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In this paper, CuCr–Zr alloys prepared by vacuum melting with adding La and Ni elementswere heat-treated and aged, followed by plastic deformation using low-energy cyclic impact tests, to simultaneously improve their mechanical and electrical properties. Results showed that the grain size of the casted Cu–Cr–Zr alloys was significantly reduced after the solid-solution aging and plastic deformation process. There were a lot of dispersed Cr and Cu5Zr precipitates formed in the alloys, and the numbers of dislocations were significantly increased. Accordingly, the hardness was increased from 78 to 232 HV, and the tensile strength was increased from 225 to 691 MPa. Electrical conductivity has not been significantly affected after these processes. The enhancement of overall performance is mainly attributed to the combined effects of solid-solution hardening, fine grain hardening, and precipitation/dislocation strengthening.

The influence of αs precipitates on electrochemical behavior and mechanical degradation of Ti-1300 alloy in artificial seawater have been studied. The results show that corrosion resistance and mechanical degradation have been significantly affected by the formation of acicular αs precipitates. The precipitated αs phase with an acicular shape around 40–60 nm in width are uniformly distributed inside β grain. Many αs precipitates are intersected each other and keep a well-defined Burgers orientation relationship with β matrix, which restricts the growth of other αs phases due to pinning effect. Within the electrolyte, the αs phases can form “microgalvanic cells” with their adjacent intergranular β phases, which dramatically deteriorate its corrosion resistance. The mechanical properties of the alloy are also degraded with the increase of immersion time due to the pitting reaction. The precipitated microstructure exhibits an inferior mechanical degradation behavior, and this is mainly because a lot of corrosion cavities are nucleate d and propagated at the interface between αs precipitates and prior β grains.

Graphene has been extensively explored to enhance functional and mechanical properties of metal matrix nanocomposites for wide-range applications due to their superior mechanical, electrical and thermal properties. This article discusses recent advances of key mechanisms, synthesis, manufacture, modelling and applications of graphene metal matrix nanocomposites. The main strengthening mechanisms include load transfer, Orowan cycle, thermal mismatch, and refinement strengthening. Synthesis technologies are discussed including some conventional methods (such as liquid metallurgy, powder metallurgy, thermal spraying and deposition technology) and some advanced processing methods (such as molecular-level mixing and friction stir processing). Analytical modelling (including phenomenological models, semi-empirical models, homogenization models, and self-consistent model) and numerical simulations (including finite elements method, finite difference method, and boundary element method) have been discussed for understanding the interface bonding and performance characteristics between graphene and different metal matrices (Al, Cu, Mg, Ni). Key challenges in applying graphene as a reinforcing component for the metal matrix composites and the potential solutions as well as prospectives of future development and opportunities are highlighted.

Tungsten-copper (W-Cu) composites are used as high temperature frictional materials under special service conditions for electromagnetic gun rail and precision guide for rolled pieces due to their good ablation resistance and electrical conductivity. However, they have poor wear resistance at elevated temperatures. In this paper, surface carburization method was applied on the W-20 wt%Cu composite to investigate the mechanisms of carburization and its effects on the high temperature friction behavior of composite. Carburization process has been done at a temperature of 1100 °C for 30 h. The obtained results showed that carburizing at 1100 °C with a dwelling time of 30 h resulted into formation of a carburized layer and a dense intermediate sub-layer on the substrate. Also, the surface carburized layer with a thickness of about 70 μm composed of mixed phases of graphite, WC and W2C. The hardness of carburized layer (~HV454) was significantly higher than that of substrate (HV223). Also, bending strength of the carburized W-Cu composites has been significantly improved, although their electrical conductivity and tensile strength was decreased slightly. The carburization mechanism of the W-Cu composites was found to be dominant by carbon atom diffusion through reaction with W atoms and formation of surface liquid copper, which promoted migration and diffusion of tungsten and carbon at high temperatures. Average coefficients of friction and wear rate of carburized W-Cu composites are all lower than these of un-carburized W-Cu composites owing to the presence of surface carburized layer. Also, formation of CuWO4 at high temperatures reduced the friction and wear resistance of the W-Cu composites.

