石墨烯纳米片增强铝基复合材料的制备与性能

采用高能球磨、放电等离子烧结以及热挤压工艺制备含量为5.0%(体积分数)的石墨烯增强铝基复合材料。分别采用X射线光电子能谱、透射电镜及拉伸试验研究挤压态复合材料的显微组织与力学性能,发现5.0%(体积分数)的石墨烯分散在铝晶界上,并且未与铝基体发生界面反应。最终,挤压态复合材料的屈服强度和抗拉强度高达462 MPa和479 MPa,分别比挤压态铝基体提高62%和60%。断口分析表明,在断裂过程中复合材料中分散的石墨烯起到明显的载荷传递的作用。上述结果表明,采用高能球磨、放电等离子烧结以及热挤压制备工艺可将高含量石墨烯分散于铝合金中,且能控制石墨烯和铝基体之间的界面反应。     

Origin of Insignificant Strengthening Effect of CNTs in T6-Treated CNT/6061Al Composites

Carbon nanotube(CNT)-reinforced 6061 Al(CNT/6061 Al) composites were fabricated via powder metallurgy combined with friction stir processing(FSP). CNTs were dispersed after FSP and accelerated the precipitation process of the CNT/6061 Al composites. However, the strengthening effect of CNTs on the T6-treated materials was insignificant,while the composites under the FSP and solution treatment conditions exhibited increased strength compared to the matrix.Precipitate-free zones(PFZs) were detected around CNTs in the T6-treated CNT/6061 Al composites, and a model was proposed to describe the effect of PFZs on strength. The calculations indicated that the strength of PFZs was similar to that of the T6-treated 6061 Al. As a result, the strengthening effect of CNTs on the T6-treated CNT/6061 Al composites was insignificant.     

Dispersion and Damage of Carbon Nanotubes in Carbon Nanotube/7055AI Composites During High-Energy Ball Milling Process

High-energy ball milling (HEBM) combined with powder metallurgy route was used to fabricate carbon nanotube (CNT) reinforced 7055Al composites.Two powder morphology evolution processes (HEBM-1 and HEBM-2) were designed to investigate the dispersion and damage of CNTs during HEBM process.HEBM-1 evolution process involved powder flattening,cold-welding and fracture,while HEBM-2 evolution process consisted of powder flattening and fracture.For HEBM-1,the repetitive fracture and cold-welding process was effective for dispersing CNTs.However,the powder flattening process in HEBM-2 was unsuccessful in dispersing CNTs due to two reasons: (1) the thickness of flaky Al powders exceeded the critical value,and (2) the clustered CNTs embedded in flaky Al powders could not be unravelled.Because of the broadening of D band and the appearance of a new defect-related D'band,product of ID/IG and full width half maximum of D band,rather than ID/IG,was used to evaluate the actual damage of CNTs.It indicates that the damage of CNTs was severe in powder flattening and fracture stages,while the damage of CNTs was small in powder cold-welding stage.     

Dispersion and Damage of Carbon Nanotubes in Carbon Nanotube/7055Al Composites During High-Energy Ball Milling Process

High-energy ball milling(HEBM) combined with powder metallurgy route was used to fabricate carbon nanotube(CNT) reinforced 7055 Al composites. Two powder morphology evolution processes(HEBM-1 and HEBM-2) were designed to investigate the dispersion and damage of CNTs during HEBM process. HEBM-1 evolution process involved powder fl attening, cold-welding and fracture, while HEBM-2 evolution process consisted of powder fl attening and fracture. For HEBM-1, the repetitive fracture and cold-welding process was effective for dispersing CNTs. However, the powder fl attening process in HEBM-2 was unsuccessful in dispersing CNTs due to two reasons:(1) the thickness of fl aky Al powders exceeded the critical value, and(2) the clustered CNTs embedded in fl aky Al powders could not be unravelled. Because of the broadening of D band and the appearance of a new defect-related D' band, product of I D/I G and full width half maximum of D band, rather than I D/I G, was used to evaluate the actual damage of CNTs. It indicates that the damage of CNTs was severe in powder fl attening and fracture stages, while the damage of CNTs was small in powder cold-welding stage.     

