Increasing the density of sinks such as grain boundaries and interfaces for irradiation-induced defects and implanted ions has been demonstrated to be an effective way to improve the irradiation ...resistance of materials. To understand the effects of grain boundaries on the degradation mechanism of nanostructured materials, nanocrystalline tungsten was fabricated by high pressure torsion (HPT-W). Morphological changes of HPT-W and coarse grain tungsten (CG-W) during helium ion irradiation were evaluated in situ in a helium ion microscopy. It has been shown that the degradation mechanisms of CG-W and HPT-W are remarkably different. Blister occurs on the surface of CG-W when the irradiation dose increases up to 5.0 × 1021 m−2, and orientation dependence of blistering has been observed. However, no blister is formed on the surface of HPT-W even when the irradiation dose increases up to 1.0 × 1023 m−2. Instead, crack formation along grain boundaries is the major degradation mechanism during helium irradiation of HPT-W, supporting a different irradiation degradation mechanism. This explains the unprecedented irradiation tolerance of HPT-W in terms of blistering. Molecular dynamics results also show that grain boundaries and helium clusters play an important role during the propagation of a crack. The zigzag crack planes are attributed to the coalescence and growth of helium blister/bubble-induced crack. The results document that grain boundaries play decisive roles in the irradiation resistance of nanostructured materials, and provide a new perspective to the design of plasma facing materials with excellent irradiation resistance. It is thus suggested that excellent irradiation resistance can be achieved by a meticulous design of grain boundaries based on “interface engineering”.
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To understand the effects of different factors such as grain size, strain rate and temperature on properties and deformation mechanisms of commercially pure titanium (CP-Ti), ultrafine grained (UFG) ...Ti was fabricated by room temperature equal channel angular pressing (ECAP). The microstructure was characterized using various techniques and the mechanical properties were evaluated within a wide range of temperature and strain rate. It has been shown that the microstructure change upon ECAP significantly impacts the mechanical properties. Strong dependence on grain size, temperature and strain rate of the mechanical properties of CP-Ti was observed. Ductility of UFG Ti increased at both cryogenic (77K) and elevated temperatures (473K), but with distinctly different deformation mechanisms. Other phenomena such as texture, deformation twinning, fracture, adiabatic shear banding, strain softening and hardening were observed and possible underlying mechanisms are given. Yield stress, elongation to failure, etc. and the related parameters, the Zener-Hollomon parameter, strain rate and temperature sensitivity, and activation volumes associated with plastic deformation were discussed. Comparison between CG and UFG CP-Ti sheds light on possible deformation mechanisms during tensile loading. Different rate controlling mechanisms such as dislocation slip and grain boundary sliding are proposed for this metal of hexagonal close-packed (hcp) structure.
Given that commercial purity titanium (CP-Ti) doped with a high oxygen concentration has recently been reported with high mechanical performance, the present work investigated the failure behavior ...and its mechanisms of the material under uniaxial impact loading. Besides the strong hardening effect of oxygen, it was found that increasing oxygen content led to increased propensity for adiabatic shear failure in CP-Ti. The texture or grain orientation was also found to have profound influence on the formation of adiabatic shear band (ASB). Microstructural examinations on the postmortem high oxygen CP-Ti suggested that uniform and equi-axed nano-grains were produced within the ASB. The orientations of these nano-grains were analyzed using the precession electron diffraction (PED) technique. It provides direct evidence of phase transformation occurring in ASB. Then we demonstrate that the nano-grains in ASB formed from parent grains by phase transformation, indicating that the α→β→α phase transformation process takes part in ASB evolution and is an underlying mechanism of grain refinement. As such, this result is a supplement for traditional well-accepted dynamic recrystallization mechanism of ASB evolution.
Uniaxial tensile properties of monocrystalline tungsten (MC-W) and nanocrystalline tungsten (NC-W) with embedded hydrogen and helium atoms have been investigated using molecular dynamics (MD) ...simulations in the context of radiation damage evolution. Different strain rates have been imposed to investigate the strain rate sensitivity (SRS) of the samples. Results show that the plastic deformation processes of MC-W and NC-W are dominated by different mechanisms, namely dislocation-based for MC-W and grain boundary-based activities for NC-W, respectively. For MC-W, the SRS increases and a transition appears in the deformation mechanism with increasing embedded atom concentration. However, no obvious embedded atom concentration dependence of the SRS has been observed for NC-W. Instead, in the latter case, the embedded atoms facilitate GB sliding and intergranular fracture. Additionally, a strong strain enhanced He cluster growth has been observed. The corresponding underlying mechanisms are discussed.
•Uniaxial tensile behavior of monocrystal tungsten (C-W) and nanocrystalline W (NC-W) have been investigated.•Dislocation-based activities dominate the plastic deformation of MC-W.•Grain boundary-based activities dominate the plastic deformation of NC-W.•H/He atoms have significant impacts on the tensile behavior of MC-W and NC-W.•Strong strain enhanced He cluster growth has been revealed.