W-based alloys are currently considered promising candidates for high heat flux components in future fusion reactors. In this paper, hot pressed W-20 wt%Cu composites were treated at room temperature using a sliding friction severe deformation (SFD) process, with a moving speed of 0.2 m/s and an applied load of 500 N. Microstructural evolution of composites after the SFD treatment was evaluated and compared with that of the untreated composites. Results showed that there was a gradient structure generated and an obvious refinement in tungsten particles size in the surface layer after the SFD process. The average particle size of tungsten in the SFD treated composites was 2.60 μm, whereas it was 4.5 μm for tungsten in the untreated composites. Fracture surfaces of the composites indicated that the SFD treatment destroyed the W skeleton and changed fracture mode from predominant inter-granular one to trans-granular one due to the decrease in contact area of W-W inter-particles. Yield strength and ultimate tensile strength of composites after the SFD treatment were 308 MPa and 553 MPa, respectively. The treated composites exhibited micro-hardness values with an average reading of about 308 HV. Analysis of the facture microstructures clearly suggested that the tungsten particles in the treated composites are consisted of dislocations and boundaries as well as dislocation tangles. The electrical conductivity of the composites was decreased from 33 IACS% to 28.5 IACS% after the SFD treatment, mainly due to loss or squeezing of copper into the inner surface.

Classification experiments using Sn0.3Ag0.7Cu lead-free solder powder prepared by supersonic gas atomization were conducted by turbo air classifier. The effects of rotor cage rotary speed on the cut size and partial classification efficiency of Sn0.3Ag0.7Cu solder powder were investigated. The relationship between cut size and rotor cage rotary speed was calculated through curve fitting. Results showed that cut size and median diameter of fine powders decreases exponentially with an increasing of the rotor rotary speed. The “fish-hook effect” existed in the turbo air classification process of Sn0.3Ag0.7Cu solder powder. The “fish-hook effect” weakens and classification efficiency increases with the rotor cage rotary speed decreasing. The efficiency of classifier was evaluated with the classification size ratio (CSR) index. It shows that higher rotor cage rotary speed is beneficial to the separation of coarse and fine powders adequately, and the classification is difficult when the size distribution of material fed is narrow.

Tungsten copper (W-Cu) composites, as a traditional refractory material, are promising materials for manufacture of electrical contacts and electrodes, heavy duty electronic contacts, welding and electro-forging dies, heat sinks, packaging material, arcing resistance electrodes and thermal management devices owing to their excellent properties. This critical review presents and discusses the current progress of W-Cu composites. Starting with an introduction of the synthesis methods for W-Cu composites, including the conventional and modern preparation approaches. After that we focus on the description of the improvement of mechanical properties and arc-erosion properties by modification techniques. Finally, the advantages of W-Cu composites in applications such as electrical contacts, electronic packaging materials, and heat sinks, as well as military materials, are described, respectively.

CuW composites were synthesized using an equal channel angular pressing (ECAP) technique. Microstructural evolution during sintering process was investigated using both optical microscopy and transmission electron microscopy (TEM), and their deformation mechanisms were studied using finite element analysis (FEA). Results showed severe plastic deformation of the CuW composites and effective refinement of W grains after the ECAP process. TEM observation revealed that the ECAP process resulted in lamellar bands with high densities dislocations inside the composites. Effects of extrusion temperature and extrusion angles on stress-strain relationship and sizes of deformation zones after the ECAP process were investigated both theoretically and experimentally. When the extrusion angle was 90°, a maximum equivalent stress of ~1001 MPa was obtained when the extrusion test was done at room temperature of 22 °C, and this value was lower than compression strength of the CuW composites (1105.43 MPa). The maximum equivalent strains were varied between 0.5 and 0.7. However, when the extrusion temperature was increased to 550 °C and further to 900 °C, the maximum equivalent stresses were decreased sharply, with readings of 311 MPa and 68 MPa, respectively. When the extrusion angle was increased to 135°, the maximum equivalent stresses were found to be 716.9 MPa, 208 MPa, and 32 MPa for the samples extruded at temperatures of 22 °C, 550 °C and 900 °C, respectively. Simultaneously, the maximum equivalent strains were decreased to 0.2–0.4. Furthermore, results showed that the maximum equivalent stress was located on the sample's external surface and the stress values were gradually decreased from the surface to the center of samples, and the magnitudes of plastic deformation zones at the surface were much larger than those at the central part of the sintered samples. FEA simulation results were in good agreements with experimentally measured ones. Keywords: Equal channel angular pressing, Finite element analysis, CuW composites, Microstructure, Deformation behavior, Powder metallurgy

