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2020 Vol. 41, No. 2
Published: 2020-04-28
93
Progress in theoretical modeling and simulation on strengthening and toughening of high entropy alloys
High entropy alloys (HEAs) have attracted extensive attention due to their excellent properties, such as high strength, high hardness, high toughness, high wear resistance, high radiation resistance, high corrosion resistance, high resistance, high heat resistance, which are expected to be used in important fields such as nuclear energy and aerospace, as well as major equipment. The experimental studies on the preparation, microstructure and properties of HEAs show that their unique properties depend on the high entropy effect, lattice distortion and diffusion hysteresis. At the micro-scale and macro-scale, theoretical models and numerical simulations provide a method for studying the micro mechanism and mechanical properties of HEAs. Establishing the connection from the microstructure and deformation mechanism of HEAs to the unique macroscopic mechanical properties is a multi-scale scientific problem. Recently, based on experimental observation, using multi-scale theory and simulation (first principle method, molecular dynamics, discrete dislocation dynamics, crystal plastic finite element, microstructure dependent theoretical model), the stacking energy, elastic modulus, diffusion coefficient and phase stability of HEAs are studied, and then the deformation and strengthening mechanisms of HEAs are revealed. In this paper, the research progress of multi-scale calculation on mechanical properties and deformation behavior of HEAs is reviewed, and in situ deformation experiment, high-throughput technology and machine learning in HEA is briefly prospected.
2020 Vol. 41 (2): 93-108 [
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109
Temperature-dependent Tensile Behavior of CrMnFeCoNi Nanocrystals
Abstract: High-entropy alloys are a new type of alloys composed of several principal elements (usually ≥5) with equiatomic or near equiatomic ratios. Experimental studies have shown that the CrMnFeCoNi high-entropy alloy has higher tensile strength and fracture toughness at room temperature than at lower temperatures. In this paper, we performed molecular dynamics simulations to investigate the tensile behavior of the nanocrystalline CrMnFeCoNi alloy with an average grain size of 6 nm at 300, 200, and 77 K, respectively. The temperature effect and toughening mechanism of nano-scale CrMnFeCoNi high-entropy alloys were revealed. Microstructure evolution analyses show that in the plastic deformation stage at low temperatures, the slip systems were less activated. With the resistance of dislocation slip increases, the yield and tensile strengths increase. When the simulation cell collapses upon deformation, the nucleation of void defects is slower; More void defects evolve into fracture, and more fracture surfaces distribute tensile strain, which makes the low-temperature toughness of nanocrystalline high-entropy alloys better.
2020 Vol. 41 (2): 109-117 [
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118
Molecular Dynamics Simulation on the One-way Shape Memory Effect of NiTi Shape Memory Alloy single crystal
The one-way shape memory effect of NiTi shape memory alloy nano-single crystal was investigated by the molecular dynamics simulations based on a second nearest neighbor modified embedded atom interaction potential. The deformation behavior and microstructural evolution of the NiTi nano-single crystal during temperature-induced martensitic transformation and stress-induced martensitic reorientation were elucidated in detail, and the effect of loading rate on the one-way shape memory effect in a nanoscale was discussed. Simulated results show that different martensite variants are coexisted when the nano-single crystal is cooled below the martensite finish temperature and then the martensitic reorientation occurs during the stress loading process. The residual strain after unloading gradually decreases with the increase of temperature, but cannot fully recover even if the temperature is much higher than the austenite finish temperature, since the irreversible plastic deformation occurs during the martensite reorientation. It is also found that the austenite transformation temperature of the single crystal during the subsequent heating after the stress-induced martensitic reorientation is much higher than that without undergoing the martensitic reorientation. On the one hand, the plastic deformation will disturb the stress field in the single crystal, resulting in a pinning effect on the martensitic reverse transformation; then, a larger driving force is needed to trigger the martensitic reverse transformation. On the other hand, since the interface morphology among various reoriented martensite variants is different from that among twinned martensite variants, the martensite interfacial energy will be changed by the martensitic reorientation, which affecting the critical temperatures of martensite transformation. In addition, it is found that the martensitic reorientation depends on the loading rate strongly. With the increase of the loading rate, the modulus and critical stress of martensitic reorientation increase gradually, which are caused by the viscosity of the movement of interfaces among different martensite variants. Finally, simulated results show that the evolutions of atomic structures are different at various loading rates in the subsequent cooling process. All the above results will provide a basis for the research on the molecular dynamics simulations on the shape memory effect of NiTi shape memory alloy polycrystal.