Realistic and real-time simulation of fluid animation is widely used to the application of virtual reality(VR) such as VR game, special effect in film, augmented reality (AR) and so on. However, fast ...simulation of complex fluid animation problem such as free interaction surface and high impact requires a large number of both physical computations and time steps. It in turn leads to high computational cost. In order to improve the problem, we design a fast tool to accelerate and simulate fluid animation using multi-node graphics processing units clusters. In this paper, we present a fluid animation model tool for VR application based on multi-GPU cluster. The model method of position-based fluid (PBF) is implemented on our tool, and some strategies for GPUs optimizations are applied to parallel system based on the character of hardware. We first present an efficient data structure for speeding up memory access. Then, an optimized parallel framework is designed to get higher performance. We adjust the size of grid sptial index, reducing the access and thread synchronization during the neighborhood search, which greatly improve the efficiency on GPU. The key work of extending the PBF method from single GPU to GPU clusters, a spatial decomposition strategy is presented based on Orthogonal Recursive Bisection(ORB) model. Finally, an effective VR tool for real-time fluid animation modeling on the GPUs cluster is designed which can create various vivid animation. The performance and efficiency of our method are demonstrated using multiple VR scenes.
This work aims to investigate the effects of confined cold rolling on the evolution of microstructure, hardness, and helium irradiation performance of high purity tungsten (W). Using a final rolling ...temperature of 450 °C, W samples were severely deformed by confined cold rolling up to equivalent strains (εeq) of 1.6 and 3.3. Experimental results indicate that the average grain size of W specimens processed by confined cold rolling has been greatly reduced, and the rolled W samples with εeq ∼3.3 do not show an “ideal texture” of (001)110 which is the expected texture of bcc metals processed by conventional cold rolling. The irradiation resistance against 60 keV He+ ions with up to a dose of 1.5 × 1022 ions·m−2 of the rolled W is compared to that of the as-received W. Results show that, due to an improvement of the metal's ductility, blister bursting with a partially opened lid forms on the surface of the rolled W, whereas blister bursting with a fully opened lid forms on the surface of the as-received W.
•Compression and tension over a wide range of strain rates are reported for AZ31B.•The stress strain behavior is significantly affected by loading path and strain rates.•Texture observations in the ...loading process suggest different deformation mechanism.•Twinning becomes more difficult as the grain size is refined under compression.•〈c+a〉 dislocations in ECAPed alloy under dynamic tensile loading is predicted.
In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c+a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.
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•We tried to uncover why tungsten is prone to impurity embrittling effect.•We considered impurity effect on both grain boundaries and dislocations.•We proposed a model that entails ...the properties of the impurities and matrix.•Low lattice constant and high elastic modulus make tungsten prone to impurity effect.•This work helps predict the impurity effect without doing expensive calculations.
It is well known that the ductility of tungsten is very sensitive to impurities while the ductility of tantalum is tolerant to them. However, the fundamental reason behind this preferential effect still remains elusive. Here, based on first-principles calculations, we demonstrated that impurities in tungsten are more likely to segregate into the investigated grain boundary region and the vicinity of straight screw dislocation core than in tantalum, thus having more chances to decrease the ductility. In turn, the presence of impurities, if deemed undesirable, will cause a greater reduction in the grain boundary separation energy for tungsten. The analyses of the chemical and mechanical effects of impurities based on an elegant model suggest that, for the deleterious impurities that have similar binding behavior with tantalum and tungsten, if their effect is repulsive at all relevant site, tungsten is more sensitive to them due to its low lattice constant and high elastic modulus despite other possible causes.
It has been a common method to improve the mechanical properties of metals by manipulating their microstructures via static recrystallization, i.e., through heat treatment. Therefore, the knowledge ...of recrystallization and grain growth is critical to the success of the technique. In the present work, by using in-situ high temperature EBSD, the mechanisms that control recrystallization and grain growth of an extruded pure Mg were studied. The experimental results revealed that the grains of priority for dynamic recrystallization exhibit fading competitiveness under static recrystallization. It is also found that grain boundary movement or grain growth is likely to show an inverse energy gradient effect, i.e., low energy grains tend to swallow or grow into high energy grains, and grain boundaries of close to 30° exhibit superior growth advantage to others. Another finding is that {10–12} tensile twin boundaries are sites of hardly observed for recrystallization, and are finally swallowed by adjacent recrystallized grains. The above findings may give comprehensive insights of static recrystallization and grain growth of Mg, and may guide the design of advanced materials processing in microstructural engineering.
Under high-strain-rate compression (strain rate approximately 10(3) s(-1)), nacre (mother-of-pearl) exhibits surprisingly high fracture strength vis-à-vis under quasi-static loading (strain rate ...10(-3) s(-1)). Nevertheless, the underlying mechanism responsible for such sharply different behaviors in these two loading modes remains completely unknown. Here we report a new deformation mechanism, adopted by nacre, the best-ever natural armor material, to protect itself against predatory penetrating impacts. It involves the emission of partial dislocations and the onset of deformation twinning that operate in a well-concerted manner to contribute to the increased high-strain-rate fracture strength of nacre. Our findings unveil that Mother Nature delicately uses an ingenious strain-rate-dependent stiffening mechanism with a purpose to fight against foreign attacks. These findings should serve as critical design guidelines for developing engineered body armor materials.
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