Graphene nanoplates (GNPs) have been adopted in titanium matrix composite manufacturing and design due to its superior mechanical properties. In this study, Graphene nanoplates reinforced Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si alloys (named as GNPs/TC21 composites) were prepared by using spark plasma sintering (SPS), and the effects of sintering temperature on the microstructure, mechanical and wear properties of GNPs/TC21 composites were systematically investigated. The results indicate that the sintering temperatures show a great influence on the interfacial structure. With the increase of sintering temperature from 850 °C to 1050 °C, the interfacial microstructures exhibit three morphographies: (1) The original TC21 particles coated with GNPs (850 °C); (2) A special TiC@GNPs band (900 °C−1000 °C); (3) A discontinuous network structure wrapped by TiC particles/plates (1050 °C). Mechanical and tribological properties of the GNPs/TC21 composites were investigated at room temperature. The optimal SPS temperature was 1000 °C, at which the GNPs/TC21 composites show the best comprehensive mechanical properties i.e. the ultimate tensile strength and yield strength are 1167 MPa and 1041 MPa, the friction coefficient is 0.16275 and the wear loss is 0.0179 g. The improvement of mechanical properties can be attributed to the synergistic strengthening of the load transfer effect of TiC@GNPs bands and the pinning effect of TiC particles/plates. The excellent wear resistance is mainly attributed to the pinning of TiC particles on the surrounding metal matrix and the strengthening and lubrication of GNPs.

Graphene nanoplates (GNPs) reinforced TC21(Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si) (GNPs/TC21) composites were prepared by using spark plasma sintering (SPS). Microstructure, mechanical and tribological properties of the sintered GNPs/TC21 composites were systematically investigated. Microstructural observations demonstrated the formation of a special sandwich interface product is in-situ TiC coated GNPs (TiC@GNPs) bands and TiC particles/plates in the microstructure of the sintered GNPs/TC21 composites. Results of mechanical testing indicated the excellent combination of strength and ductility of GNPs/TC21 composites owing to the dispersion strengthening effects of TiC particles/plates as well as the load transfer between TiC@GNPs bands and TC21 matrix. GNPs/TC21 composite with 0.05 wt% GNPs addition exhibits the relatively excellent mechanical properties, i.e., yield strength of 1017.2 MPa, ultimate tensile strength of 1161.99 MPa and an elongation about 13.2%. Among them, the dispersion strengthening effect of TiC particles/plate was the dominant factor in 0.05 wt% GNPs /TC21 composites. When the GNPs content exceeds 0.1 wt%, TiC particles/plates and TiC@GNPs bands formed at grain boundaries. The excellent combination of strength and ductility of GNPs/TC21 composites owing to the dispersion strengthening effects of TiC particles/plates as well as the load transfer between TiC@GNPs bands and TC21 matrix. Furthermore, additions of GNPs have significantly improved the tribological properties of the sintered GNPs/TC21 composites. The wear loss of 0.05 wt% GNPs/TC21 composite decreased by 21% compared to that of the sintered TC21 alloy. The superior mechanical and tribological performances of GNPs/TC21 composites can be attributed to the unique strengthening effect and the lubricating efficiency of GNPs. This demonstrates that GNPs is an ideal filler for TC21 matrix composites, as the effective lubricant and favorable reinforcement.

Successful applications of WCu alloys in high voltage electrical switches require their high strength and excellent conductivity. Unfortunately, the strategies for increasing their strength such as doping with fine particles and alloying often significantly decrease their conductivity. In this paper, we developed a new pathway for fabricating WCu alloys using spark plasma infiltrating sintering of copper-coated graphene (Cu@Gr) composite powders. Cu@Gr was found to partially prevent the formation of WC after sintering, and graphene was uniformly distributed on the surfaces of network Cu phases. Electrical conductivity of 38.512 M·S/m, thermal conductivity of 264 W·m−1·K−1 and microhardness of 278 HV were achieved for the sintered WCu composites doped with only 0.8 wt.% Cu@Gr powders, which showed 95.3%, 24.3%, 28% enhancement compared with those from the conventional sintering using the undoped WCu powders.

