Abstract
Weyl semimetals exhibit unusual surface states and anomalous transport phenomena. It is hard to manipulate the band structure topology of specific Weyl materials. Topological transport ...phenomena usually appear at very low temperatures, which sets challenges for applications. In this work, we demonstrate the band topology modification via a weak magnetic field in a ferromagnetic Weyl semimetal candidate, Co
2
MnAl, at room temperature. We observe a tunable, giant anomalous Hall effect (AHE) induced by the transition involving Weyl points and nodal rings. The AHE conductivity is as large as that of a 3D quantum AHE, with the Hall angle (
Θ
H
) reaching a record value (
$$\tan {\Theta }^{H}=0.21$$
tan
Θ
H
=
0.21
) at the room temperature among magnetic conductors. Furthermore, we propose a material recipe to generate large AHE by gaping nodal rings without requiring Weyl points. Our work reveals an intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for developing spintronic devices.
The deformation mechanism of an eutectic high-entropy alloy (EHEA) AlCoCrFeNi2.1 at elevated temperature has been studied by tensile test, microstructure analysis and calculation. The ratio of the ...FCC and BCC phases has changed greatly after deformation. The formation enthalpies of L12-type Ni3Al and B2-type NiAl are calculated by using Miedema model. Testing and calculation suggest that B2Ⅱprecipitates (NiAl) have a priority in the precipitation order, which is critical to phase change. Besides, TEM results reveal that the function of B2Ⅱ precipitates in FCC matrix during deformation is similar to Orowan by-passing mechanism, and the role of L12,Ⅱ precipitates in B2 matrix is regarded as dislocation cutting. Dynamic recrystallization, occurring in the FCC matrix, reduces stress concentration caused by dislocation motion, but it may be suppressed by B2Ⅱ precipitates. It has been identified that the softening mechanism of dynamic recrystallization and precipitation strengthening have an effect on the high temperature deformation process of the AlCoCrFeNi2.1 EHEA.
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•Eutectichigh-entropy alloy exhibits ultra high ductility at elevated temperature.•The dynamic recrystallization takes place in the FCC matrix and it plays a softening role in the deformation process.
In this work, the microstructure and tensile properties of DD32 single-crystal (SC) superalloy repaired by laser metal forming (LMF) using pulsed laser have been studied in detail. The ...microstructures of the deposited samples and the tensile-ruptured samples were characterized by optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscope (SEM). Due to high cooling rate, the primary dendrite spacing in the deposited area (17.2 μm) was apparently smaller than that in the substrate area (307 μm), and the carbides in the deposited samples were also smaller compared with that in the substrate area. The formation of (γ+γ′) eutectic in the initial layer of repaired SC was inhibited because of the high cooling rate. As the deposition proceeded, the cooling rate decreased, and the (γ+γ′) eutectic increased gradually. The (γ+γ′) eutectic at heat-affected zone (HAZ) in the molten pool dissolved partly because of the high temperature at HAZ, but there were still residual eutectics. Tensile test results showed that tensile behavior of repaired SC at different temperatures was closely related to the MC carbides, solidification porosity, γ′ phase, and (γ+γ′) eutectic. At moderate temperature, the samples tested fractured preferentially at the substrate area due to the fragmentation of the coarse MC carbide in the substrate area. At elevated temperature, the (γ+γ′) eutectic and solidification porosity in the deposited area became the source of cracks, which deteriorated the high-temperature properties and made the samples rupture at the deposited area preferentially.
Herein, alumina green bodies are fabricated by three dimensional (3D) printing technology, then, the influence of debinding holding time under vacuum and argon on mechanical properties is ...systematically investigated by comparing the changes in microstructure, bulk density, open porosity, grain connection situation and flexural strength of ceramics. The flexural strength of alumina ceramics acquired the maximum values of 26.4 ± 0.7 MPa and 25.1 ± 0.5 MPa after debinding under vacuum and argon for 120 min and 180 min, respectively. However, the alumina ceramics rendered the flexural strength of 19.4 ± 0.6 MPa and 9.5 ± 0.4 MPa under vacuum and argon without extended holding time, respectively. The relatively low mechanical properties can be mainly attributed to the weak interlayer binding force, which is caused by layer-by-layer forming mode during 3D printing process and anisotropic shrinkage during the sintering process. Moreover, the alumina ceramics exhibited moderate bulk density and open porosity of 2.4 g/cm3 and 42% after the sintering process, respectively, which are mainly influenced by the microstructural evolution of alumina ceramics during thermal treatment. Also, the diffusion of gases is achieved by curing of photosensitive resin and influenced by different holding times during debinding, affecting the mechanical properties of sintered ceramics. The mechanical properties of as-sintered ceramics are suitable for the utilization of ceramic cores in the manufacturing of hollow blades.