2020 Vol. 41 (2): 118-126 [
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127
Two-dimensional fretting contact analysis of magneto-electro-elastic materials under a rigid conducting cylindrical punch
Magneto-electro-elastic composite materials (MEEMs), comprised of piezomagnetic and piezoelectric phases, possess superior mechanical performance and intrinsic electro-magnetic-mechanical coupling effects. MEEMs have been widely used in various hi-tech smart structures and devices, such as, micro power generators, transducers and actuators. In practical engineering applications, due to the intrinsic brittleness of MEEMs, the smart devices and structures made of MEEMs will easily suffer from surface contact damage when subjected to the highly concentrated local contact loads and friction forces. In addition, these structures are often served in the vibration environment. Therefore, fretting contact damage and fatigue failure inevitably occur in these smart devices. In this paper, the two-dimensional fretting contact between an MEEM half-plane and a rigid conducting cylindrical punch are investigated to improve the resistance to fretting contact damage and electromagnetic failures. The punch is assumed to be a perfect electro-magnetic conductor with constant electric potential and constant magnetic potential within the contact region. Since the fretting contact problem is loading history dependent, the two bodies are brought into contact first by a monotonically increasing normal load, and then by a cyclic tangential load, which is less than that necessary to cause complete sliding. It is assumed that the whole contact region contains an inner stick region and two outer slip regions where Coulomb’s friction law is applied. By using the Fourier integral transform technique, the problem is reduced to a set of coupled Cauchy singular integral equations. An iterative method is used to determine the unknown stick/slip region, normal contact pressure, electric charge, magnetic induction and tangential traction. The effects of the friction coefficient, total electric charge, total magnetic induction and conductivity of the punch on the surface electromechanical fields are discussed for different loading phases. It is found that the peak value of tangential traction for the insulating punch is larger than that of the conducting punch, but the size of the stick region is smaller than that of the conducting punch. The maximum values of the in-plane tensile stress, the in-plane electric displacement and the in-plane magnetic induction occur at the edges of the contact region during the tangential loading phase, which implies a possible site of the contact damage and fretting crack initiation.
2020 Vol. 41 (2): 127-141 [
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142
Mechanical control of topological magnetic structures in multiferroic composites
Magnetic skyrmions is a kind of important topological magnetic structure in ferromagnetic materials. Due to its novel multi-physics coupling between the magnetic, electric, mechanical and thermal fields, magnetic skyrmions has potential applications in the future spintronic devices. However, the magnetic skyrmions are stable only at an external magnetic field, which limits its potential in applications. In this work, based on a real-space multiferroic phase field model, we demonstrate that the magnetic skyrmions can be stabilized by the polar skyrmions via interfacial deformation in the multiferroic composite. The multiferroic composite thin film consists of MnSi, BaTiO3 and SrTiO3 components. When an electric field is applied and then removed from the multiferroic thin film, an out-of-plane upward polar vortex shows in the BaTiO3 component. After applying a reverse electric field to the thin film, the polarizations outside the vortex in the SrTiO3 become downward. Hence, the polar skyrmions are obtained in the ferroelectric layer of the multiferroic composite. Because of the electrostrictive effect, the nonuniform polarizations distributions in the ferroelectric layer generate a periodic non-uniform deformation in the interface between ferroelectric and ferromagnetic layers. This nonuniform interfacial deformation can stabilize the magnetic skyrmions in the ferromagnetic layer without an external field, via magnetostrictive effect. The present work shows that the topological magnetic structures in the multiferroic composites can be controlled by the electro-magneto-elasticity coupling, which provides new ideas for the designs of the topological magnetic structure-based spintronic devices.
2020 Vol. 41 (2): 142-150 [
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151
Strain response of piezoelectric ring multilayer actuators based on reversible non-180 ° domain switching
Piezoelectric actuators have been playing a very important role in modern industries. However, the current used PZT acutaors are based on linear piezoelectric effect, with the maximum output strain is only 0.1-0.15%. Realization of large strain is always the goal of scholars in this field. In this work, we proposed two types of PZT ring multilayer actuators based on reversible non-180 ° domain switching. One is the radially-poled, partially electroded (RPPE) 4-layer ring, the other is the periodically orthogonal poled (POP) 4-layer rings. Experimental results show that under the same driving field (2kV / mm, 0.1Hz), the maximum actuation strain of 4-layer RPPE actuator is 0.27%, about twice that of conventional PZT ring, but the surface deformation is not uniform. As to the 4-layer POP ring actuator, the maximum output strain is 0.36%, about 2.7 times strain of conventional PZT ring, and the deformation is very uniform over the surface. The actuation strain of both actuators decreases with the increasing driving frequency. The strain of the RPPE actuator was stabilized at 0.19% when the driving frequency exceeds 1Hz, and that of the POP actuator keeps at 0.2% when the driving frequency exceeds 5Hz. Furthermore, the actuation strain of the 4-layer POP ring actuator is very stable and keeps unchanged after 20k cycles of operation. The POP ring multi-layer actuator has the advantages of large output strain and stable configuration, and may get wide applications in the actuation field.
