Highlights
Establish processing-composition-microstructure-property relationships governing binderless tungsten carbide (BTC).
Highlight the densification improving strategies and toughening methods ...for BTC.
Provide key challenges as well as the outlook for superior peformance associated with BTC.
WC-Co alloys have enjoyed great practical significance owing to their excellent properties during the past decades. Despite the advantages, however, recently there have been concerns about the challenges associated with the use of Co, i.e. price instability, toxicity and properties degeneration, which necessitates the fabrication of binderless tungsten carbide (BTC). On the other hand, BTC or BTC composites, none of them, to date has been commercialized and produced on an industrial scale, but only used to a limited extent for specialized applications, such as mechanical seals undergoing high burthen as well as high temperature electrical contacts. There are two challenges in developing BTC: fully densifying the sintered body together with achieving a high toughness. Thus, this review applies towards comprehensively summarize the current knowledge of sintering behavior, microstructure, and mechanical properties of BTC, highlighting the densification improving strategies as well as toughening methods, so as to provide reference for those who would like to enhance the performance of BTC with better reliability advancing them to further wide applications and prepare the material in a way that is environment friendly, harmless to human health and low in production cost. This paper shows that the fabrication of highly dense and high-performance BTC is economically and technically feasible. The properties of BTC can be tailored by judiciously selecting the chemical composition coupled with taking into careful account the effects of processing techniques and parameters.
The effects of chromium additions on three-body abrasive wear behavior of Fe-3.0 wt% B alloy have been systematically investigated using a moving wedge type of apparatus and two types of abrasives ...(SiC and SiO2 grains). Features of both the worn surface and subsurface have been discussed with regard to the respective wear rates. As chromium increased, the wear rate of Fe-3.0 wt% B alloy first decreases to a minimum and then increases slightly for both abrasives. That behavior exactly reflects an opposite trend to the fracture toughness of the boride. The wear rate against SiC is evidently higher than that against SiO2. Mixed layers comprised of boride fragments and fatigued metallic matrix can be observed on the worn surfaces of the sample without Cr. Furthermore, the surface roughness resulting from wear first decreases and then increases using SiC; however, no obvious change takes place in the case of wear by SiO2. In the worn subsurface, serious fracture of the boride phase appears in 0 wt% Cr sample while nearly no such cracks can be found in the 2 wt% Cr sample. It is concluded that during three-body abrasive wear process involving both impact and indentation, the fracture toughness of the boride phases, apart from the bulk material hardness, plays a key role in the wear behavior of Fe-3.0 wt% B alloy.
To simultaneously improve the toughness and strength, MoAlB ceramics have been prepared by adding different Si contents. The effects of Si addition on the microstructure and mechanical properties ...have been systematically investigated and the corresponding mechanism has been discussed in the present study. Si element cannot only dissolve into MoAlB crystal grain but also contribute to the phase composition transformation from MoAlB+Al8Mo3 +Al2O3 to MoAlB+ Mo(Si, Al)2 +Al2O3. Additionally, MoAlB crystal grains can be effectively refined after Si addition. Accordingly, the Vickers hardness (10 N) can be greatly improved, reaching about 14 GPa. Flexural and compressive strengths increase firstly and then decrease, acquiring the highest values of 783.2 and 2760 MPa, respectively. Likely, fracture toughness of MoAlB can be improved to 7.46 MPa·m1/2 with Si addition of 0.05 mol. Si dissolution, grain refinement and phase composition transformation synergistically affect the mechanical properties of MoAlB ceramics.
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•The strength and fracture toughness of MoAlB ceramic can be simultaneously improved by adding appropriate Si element.•Si addition cannot only dissolve into MoAlB grains but also form a new Mo(Si, Al)2 phase;•The improved mechanical properties of MoAlB are attributed to Si solution, grain refinement and phase transformation.