W70Cu30(W-30 wt.% Cu) alloys were fabricated using cold pressing and infiltration sintering methods from two types of powders, i.e., mixed copper-tungsten (M-Cu-W) powders and our newly developed copper-coated tungsten composite (Cu@W) powders. Microstructure, mechanical and arc-ablation properties of the W70Cu30 alloys were investigated, and the mechanism of enhanced physical/mechanical properties and arc-erosion resistance of the W70Cu30 alloys was discussed. For the W70Cu30 alloys prepared using the Cu@W powders, their physical properties, including hardness, electrical conductivity and relative density were much better than those prepared from the M-Cu-W powders. The W70Cu30 alloys fabricated from the Cu@W powders were free of cracks, and showed homogenous distributions of W and Cu network structures. Whereas for the alloys prepared from the M-Cu-W powders, segregation of Cu was observed and the segregation size was about 40–100 μm. Characterization of arc-erosion morphologies of the W70Cu30 alloys prepared with the Cu@W powders revealed the occurrence of evaporation of Cu phase; whereas that of W70Cu30 alloys prepared with the M-Cu-W powders revealed the occurrence of the sputtering of Cu. After arc breakdown for 200 times, mass loss of alloys made using the mixed powders was twice as much as those made using the coated composite powders. Based on the experimental results and theoretical analysis, an arc breakdown mechanism of the WCu-C alloys using the composite powders was proposed which is attributed to the formation of a homogeneous Cu-Cu network structure to uniformly disperse arc energy and dissipate the generated heat, thus prolonging the service life of the WCu alloy contacts.

The aim of this work was to join Molybdenum (Mo) and a graphite to obtain laminated structure using a high-temperature and hot press welding method with a Ti-Zr foil as the binder in a vacuum condition. The microstructure and composition changes of the interface were characterized by scanning electron microscopy and X-ray diffraction, respectively. The properties of the interface were studied using shear tests and hardness measurements. The optimal processing conditions for Mo and graphite joints involved a pressure of 0.5–1 MPa, holding time of 10–30 s, and brazing time of 5 min with a melting current of less than 150 A under a 1.3 × 10 −3 Pa vacuum condition. A transition zone with a width of approximately 0.3 mm and an average micro-hardness reaching 565 HV was formed, which is much higher than that of graphite and Mo. The main reason is the formation of different carbide and solid solutions such as MoC, Mo 9 Ti 4 , and Ti 2 C 0.06 . The shear bond strength of the graphite and molybdenum connection is 15.2 MPa, which is higher than that of graphite. The connection mechanism of Mo and graphite was the interlayer reacting with the Mo or graphite substrate at a high temperature to form compounds and solid solutions, respectively, covering the surface of the substrate. Therefore, an effective connection of Mo and graphite was successfully achieved. A model of the Mo and graphite connection structure was introduced.

W-Cu alloy sheets were fabricated in this study by multiple high temperature and high pressure (multi-HTHP) deformation technique. Crystalline structures and phase composition of W-Cu alloy sheets were investigated by transmission electron microscopy and X-ray diffraction, respectively. Results showed that W-Cu alloys possessed only W and Cu phases without other intermediate phases or solid solution of W and Cu. But shape, size and distribution of the two phases were changed after multi-HTHP deformation. Phase composition was also found without any changes after the multi-HTHP deformation treatments. Additionally, W-Cu alloy sheets with a relative density of 99.97% can be successfully obtained by multi-HTHP techniques. Cu phases are closely surrounded by W particles to form a net-work structure and W particles were also refined during the process of multi-HTHP, as a result, W-Cu alloy sheets with a higher properties in electrical conductivity (Ec) and micro-hardness reach 46.8% IACS and 450 HV, respectively, was obtained.