2020 Vol. 41 (2): 151-158 [
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159
Electromechanical coupling model and performance analysis of the unimorph cantilever beam-based flexoelectric energy harvester
Flexoelectric effect refers to the strain gradient induced electric polarization, and it is a universal electromechanical coupling effect in all solid dielectrics due to the inversion symmetry breaking by the strain gradient. Since the strain gradients are inversely proportional to the characteristic dimensional of materials, flexoelectric effect is expected to beyond the piezoelectric effect to dominant the electromechanical coupling phenomenon of materials at the nano scale. Mechanical Energy harvesters based on flexoelectric effect are considered one of the most promising applications in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). In the present work, a theoretical model for the flexoelectric energy harvester is established. The governing equations and corresponding boundary conditions are derived from the energy variation principle. In addition, the performance of the flexoelectric unimorph cantilever beam based energy harvester are analyzed based on the theoretical model. The effects of the resonance frequency, the resistance of the circuit, the thickness of the flexoelectric layer, and the Young’s modulus of the elastic layer on the output voltage frequency response and the output power density frequency response are discussed. Particularly, numerical analysis for cantilever beam based flexoelectric energy harvester fabricated by PVDF polymer thin film and epoxy substrate are obtained. It is found that the maximum output voltage frequency response and output power density frequency response appear at the resonance frequencies of the cantilever energy harvester. The output voltage and the output power density increase with the increase of the resonance frequencies at each mode. The numerical results also showed that there is an optimum resistance. Furthermore, the output power density increases with the decrease of the thickness of the flexoelectric layer when the resistance near its optimum value. Moreover, it is found that the output voltage decreases with the increase of the Young’s modulus of the elastic layer. The numerical results in this paper is helpful in designing cantilever beam-based flexoelectric energy harvesters.
2020 Vol. 41 (2): 159-169 [
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170
Mechanism and Experimental Study on Crack Repair of Shape Memory Alloy Intelligent Concrete
As for the concrete structures, the inevitable crack generation and expansion has always been a difficult problem should be solved. In this paper, the shape memory alloy intelligent concrete design scheme is proposed, which implants shape memory alloys into the concrete to form a shape memory alloy intelligent concrete material. Shape memory alloy intelligent concrete materials will have the advantages of crack repair due to the characteristics of pseudo-elasticity and shape memory effect. In this paper, the crack repair mechanism and the crack repair test of shape memory alloy intelligent concrete materials are carried out, respectively. This paper will provide theoretical guidance for the further optimization design and application of the shape memory alloy intelligent concrete.
2020 Vol. 41 (2): 170-181 [
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182
Effect of Irradition induced Quasi-vacancy on Toughening of Metallic Glasses
The effect of low energy ion irradiation on metallic glasses（MGs） has been suggested to be described in terms of “vacancy-like” defects，which we call “quasi-vacancy”．Using molecular dynamics simulations，we investigate the plastic deformation behavior of Cu50Zr50 MGs with quasi-vacancies introduced．The effects of volume fraction and distribution area of quasi-vacancies on toughening of MGs were discussed．With quasi-vacancies introduced，the deformation behavior of MGs under different cyclic loads was also analyzed．Our results reveals that the introduction of quasi-vacancies can effectively enhance the ductility of MGs．As the volume fraction of quasi-vacancy increases，the deformation mode of MGs has noticeably changed from localized shear banding to homogeneously plastic deformation．The critical volume fraction of quasi-vacancy for the ductile-brittle transition of MGs is suggested to be about 0．5％．Besides，the more uniform quasi-vacancies are distributed，the better ductility of MGs will be enhanced．The enhanced ductility of MGs by introducing quasi-vacancies can be well maintained under different cyclic loads．When the strain reaches 30％ after several cycles of tension-tension or tension-compression loadings，the ductilie response of MGs remains unchanged．For general metals and alloys，the vacancies introduced by irradiation and the cavities，precipitates，dislocation loops or stacking fault tetrahedrons formed by further evolution usually make the materials brittle．However，for MGs，the vacancy introduced by irradiation is hard to diffuse and further evolve，but may increase the ductility of the material．
2020 Vol. 41 (2): 182-192 [
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