For the water-splitting process, it is essential to improve the search for low-cost, highly active materials that can catalyze the oxygen evolution reaction (OER). Based on this, the current work ...suggests a unique method for creating g-C
3
N
4
/CdS nanocomposite. The g-C
3
N
4
/CdS nanocomposite loaded on nickel foam (NF) required overpotential of 279.6 mV for the OER, which is higher than overpotentials required by g-C
3
N
4
and CdS for attaining desired standard current density of 10 mA cm
−2
under the same circumstances. Findings elucidate high activity of composite material through increased active sites with ECSA of 130 cm
2
, very low charge-transfer resistance Rct value of 2.5 (Ω) and decreased tafel slope value of 44.5 mV dec
−1
. All these factors are accountable for the improved OER activity and faster reaction kinetics. The material was characterized through XRD, FTIR, SEM, EDS, and XPS techniques.
Graphical abstract
The microstructure and crystallography of M7C3 carbide in 20 wt.% Cr hypereutectic cast iron have been investigated. The results show that primary M7C3 carbide subjected to fast cooling displays an ...irregular hollow hexagonal structure. Rapid solidification has remarkable effects on the morphology and orientation of primary M7C3 carbide. Transmission electron microscopy (TEM) results indicate that M7C3 carbide has hexagonal structure and crystallizes on {011−0} planes with some stacking faults generating along 0001 orientation. The twins in M7C3 are formed on (011−0) twin plane accompanying self-perpetuating sub-steps which can impellingly promote M7C3 carbide growth. The chemical formulas of primary and eutectic M7C3 carbides are determined to be Cr3.78Fe3.22C2.99 and Cr3.75Fe3.25C2.99, respectively. High-resolution transmission electron microscopy results reveal that primary M7C3 carbide emerges outcrop and promptly grows after nucleating in liquid phase. The existences of sub-steps and stacking faults show that crystalline defects can act as an important role in M7C3 carbide growth. Moreover, an outflanking growth model of primary M7C3 carbide is discussed based on the crystalline defects and growth condition, which results in the formation of hexagonal carbides.
•Directionally solidified M7C3 displays strong textured structures.•The chemical formulas of Cr-rich M7C3 are determined.•Crystal planes of primary M7C3 are {011−0} and its growth is discussed.•Crystalline defects impel the growth of primary M7C3 carbide.
In situ small-sized TiB2/Al-4.5Cu composites with high strength and good ductility were prepared via an ultrasound assisted re-melting and diluting (RD) approach in this research. The experimental ...results showed that small-sized TiB2 particles with an average size of 100 nm were synthesized in the metal-salts reaction. Both the Al-TiB2 mater composite and TiB2/Al-4.5Cu composites with homogeneous microstructures were obtained due to the effects of ultrasound. The α-Al grains in Al-4.5Cu alloy transferred from large-sized dendrite to small-sized spherical in morphology by adding the TiB2 particles. Compared with the original Al-4.5Cu alloy, the 5 wt% TiB2/Al-4.5Cu composites exhibited the best comprehensive mechanical properties, and the yield strength, ultimate tensile strength and elongation were improved by 39.7%, 39.9%, and 7.9%, respectively. Furthermore, the mechanisms of the improved mechanical properties, including the strength and ductility, of the composites were discussed.
•The average of in situ formed TiB2 particles was about 100 nm.•Ultrasound leaded to the uniform distribution of TiB2 particles in the matrix.•Al-Cu alloy matrix was refined by TiB2.•The strength and ductility of composites were improved simultaneously.
Stellite 6 alloy has been considered a promising coating for hydro-turbine parts owing to excellent erosion resistance. In the present work, Stellite 6 coatings have been prepared by the high-speed ...laser cladding technique with the inside-beam powder feeding method. Microstructure, hardness and erosion-wear behaviors of the Stellite 6 coatings have been systematically investigated. The results indicate that, there are no visible defects in all the Stellite 6 coatings; the coating thickness gradually decreases with the cladding speed while it increases with the laser power, ranging from 429.35 to 842.19 μm. The coatings are mainly composed of γ-Co and carbides, and the microstructure of γ-Co gradually displays planar, columnar, dendrite and equiaxed crystals from the substrate interface to coating surface. The second dendrite arm spacing (SDAS) of γ-Co shows positively linear relationship with the line energy density (LED). The dilution rate of Stellite 6 coating increases with the laser power and cladding speed, while the microhardness shows the opposite variation trend. The highest hardness of 610.26 HV can be obtained when the laser power and cladding speed are 1600 W and 4000 mm/min. The erosion-wear resistance of Stellite 6 coating is approximately 2–5 times higher than that of the 45# steel substrate, and it mainly depends on the coating hardness. Microcutting and impacting from abrasive particles are the predominant damage mechanisms while the corrosion attack from 3.5% NaCl solution can be negligible. Thus, the erosion resistance of Stellite 6 laser cladding coating can be further improved by enhancing the coating's hardness.