CNTs/Cu composite powders were prepared by an electroless plating and planetary ball milling process. The structure of composite powders was investigated. Results showed that Cu particles are homogeneously deposited on CNTs surface, and CNTs are deeply embedded into Cu matrix after milling, which play an important role in the dispersion and the wettability of CNTs within matrix and subsequently in the mechanical properties of composites. And the bulk CNTs/Cu was fabricated by vacuum sintering, re-pressing and re-sintering, and cold drawing process. It revealed that comparing pure Cu and composite without electroless plating, the tensile strength of composite with electroless plating increases obviously, up to 207 MPa. SEM images revealed that agglomeration is the main reason of deteriorating mechanical properties of composites. Finally, higher strength composites were successfully obtained by cold drawing. Results demonstrated that it can significantly improve the mechanical properties of the composites. Also, the tensile strength and the yield strength are up to 311 MPa and 185 MPa, respectively. SEM images clearly revealed that it is the dispersion and directional distribution of CNTs in matrix that further improve the strength of composites in drawing orientation. The load transfer theory was also employed to testify the results, effectively.

W–Cu composites, as a traditional material, have attracted tremendous research interest in fields such as electric engineering, electronic information, aerospace, weapons, etc., owing to their excellent properties. This critical review presents and discusses the current development of W–Cu composites. After introduction of the synthesis methods for W–Cu composites, including the conventional and modern preparation approaches, we focus on the description of the improvement of mechanical properties and arc-erosion properties by modification techniques. Finally, the advantages of W–Cu composites in applications such as electrical contacts, electronic packaging materials, and heat sinks, as well as military materials, are described.

In this work, graphene was chosen to add into W70Cu30 alloys to develop properties of arc erosion. The effect of graphene addition on arc ablation behavior of W70Cu30 contacts was investigated by the measurement of weight-loss and the vacuum electrical breakdown tests. Also, the surface morphology of W70Cu30 contacts was analyzed by using scanning electron microscopy (SEM) after arc breakdown the first and 100 times. The results reveal that addition of graphene can enhance the breakdown strength a lot, which reach up to ∼45.5%, with a maximum value of 8.5 × 106 V/m. Compared with W70Cu30 alloys without any additives, the surface morphology of W70Cu30 alloys with graphene addition have a slight splash of molten copper and a small, flat cathode craters, thus, avoiding the concentration erosion on the surface of contacts. The main ablation mechanisms of W70Cu30 alloys with graphene addition was considered to be electron transition, energy consumption and difference work function of W, Cu, graphene, respectively.

Graphene is considered as an excellent reinforcement in composite materials because of its unique physical and mechanical properties. In this study, graphene was successfully prepared at the reducing temperature of ambient temperature by oxidation reduction process using a green reducing agent and an attempt was made to fabricate graphene/W70Cu30 composites by mechanical alloy and pressureless infiltration sintering technology. Effect of graphene addition on W70Cu30 powders and W70Cu30 composites were characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, Optical microscopy (OM) and HB-3000 hardness tester. The results show that the graphene could remain after ball milling and sintering process. But, the carbides (like WC and W 2 C) were also formed in bulk composites, which revealed that some of graphene would react with W during the sintering process, but the graphene coated with Cu phase in W70Cu30 alloy still keep intrinsic structures. And W grain size is efficiently refined with addition of graphene. The relative density of graphene/W70Cu30 composites enhance with the content of graphene increasing. When the graphene content is 1.0 wt%, the relative density of composites reach up to 98.4%, which is considered that graphene could improve the wettability of W and Cu. The incorporation of graphene into W7Cu30 alloy gradually increases the hardness of composites, which is enhanced about 21% compared with the pure W70Cu30 when the graphene content is 1.0 wt%. The electrical conductivity of composites increase gradually firstly and decrease sharply with the increase of graphene content. When the 0.5 wt% graphene was added to the W70Cu30 alloys, the maximum conductivity reaches ∼46% IACS.