高能球磨结合粉末冶金法制备碳纳米管增强7055Al复合材料的微观组织和力学性能

采用高能球磨结合粉末冶金工艺制备了碳纳米管(CNT)含量(体积分数)分别为0、1%和3%的CNT/7055Al复合材料。采用OM、SEM、TEM以及拉伸实验等方法研究了CNT/7055Al复合材料的CNT分布、晶粒结构、近界面结构及力学性能,分析了复合材料的强化机制和各向异性。结果表明,CNT/7055Al复合材料为无CNT的粗晶区与富集CNT的超细晶区组成的双模态晶粒结构;CNT在Al基体的超细晶区中分散良好,CNT-Al界面干净清洁,界面反应产物少;3%CNT/7055Al复合材料沿挤压方向的抗拉强度达到816 MPa,但延伸率仅为0.5%。细晶强化和Orowan强化是CNT/7055Al复合材料主要的强化机制。由于CNT沿不同方向的增强效率不同以及粗晶条带组织的存在,复合材料表现出比基体合金更强烈的各向异性,在垂直挤压方向的拉伸性能要弱于沿挤压方向的拉伸性能。     

The Effect of TaC Reinforcement on the Oxidation Resistance of CNTs/SiC CMCs

This study focuses on a two-stage spark plasma sintering (SPS) of TaC and/or carbon nanotubes (CNTs)-reinforced SiC ceramic matrix composites (CMCs). The oxidation mechanism of SiC-based CMCs with CNTs reinforcement as well as the TaC additives effect on the thermal oxidation resistance of the SiC-CNTs-TaC systems are investigated. The oxidation behavior up to 1500 °C is characterized in terms of mass changes, oxide layer formation, and thickness. The results showed that more disorder occurred in the CNT network with increased oxidation temperature. TaC additives exhibited an enhanced protective effect in increasing the oxidation temperature of CNTs from 460 to 550 °C, and this protective effect was effective at 1200 °C achieved by the crystalized Ta₂O₅ which grew with a preferred orientation giving rise to the phase separation in the glassy protective layer. Degraded oxidation resistance was found at 1500 °C.     

Effect of Carbon Nanotube Orientation on Mechanical Properties and Thermal Expansion Coefficient of Carbon Nanotube-Reinforced Aluminum Matrix Composites

Carbon nanotube-reinforced 2009Al （CNT/2009Al） composites with randomly oriented CNTs and aligned CNTs were fabricated by friction stir processing （FSP） and FSP-rolling, respectively. The CNT/2009A1 composites with aligned CNTs showed much better tensile properties at room temperature and elevated temperature compared with those with the randomly oriented CNTs, which is mainly attributed to larger equivalent aspect ratio of the CNTs and avoidance of preferential fracture problems. However, much finer grain size was not beneficial to obtaining high strength above 473 K. The aligned CNTs resulted in tensile anisotropy, with the best tensile properties being achieved along the direction of CNT aligning. As the off-axis angle increased, the tensile properties were reduced due to the weakening of the load transfer ability. Furthermore, aligned CNTs resulted in much lower coefficient of thermal expansion compared with randomly oriented CNTs.     

Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α₂-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α₂/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.     

Enhanced Homogenization of Vanadium in Spark Plasma Sintering of Ti-10V-2Fe-3Al Alloy from Titanium and V-Fe-Al Master Alloy Powder Blends

Strong and ductile powder metallurgy (PM) Ti-10V-2Fe-3Al alloy has been fabricated by spark plasma sintering (SPS) of titanium and V-Fe-Al master alloy powder blends at 1100°C for 30 min under 30 MPa. The homogenization of vanadium, which dictates the realization of a uniform microstructure of the Ti-10V-2Fe-3Al alloy, was markedly accelerated by SPS. The mechanism is attributed to the intensive Joule heating effect produced by the direct current passing through the electric conducting powder blends, rather than through spark plasma discharge, because homogenization occurred mainly after near full identification had been achieved. The chemical and microstructural homogeneity ensured the achievement of excellent tensile properties of PM Ti-10V-2Fe-3Al in the as-sintered state, with tensile strength >1250 MPa and elongation >10%.     

