without irradiation effect and hardness-depth relationship with/without irradiation effect can match well with corresponding experimental data. The dominant features as observed in the nano-indentation of ion-irradiated materials can be effectively characterized by the proposed theoretical framework, which include the indentation size effect, depth-dependent irradiation hardening, and unirradiated substrate softening effect. Moreover, the evolution of different hardening mechanisms during the surface nano-indentation is systematically analyzed to help understand the macroscopic deformation behavior. Highlights • A strain-gradient crystal plasticity theory with irradiation effect is proposed. • Surface nano-indentation of ion-irradiated FCC metals is simulated by ABAQUS. • Indentation size effect and depth-dependent irradiation hardening can be characterized. • Numerical results can match well with corresponding experimental data. [ABSTRACT FROM AUTHOR]

Abstract Lath martensitic steels possess excellent mechanical properties because of the special microstructure, i.e. laths, blocks and packets with particular crystallography in a prior austenite grain. To theoretically analyze the mechanical behavior of lath martensitic steels, a hierarchical model is proposed combining the crystal plasticity theory and micromechanical method. Within the crystal plasticity theory, an interfacial dislocation model is proposed at the block level to physically describe the effect of lath boundary on the deformation behavior. Furthermore, The scale transition among block, prior austenite grain and lath martensitic steel is accomplished by the elastic-viscoplastic self consistent theory. Based on the proposed model, the deformation behavior of lath martensitic steel with hierarchical structures has been theoretically analyzed. It is revealed that the deformation anisotropy of prior austenite grain is weak due to the particular crystallography of microstructures, and the influence of temperature on yield stress is dominated by the thermally related intrinsic lattice friction and elastic modulus. Moreover, the hierarchical model is generalized to study the mechanical behavior of irradiated lath martensitic steel, and it is found that the change of temperature has a limited effect on the irradiation hardening behavior, and the increase of yield stress induced by irradiation-induced defects can accelerate the evolution rate of dislocation density. Numerical results with/without irradiation effect can both match well with corresponding experimental data, indicating the good accuracy and rationality of the proposed model. Highlights • An interfacial dislocation model is proposed to describe the effect of lath boundary on the deformation behaviors. • A hierarchical theoretical model is proposed to study the hierarchical mechanical behavior of lath martensitic steel. • Based on the proposed model, the mechanical behaviors of unirradiated/irradiated lath martensitic steel are analyzed. [ABSTRACT FROM AUTHOR]

A mechanistic model was developed for modeling the depth-dependent hardness in ion irradiated metallic materials. The model is capable of capturing the indentation size effect, ion irradiation induced damage gradient effect, and effect of unirradiated region acting as a soft substrate. A procedure was developed and described in detail to parametrize the model based on experimentally obtained hardness vs. indentation depth curves. Very good agreement was observed between our model predictions and experimental data of several different stainless steels subjected to various ion irradiation conditions. In addition, two hardening mechanisms are revealed in the new model. One is the well-known indentation size effect arising from the creation of geometrically necessary dislocations as the indenter pierces into the materials. The other is the irradiation hardening due to the presence of irradiation-induced defects. As a function of indentation depth h , the hardening due to indentation size effect is described by h ¯ ∗ / h , while the hardening due to irradiation first follows a power law form P h n , then changes to Z / h − Q / h 3 , where h ¯ ∗ , P , n , Z and Q > 0 are constants. This transition occurs at the indentation depth when the plastic zone reaches the end of the irradiated layer. [ABSTRACT FROM AUTHOR]

MAX phases have exhibited excellent irradiation resistance regarding structure stability, but limited knowledge is available on the irradiation induced change in the mechanical performance of MAX phases. In this work, Cr 2 AlC single crystal samples were irradiated with 6 MeV C ions, and smaples' mechanical properties were measured by micropillar compression test and nanoindentation technique. After irradiation, new slip traces are activated, which means the deformation behavior changed by irradiation. More important is that both the yield strength and Young's modulus decrease gradually with the increasing irradiation doses up to 0.098 dpa, suggesting a significant radiation softening effect but not the radiation hardening effect which is very popular in other materials. This radiation softening effect is possible the result of the irradiation-induecd vacancies, which is supported by the DFT calculation. These results indicate that MAX phases like Cr 2 AlC have an excellent irradiation tolerance regarding mechanical properties and is a new class of promising candidate material for the advanced nuclear systems. • Micropillar compression test has application on Cr 2 AlC MAX phase, and show the surprising irradiation softening effects, which is less pronounced in the low dose regime. [ABSTRACT FROM AUTHOR]