Types and sizes of nanoparticles as the secondary phases of metal matrix composites (MMCs) significantly affect their microstructures and mechanical properties. In literature, graphene nanoplates (GNPs) have been introduced into Ti matrix composites (TiMCs) but it is still a contradictory issue on how to simultaneously increase both the strength and toughness of the TiMCs using these graphene nanosheets. In the present work, graphene oxide nanosheets (GONs) were chosen as the reinforcement agent to prepare GONs/Ti matrix composites through a combined process of powder metallurgy and spark plasma sintering (SPS). Microstructures and mechanical properties of the TiMCs were investigated at both room temperature and high temperatures in order to evaluate strengthening and toughening effects of the GONs. It was revealed that 0.2% yield strength and ultimate tensile strength of the Ti-0.6 wt% GONs composite were increased by 7.44% and 9.65% as compared to those of pure Ti, though their elongation was slightly decreased to 22.9%, compared with 31.3% of the pure Ti. All the synthesized samples exhibited typical characteristics of ductile fracture with dimple patterns and pulling-out of the GONs. The Ti-0.6 wt% GONs composite demonstrated an enhancement of 31.66% in the 0.2% yield compressive strength measured at a temperature of 700 °C. Based on both theoretical analysis and experimental verification, the strengthening and toughening mechanisms of the nanocomposites were attributed to the synergistic effects of in-situ TiCx dispersion strengthening from the GONs and effective load transfer capability due to the well-formed interfacial structures.

Titanium alloys have been applied for many lightweight structural components in the fields of aerospace, automobiles and biomedical implants owing to their light-weight, good mechanical properties and biocompatibility. However, poor tribological performance often restricts their wide-range applications. In this study, we synthesized Cu modified Ti-6Al-4 V (TC4) powders with various Cu contents (0, 1, 3, 5, 10 wt%), which was further strengthened with 0.3 wt% graphene oxide nanosheets (GONs) using a powder metallurgy technology. These composite powders were then synthesized into titanium matrix composites using spark plasma sintering. Effects of Cu contents on microstructure evolution, phase composition and tribological properties of Ti matrix composites were systematically investigated. The synthesized composites were consisted of α-Ti, β-Ti, Ti2Cu, in-situ-formed TiC and remained GONs, and showed better tribological properties than those of TC4 alloy. The average coefficient of friction was reduced from 0.168 to a minimum value of 0.120 as the copper content increased from 0 to 3 wt%, meanwhile the wear volume loss was reduced by 49.3%. Whereas further increasing Cu contents resulted in the increases of both coefficients of friction and wear volume loss. These improvements are mainly attributed to the hardness strengthening effects by Ti-Cu intermetallics and TiC@GONs structure, as well as the self-lubricating effect of GONs. Compared with traditional surface modification processes, the new method proposed in this work is cost-effective and promising for improving the tribological performance of titanium alloys in industry applications.

In this study, the graphene was successfully prepared via the reduction of graphene oxide (GO) used thiourea dioxide as a new reductant. The graphene oxide after reduction with thiourea dioxide has been characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman photoelectron spectroscopy and transmission electron microscopy (TEM). The factors affecting reduction degree of graphene were discussed in the process of reduction, such as reduction time, temperature and the amount of reducing agent. The result shows that reduction degree of graphene is mainly controlled by temperature and reaction time. Finally, it is founded that the stronger reduction capacity of thiourea dioxide was attributed to the synergistic effect of its hydrolysis products, such as HSO3− and catalyze the ring-opening reaction.

QAl9-4 aluminum bronze and 304 stainless steel composites were fabricated by vacuum smelting-casting at 1150 °C, in a vacuum of 4.0 × 10 −2 Pa. Microstructure and chemical composition were characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). Properties of interface were studied by shear tests and hardness measurement. Between the QAl9-4 aluminum bronze and 304 stainless steel, a transition zone that the width is about 133 μm and the maximum micro-hardness value is 467 HV was observed. It is much higher than that of QAl9-4 aluminum bronze and 304 stainless steel, the main reason is considered to the formation of the brittle intermetallic compounds such as AlCrFe 2 , Al 4 Cu 9 and AlNi 3 , but these intermetallic compounds are very easy to cause the interfacial fracture. The interfacial shape close to 304 stainless steel is flat, but to QAl9-4 aluminum bronze is zigzag. The shear strength of the composite is 278 MPa. Microstructure formation mechanism of interfacial layer can be mainly considered as Fe, Cr atoms of steel substrate are given a priority in molten copper liquid compared with impurity elements (Mn, Si), eventually forming a good metallurgical bonding state on both sides of the substrate through inter-diffusion of interfacial atoms.