An eutectic high-entropy alloy consisting Al, Co, Cr, Fe and Ni elements was prepared by vacuum directional solidification technology. The alloy exhibits excellent comprehensive mechanical ...performance during tension at temperature range of 600–700 °C. The microstructure reveals the intersection of twin-twin is the prevailing deformation mechanism and the twins play a dual role in strengthening and toughening the alloy in the thermomechanical process. The deformation twin variants I and Π were formed by the edge dislocation 112¯ and the mixed dislocation 211¯ on the {111} crystal planes, respectively. Besides, the dislocation jogs and kinks caused by twin intersection on the slip planes can strengthen the alloy, which may contribute to the high strength (the tensile strengths at the 600° and 700° tensile tests are respectively780 MPa and 630 MPa.). Moreover, the coherent twin boundary migration has the function of coordinating deformation and contributes to the high ductility of the alloy.
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•The Pt-Al bond coat was detrimental at 900 ℃ or under high stresses at 760 ℃, while was negligible under low stresses at 760 °C.•At 760 °C, the protection of crack-induced oxide ...disappeared and the tip cracks formed with amplitude stress increasing.•At 900 °C, the fatigue property degradation was related to the coat oxidation, oxide cracking and interfacial microstructure degradation.•An empirical life prediction method on coated superalloy was developed, matching well with the test results.
The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.
Abstract
In view of the low detection efficiency of traditional aerial image-based self-explosive insulators and the need to manually extract the features of self-explosive insulators, a ...self-explosive insulator detection algorithm based on the improved YOLO v4 is proposed: We use the hybrid data augmentation method to increase the number of defective samples of self-explosive insulators and increase the diversity of self-explosive insulators to improve the detection accuracy of the algorithm. In addition, the channel attention mechanism is combined with the YOLO v4 algorithm to improve the feature extraction capability of algorithms to self-explosion insulators. The proposed algorithm is verified using the expanded self-explosive insulator dataset and the traffic signs dataset TT100K. The experimental results show that the proposed algorithm has a mAP of 92.0 % on the self-explosive insulator dataset and has an F1-score of 78.70% on TT100K dataset, which has positive significance for the research on self-explosive insulator detection.
Vehicular ad hoc networks (VANETs) are the communication foundation for future intelligent transportation systems and guarantee safe driving of intelligent networked vehicles. Moreover, VANETs face a ...series of security challenges related to the protection of vehicle privacy, authenticity of transmitted information, and bandwidth limitations. To resolve these contradictions, many certificateless aggregate signature (CLAS) schemes have been proposed. However, the majority of them can neither resist malicious-but-passive key generation center attacks, replay attacks, and link attacks, nor track the actual identities of malicious vehicles. Meanwhile, the security of previous CLAS schemes in VANETs is only formally provided in the random oracle model (ROM), which might be insecure in actual implementation. In addition, most CLAS schemes still have problems of large verification delays and high communication overhead. To address the above-mentioned problems, a new conditional privacy-preserving CLAS scheme in VANETs is proposed, which adopts full aggregation technology to reduce computation and bandwidth resources. According to the formal security proofs under the computational Diffie-Hellman problem (CDHP) given in the standard model (SM), the security level of this scheme is higher than that of other CLAS schemes under ROM in practical applications. Additionally, the use of pseudonym mechanism realizes conditional privacy protection in VANETs. The performance analysis shows that this scheme has a higher efficiency in terms of computation and communication overhead compared with several previous CLAS schemes.
Machine type communication (MTC) is an important mobile communication approach in the long-term evaluation-advanced (LTE-A) networks. To meet the MTC security requirements, the access authentication ...processing of MTC devices needs to follow the evolved packet system-authentication and key agreement (EPS-AKA), a protocol defined in the third generation partnership project (3GPP) standard. However, in the emergence of group-based communication scenarios, an independent authentication processing for each MTC device will cause signal congestion in the networks. In addition, the access-policy updating has always been an issue when constructing authentication schemes. In this paper, we propose a group-based AKA (GR-AKA) protocol with dynamic policy updating. Specifically, we choose an asynchronous secret share scheme combining with Diffie-Hellman key exchange scheme to implement distributed authentication and session key establishment in the LTE-A networks, and to achieve dynamic MTC-device access authority updating. Compared with other authentication protocols in the LTE-A networks, our method could not only authenticate several MTC devices simultaneously but also dynamically update the access-policy to control the access authority of MTC devices. Extensive analysis and experiment results have shown the efficiency and efficacy of proposed protocol.
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