Thermal Conductivity and Tensile Properties of Carbon Nanofiber-Reinforced Aluminum-Matrix Composites Fabricated via Powder Metallurgy: Effects of Ball Milling and Extrusion Conditions on Microstructures and Resultant Composite Properties

Carbon nanofiber(CNF)-reinforced aluminum-matrix composites were fabricated via ball milling and spark plasma sintering(SPS), SPS followed by hot extrusion and powder extrusion. Two mixing conditions of CNF and aluminum powder were adopted: milling at 90 rpm and milling at 200 rpm. After milling at 90 rpm, the mixed powder was sintered using SPS at 560 °C. The composite was then extruded at 500 °C at an extrusion ratio of 9. Composites were also fabricated via powder extrusion of powder milled at 200 rpm and 550 °C with an extrusion ratio of 9(R9) or 16(R16). The thermal conductivity and tensile properties of the resultant composites were evaluated. Anisotropic thermal conductivity was observed even in the sintered products. The anisotropy could be controlled via hot extrusion. The thermal conductivity of composites fabricated via powder extrusion was higher than those fabricated using other methods. However, in the case of specimens with a CNF volume fraction of 4.0%, the thermal conductivity of the composite fabricated via SPS and hot extrusion was the highest. The highest thermal conductivity of 4.0% CNF-reinforced composite is attributable to networking and percolation of CNFs. The effect of the fabrication route on the tensile strength and ductility was also investigated. Tensile strengths of the R9 composites were the highest. By contrast, the R16 composites prepared under long heating duration exhibited high ductility at CNF volume fractions of 2.0% and 5.0%. The microstructures of composites and fracture surfaces were observed in detail, and fracture process was elucidated. The results revealed that controlling the heating and plastic deformation during extrusion will yield strong and ductile composites.     

高能球磨法制备的碳纳米管增强铝基复合材料的微观组织和力学性能

采用高能球磨法制备了不同体积分数的碳纳米管(CNT)与Al粉的混合粉末,用粉末冶金工艺制备了CNT/A1复合材料.微观结构分析表明.球磨可以分散一定含量的CNT到Al基体中,并与其产生良好结合.在适当的球磨工艺下.球磨不会造成CNT的严重损伤.拉伸实验表明,CNT体积分数为1.5%时,力学性能达到了最高值,屈服强度相对于纯A1基体提高了53.6%.而CNT体积分数为3%时,形成了大量的CNT团聚,力学性能迅速下降.CNT/A1复合材料的主要强化机制为细晶强化和载荷传递.     

Fabrication of Carbon Nanotube-Reinforced 6061Al Alloy Matrix Composites by an In Situ Synthesis Method Combined with Hot Extrusion Technique

Carbon nanotube(CNT)-reinforced 6061 Al alloy matrix composites were prepared by chemical vapor deposition(CVD) combined with hot extrusion technique. During the preparation process, the 6061 Al flakes obtained by ball milling of the 6061 Al spherical powders were subjected to surface modification to introduce a hydrophilic polyvinyl alcohol(PVA) membrane on their surface(6061Al@PVA) to bond strongly with nickel acetate [Ni(II)]. Then the6061Al@PVA flakes bonded with Ni(II) were calcined and reduced to Ni nanoparticles, which were then heat-treated at580 °C to remove PVA for obtaining even Ni/6061 Al catalyst. After that, the as-obtained Ni/6061 Al catalyst was employed to synthesize CNTs on the surface of the 6061 Al flakes by CVD. After hot extrusion of the CNT/6061 Al composite powders, the as-obtained CNT/6061 Al bulk composites with 2.26 wt% CNTs exhibited 135% increase in yield strength and 84.5% increase in tensile strength compared to pristine 6061 Al matrix.     