First-principles calculations based on density functional theory (DFT) have been performed to explore the effects of Si, Cr, W, and Nb elements on the stability, mechanical properties, and electronic ...structures of MoAlB ternary boride. The five crystals, with the formulas of Mo4Al4B4, Mo4Al3SiB4, Mo3CrAl4B4, Mo3WAl4B4, and Mo3NbAl4B4, have been respectively established. All the calculated crystals are thermodynamically stable, according to the negative cohesive energy and formation enthalpy. By the calculation of elastic constants, the mechanical moduli and ductility evolutions of MoAlB with elemental doping can be further estimated, with the aid of B/G and Poisson’s ratios. Si and W doping cannot only enhance the Young’s modulus of MoAlB, but also improve the ductility to some degree. Simultaneously, the elastic moduli of MoAlB are supposed to become more isotropic after Si and W addition. However, Cr and Nb doping plays a negative role in ameliorating the mechanical properties. Through the analysis of electronic structures and chemical bonding, the evolutions of chemical bondings can be disclosed with the addition of dopant. The enhancement of B-B, Al/Si-B, and Al/Si-Mo bondings takes place after Si substitution, and W addition apparently intensifies the bonding with B and Al. In this case, the strengthening of chemical bonding after Si and W doping exactly accounts for the improvement of mechanical properties of MoAlB. Additionally, Si doping can also improve the Debye temperature and melting point of the MoAlB crystal. Overall, Si element is predicted to be the optimized dopant to ameliorate the mechanical properties of MoAlB.
As well-known, the paradigm of federated learning (FL) operates on the principle that without centralizing data into a single server, server only trains and updates global models based on the local ...model from multiple clients. Compared with traditional machine learning, FL enables that data availability and invisibility and preserves the data security. However, in the process of FL, recently gradient inference attack can be launched by malicious attacker to grasp gradient information, and then infer sensitive information by analyzing this gradient information. By analyzing existing schemes for combating gradient inference attacks, it can be seen that the resistance effect is not good. Designing an effective method for resisting gradient inference attacks is a challenge. In this paper, a privacy-enhanced federated learning method is proposed to efficiently defend against gradient inference attacks and improve the accuracy of the training model. In this method, the Gaussian noise has been used to enhance the training model's ability to resist gradient inference attacks, and the Adam gradient descent method has been applied to enhance the accuracy of the training model. Finally, this paper conducted experiments on the CIFAR-10 dataset and MNIST dataset, and the experimental results showed that compared to alternatives, the proposed method has stronger resistance to gradient inference attacks and higher model accuracy.
Material design was performed to solve the twin problems of densification together with brittleness of binderless TiC (BTC). Novel binderless TiC composites reinforced by multilayer graphene (MLG) ...were fabricated through two-step sintering (TSS) with the auxiliary of nano-sized TiC and WC as sintering aids. The synergistic effects of multilayer graphene and two-step sintering on densification behavior as well as mechanical properties has been investigated. The results demonstrated that excellent mechanical properties were achieved for 0.45 wt% MLG addition with a hardness of 24.36 GPa, a flexural strength of 708.9 MPa as well as a fracture toughness of 7.28 MPa·mm1/2. MLG exerted multifaceted influence on the hybrid TiC-based materials: 1) enhancing the densification and suppressing the grain growth simultaneously; 2) improving the uniform distribution of nano TiC and WC particles throughout the micro TiC matrix; 3) facilitating the load transferring from the matrix to MLG by introducing multi-phase and multi-scale strong-weak hybrid interfaces; 4) inducing efficient toughening mechanisms.
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•MLG-reinforced BTC were prepared by TSS.•TSS was employed to avoid the MLG structural damage during sintering process.•Solid-solution activated sintering and liquid-phase intensified sintering were combined to densifying BTC.•MLG induced toughening/ strengthening mechanisms were studied.
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