In this paper, arc ablation behavior of tungsten–copper (WCu) alloy contacts was investigated by the measurement of weight-loss, or erosion rate, and microstructures of tungsten–copper alloy contacts were examined by using scanning electron microscopy after electrical breakdown process. Experimental results showed that the arc erosion process started at the tiny sharp edges, then the arc spreaded on the second phases, or copper phase on the surface. Because of electrons/ions bombardment, the rapid evaporation and sputtering of liquid copper droplets were serious. As a result, an even contact surface became rugged, and tungsten skeleton remained because of being less eroded. With the increase of applied breakdown voltage, the ablation of tungsten–copper alloy became more significant. The WCu alloys with a higher content of copper showed more severe erosion rate as compared with that with a higher tungsten content under the same erosion conditions. Furthermore, when the number of breakdown is less, WCu alloys with a high copper content are easier for erosion for the conditions of fewer numbers of breakdowns. Whereas, WCu alloys with a high content of tungsten are easier for ablation for the conditions with many times of arc breakdown.

In this study, titanium matrix composites reinforced by graphene nanoplates (GNPs) were successfully prepared via an in-situ processing strategy. Both TiC nanoparticles and TiC@GNPs strips are in-situ formed at the grain boundaries, and enhance interfacial bonding strength between GNPs and Ti matrix by acting as rivets in the microstructure. The GNPs can be retained in the center of TiC layer, which provides a shielding protection effect for the GNPs. These in-situ formed TiC nanoparticles are linked together to form a discontinuous and three-dimensional (3D) network structure. Due to the formation of 3D network architecture and improved interfacial bonding, the composites show both high strength and good ductility. The significant strengthening effect reinforced by the GNPs can be attributed to a homogeneous distribution of in-situ formed TiC nanoparticles and TiC@GNPs strips, resulting in TiC interface/particle strengthening and excellent interfacial load transfer capability.

In order to improve the mechanical properties of Ti materials, SiC nanowires (SiCNWs) as reinforcement phase were added into Ti matrix. The SiCNWs reinforced Ti matrix (SiCNWs/Ti) composites were prepared by spark plasma sintering method. The effect of SiCNWs content on the microstructure and mechanical properties of the composites was investigated. The average grain size of Ti matrix in the composites decreased significantly compared with that of pure Ti after introducing SiCNWs due to grain refinment strengthening effect. XRD and TEM analysis results reveal that SiCNWs were closely bound to Ti matrix, and no reaction production was formed between them in the composites, indicating that the structural integrity of SiCNWs in the composites was preserved after sintering. The ultimate tensile strength of SiCNWs/Ti composites increases at first and then decreases with an increase of weight fraction of SiCNWs. Compared with the pure Ti, the tensile strength of the SiCNWs/Ti composites containing 0.50 wt% SiCNWs increased by 52%. The excellent tensile strength of the SiCNWs/Ti composites was mainly attributed to the grain refinement of Ti matrix, bridging, pullout and breaking of the SiCNWs.

Nanocomposite powders based on metal-coated graphene were synthesized using an in-situ co-reduction method in order to improve wettability and interfacial bonding between graphene and metal. Graphene oxide (GO) of 2~3 atomic layers was synthesized using the Hummer&rsquo, s method with graphite as a raw material and then dispersed into a dispersing agent solution mixed with N-Methyl pyrrolidone and deionized water to form a homogeneous GO suspension, which was finally added into electroless plating solutions for the reduction process. Copper-coated graphene (Cu@graphene) and nickel-coated graphene (Ni@graphene) were synthesized using this one-step and co-reduction method by mixing salt solutions containing metal ions and GOs into the plating solution. The Cu ions or Ni ions were adsorbed and bonded onto the edges and surfaces of graphene, which was reduced from the GOs using a strong reducing agent of ascorbic acid or sodium borohydride. Crystalline Cu particles with an average size of about 200 nm were formed on the surface of graphene, whereas amorphous or nanocrystalline Ni particles with an average size of 55 nm were formed on the surface of graphene. Distribution of these metal particles on the graphene is homogeneous and highly dispersed, which can effectively improve the sinterability of composite powders. Cohesive energy distribution between graphene and metal interface was analyzed using first-principle calculation method. Formation mechanism of metal coated graphene was identified to be that both the GO and metal ions were simultaneously reduced in the reducing agents and thus a chemical bonding of graphene/metal was formed between the metal particles and graphene.