Effects of Spark-Plasma Sintering Treatment on Cold-Sprayed Copper Coatings

Cold-spray is well known as an effective coating technique to make thick metallic coatings. However, cold-sprayed metallic coatings usually have low tensile strengths due to low adhesion strength between particles, and low ductility due to low adhesion strength between particles and work hardening. Spark-plasma sintering (SPS) is a pressure-sintering technique that employs a large pulsed direct current. Compared to annealing heat treatment (AHT), SPS is expected to effectively improve the adhesion strength between particles in cold-sprayed metallic coatings. In order to investigate the effects of SPS, cold-sprayed Cu coatings were treated by both SPS and AHT under a wide range of temperatures. The microstructures and mechanical properties of the treated specimens were investigated primarily by scanning electron microscopy, electron backscatter diffraction analysis, hardness tests, and tensile tests. Despite comparable values for porosity, crystal grain size, plastic strain distribution, hardness, and yield stress, the tensile strength and ductility of the specimen treated by SPS at 400 °C (SPS400) were significantly higher than those of the specimen treated by AHT at 450 °C. Based on these results, it was determined that SPS treatment is more effective in improving the adhesion strength between the particles in cold-sprayed Cu coatings than AHT.     

Fabrication of carbon nanotube reinforced AZ91D composite with superior mechanical properties

AZ91D alloy composites with 1.0% CNTs have been fabricated by a method combined ball milling with stirring casting. The composite was investigated using optical microscopy(OM), X-ray diffraction(XRD), Fourier transform infrared spectroscope (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM) and room temperature (RT) tensile test. The results show that CNTs were homogeneously distributed in the matrix and maintained integrated structure. The yield strength and ductility of AZ91D/CNTs composite were improved by 47.2% and 112.2%, respectively, when compared with the AZ91 alloy. The uniform distribution of CNTs and the strong interfacial bonds between CNT and the matrix are dominated to the simultaneous improvement of yield strength and ductility of the composite. In addition, the grain refinement as well as the finer β phase (Mg₁₇Al₁₂) with homogenous distribution in the matrix can also slightly assist to the enhancement of the mechanical properties of the composite.     

含微纳B4C/Ti颗粒铜基复合材料的微观组织与力学性能

采用高能球磨(HEBM)和放电等离子烧结(SPS)工艺成功制备出微纳B₄C/Ti颗粒增强铜基复合材料(CTBCs),通过X射线衍射(XRD)、光学显微镜(OM)、扫描电子显微镜(SEM)以及能谱(EDS)等测试手段对其微观组织形貌进行表征。结果表明,(B₄C+Ti)颗粒在基体中均匀分布,增强体与铜基体界面结合良好,且其结合形式为冶金结合和机械结合并存。采用阿基米德排水法测定出烧结态试样的致密度。复合材料的显微硬度、拉伸屈服强度、抗拉强度和延伸率等力学性能相较于纯铜试样得到显著提高,这主要归因于载荷传递、细化晶粒与热错配等强化机制。复合材料的拉伸断口表现出明显的韧性断裂特征。     

钛对碳纳米管增强铝基复合材料组织与性能的尺寸效应

通过粉末冶金的方法,制备了致密和较高强度的CNT/Al复合材料,并系统地研究了在制备粉末阶段时引入不同粒径的钛粉后,对复合材料的组织结构与力学性能的影响。结果表明,在一定范围内,钛颗粒尺寸与制备的CNT-Ti/Al复合材料力学性能成反比。当加入的钛颗粒粒径为80 nm时,CNT-Ti/Al复合棒材力学性能最佳。其主要原因包括两个方面:一是钛颗粒有助于碳纳米管的分散,同时自身作为一种第二相强化基体;二是制备过程的热反应,使复合材料组织中生成了一种核壳结构,极大地增强了其界面结合与碳纳米管的载荷转移。     

Tribological Performance of Ni<Subscript>3</Subscript>Al Matrix Self-Lubricating Composites Containing Multilayer Graphene Prepared by Additive Manufacturing