Nano-metallic-multilayers (NMMs) have attractive irradiation-tolerant properties due to the high ratio of interfaces acting as effective sinks for irradiation-induced defects, like helium bubbles. The mechanical behaviour of NMMs can be affected by both interfaces and He bubbles, which is closely related to the layer thickness. In this work, the macroscopic mechanical behaviours of Cu/Nb multilayers with irradiation-induced He bubbles are explored by the crystal plasticity model (CPM) coupled with the elastic-viscoplastic self-consistent (EVPSC) method. At the microscale, hardening effects of He bubbles, interfaces and forest dislocations are evaluated by considering their influences on the critical shear resistance of dislocation movement. At the grain level, the equivalent mechanical properties of a composite grain consisting of different individual grains and interfaces are obtained by the Mori–Tanaka method. At the macroscale, the self-consistent scheme is adopted to predict the macroscopic mechanical behavior of NMMs which are made of composite grains. Numerical results of the proposed model match well with corresponding experimental data. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical model is proposed to characterize the mechanical behaviors of irradiated face-centered-cubic (FCC) polycrystals with nanotwins. At the grain level, the spatial-dependent interactions among twin boundaries (TBs), irradiation-induced defects and dislocations are considered in the crystal plasticity model. At the polycrystal level, the elastic-viscoplastic self-consistent (EVPSC) method is applied to bridge the property of microscopic individual grains to macroscopic mechanical behaviors of polycrystals with nanotwins. Applications of the proposed model to nanotwinned (nt) copper show that the numerical results match well with corresponding experimental data. It is found that: without irradiation, the mechanical properties of nt metals are closely related to the loading direction with respect to the TBs and the width of twin spacing. There exists a critical twin spacing for nt polycrystals: above which, the impediment of slip dislocations due to TBs leads to the hardening mechanism; below which, the slip of partial dislocations along the TB planes becomes dominant and leads to the softening mechanism. With irradiation, the irradiation-induced defects acting as obstacles could impede the movement of dislocations, which leads to irradiation hardening. However, TBs can effectively absorb irradiation-induced defects and reduce the defect density. Therefore, the irradiation hardening effect may be weakened due to the effects of TBs. The proposed model may offer an effective theoretical method to study the mechanical behaviors of irradiated polycrystals with nanotwins. [ABSTRACT FROM AUTHOR]

Crystal plasticity model (CPM) for BCC iron to account for radiation-induced strain softening is proposed. CPM is based on the plastically-driven and thermally-activated removal of dislocation loops. Atomistic simulations are applied to parameterize dislocation-defect interactions. Combining experimental microstructures, defect-hardening/absorption rules from atomistic simulations, and CPM fitted to properties of non-irradiated iron, the model achieves a good agreement with experimental data regarding radiation-induced strain softening and flow stress increase under neutron irradiation. [ABSTRACT FROM AUTHOR]

• Micropillar compression test has application on HfNbZrTi high entropy alloy, and show the irradiation strengthening mechanism, which is less pronounced in the low dose regime. • The examined HEA alloy exhibits superior radiation resistance on yield strength compared to conventional nuclear structural materials. High entropy alloys (HEAs) have exhibited excellent irradiation resistance regarding structure stability. However, limited knowledge has been available on the tendency of mechanical properties of HEAs with body-centered cubic structures after ion irradiation. In this study, the evolution of mechanical properties of the HfNbZrTi irradiated with 3 MeV C ions is studied based on micropillar compression tests. The yield strength increases almost linearly from 974 to 2068 MPa with increasing irradiation doses up to 0.98 dpa. The correlation between the mechanical and microstructural changes agrees well with the prediction of Δ δ y = α M G b (N D) 1 2 , indicating a similar strengthening mechanism with conventional alloys. Nonetheless, the irradiation strengthening effect in this alloy is less pronounced in the low dose regime, and the saturation is postponed, compared with the stainless steel and the 3d metal high entropy alloys with face-centered cubic structures, exhibiting promising irradiation resistance regarding mechanical properties. [ABSTRACT FROM AUTHOR]