Ti matrix composites reinforced with 0.6 wt% reduced graphene oxide (rGO) sheets were fabricated using spark plasma sintering (SPS) technology at different sintering temperatures from 800 °C to 1100 °C. Effects of SPS sintering temperature on microstructural evolution and mechanical properties of rGO/Ti composites were studied. Results showed that with an increase in the sintering temperature, the relative density and densification of the composites were improved. The Ti grains were apparently refined owing to the presence of rGO. The optimum sintering temperature was found to be 1000 °C with a duration of 5 min under a pressure of 45 MPa in vacuum, and the structure of rGO was retained. At the same time, the reaction between Ti matrix and rGO at such high sintering temperatures resulted in uniform distribution of micro/nano TiC particle inside the rGO/Ti composites. The sintered rGO/Ti composites exhibited the best mechanical properties at the sintering temperature of 1000 °C, obtaining the values of micro-hardness, ultimate tensile strength, 0.2% yield strength of 224 HV, 535 MPa and 446 MPa, respectively. These are much higher than the composites sintered at the temperature of 900 °C. The fracture mode of the composites was found to change from a predominate trans-granular mode at low sintering temperatures to a ductile fracture mode with quasi-cleavage at higher temperatures, which is consistent with the theoretical calculations.

Copper-aluminum (Cu-Al) based lamellar composites were prepared using a solid-liquid compound casting (SLCC) technology. Characterization results showed that the Cu-Al composites were fully-sintered at 700 °C under an argon atmosphere using the SLCC technology. Cu-Al interfacial bonding was uniform with a well-defined transitional and inter-diffusion region. Intermetallic compounds and solid solutions of CuAl2, CuAl, Cu9Al4, CuAl3 and Cu3Al2 were detected at the interfacial region. With the increase of annealing temperature, the width of the Cu-Al interfacial region was increased, and the interfacial bonding strength was also increased, whereas the types of the intermediate phases were changed. With the increase of dwelling time at a given annealing temperature, the width of Cu-Al interfacial region was increased, the interfacial bonding strength was decreased and the mesophases were changed. The bonding strength of the as-prepared composite was 30 MPa, whereas those of specimens annealed at 200 °C for 2 h, 300 °C for 2 h, 400 °C for 2 h, 300 °C for 30 min and 300 °C for 1 h were 59, 39, 74, 56, and 49 MPa, respectively. The Cu-Al interfacial bonding mechanisms were identified to be rapid inter-diffusion of copper and aluminum and formation of interfacial and graded microstructures. The formation of copper-aluminum interface is a combined result of inter-atomic diffusion and interfacial chemical reactions, the latter of which is more dominant in the diffusion process.

Al 2 O 3 /Cu (with 30 wt% of Cu) composites were prepared using a combined liquid infiltration and spark plasma sintering (SPS) method using pre-processed composite powders. Crystalline structures, morphology and physical/mechanical properties of the sintered composites were studied and compared with those obtained from similar composites prepared using a standard liquid infiltration process without any external pressure. Results showed that densities of the Al 2 O 3 /Cu composites prepared without applying pressure were quite low. Whereas the composites sintered using the SPS (with a high pressure during sintering in 10 min) showed dense structures, and Cu phases were homogenously infiltrated and dispersed with a network from inside the Al 2 O 3 skeleton structures. Fracture toughness of Al 2 O 3 /Cu composites prepared without using external pressure (with a sintering time of 1.5 h) was 4.2 MPa m 1/2 , whereas that using the SPS process was 6.5 MPa m 1/2 . These toughness readings were increased by 18% and 82%, respectively, compared with that of pure alumina. Hardness, density and electrical resistivity of the samples prepared without pressure were 693 HV, 82.5% and 0.01 Ω m, whereas those using the SPS process were 842 HV, 99.1%, 0.002 Ω m, respectively. The enhancement in these properties using the SPS process are mainly due to the efficient pressurized infiltration of Cu phases into the network of Al 2 O 3 skeleton structures, and also due to high intensity discharge plasma which produces fully densified composites in a short time.