In order to improve tribological performance of Ni₃Al-based alloy, Ni₃Al matrix composites containing 1.5 wt.% multilayer graphene (MLG) are prepared through additive manufacturing (AM) and spark plasma sintering (SPS), which are denoted as NMAM and NMSPS, respectively. Tribological behaviors of NMAM and NMSPS against Si₃N₄ balls are researched under constant speed (0.2 m/s) and varied loads (from 4 to 16 N) for evaluating the tribological properties of NMAM and NMSPS. The results present that NMAM exhibits the excellent tribological properties [low friction coefficients (0.26-0.40) and considerable wear resistance (2.8-4.6 × 10^?5 mm³ N^?1 m^?1)] as compared to NMSPS, which attributes to the uniform enrichment of MLG with properties of high tensile strength and being easily sheared off on the worn surfaces. Owing to the use of spherical prealloyed powder containing multilayer graphene and the characteristics of layer by layer depositing in the AM process, NMAM has a more compact and uniform substrate, which persistently provides a source of the formation of continuous and stable frictional layer. Due to the characteristics of AM rapid solidification, NMAM has the small grain size and well-compacted microstructure, which can effectively reduce the probability of spalling wear and lead to the increase in wear resistance of materials. The research can offer the reference for self-lubricating materials prepared by AM technology.     

The mechanisms of microstructure formation in a nanostructured oxide dispersion strengthened FeAl alloy obtained by spark plasma sintering

A bulk dense nanostructured material, obtained by spark plasma sintering (SPS) of Y₂O₃ dispersion strengthened milled Fe–40Al powder, is characterized in detail using scanning (SEM) and transmission electron microscopies (TEM) in order to investigate the mechanisms of its microstructure formation. The sintered material displays a fairly heterogeneous microstructure that covers nano- and ultrafine together with large micrometric grains. The fine grains result from recovery and recrystallization, while the larger ones from grain growth or local melting. Under the present SPS conditions, large temperature differences in the range 570–670 °C, due to rapid heating–cooling and also to no holding stage applied, essentially account for such a structural heterogeneity. Controlling SPS of the milled powder thus provides a feasible processing route to get dense hetero-nanostructured material. In addition, complex oxide particles formed in the material are analyzed to be related to precipitation reaction and oxide evolution at different sintering temperatures.     

碳纤维-FeCoCrNiAl高熵合金颗粒混杂增强7075铝基复合材料的力学性能及断裂行为

为探究双相增强体对铝基复合材料拉伸性能和断裂行为的影响,采用真空热压烧结工艺在580 ℃,30 MPa条件下保温10 min制备了FeCoCrNiAl高熵合金颗粒增强7075铝基复合材料（HEA_p/Al）,Ni-Co-P镀层修饰碳纤维增强7075铝基复合材料（CF/Al）和FeCoCrNiAl高熵合金颗粒及Ni-Co-P镀层修饰碳纤维混杂增强铝基复合材料（CF-HEA_p/Al）。并对不同复合材料微观结构及拉伸性能进行分析表征及比较。结果表明：CF-HEA_p/Al复合材料的屈服强度（YS）与极限拉伸强度（UTS）随纤维含量的升高（体积分数由0至40%）呈现先增大后降低的变化,延伸率则逐渐降低。鉴于Ni-Co-P镀层修饰碳纤维与FeCoNiCrAl高熵合金颗粒的混杂强化效应, CF-HEA_p/Al复合材料的YS和UTS较HEA_p/Al与CF/Al复合材料明显提高,且其断口表现出基体韧性断裂及纤维拔出与断裂的多种失效特征。     

Correlation between microstructure and tensile behavior of metal–intermetallic laminate compound with different initial Ni foil thickness

Ni-base metal–intermetallic laminate composites were obtained from in situ reaction synthesis between Ni and Al foils by utilizing plasma activated sintering. The effects of Ni foil thickness on the microstructure and tensile properties of the composites were investigated. The results show that the phases forming during reaction synthesis are independent of the starting thickness of the Ni foils. However, thicker reacted layers are obtained in the samples fabricated from 100 lm Ni foils(Ni100) than those obtained in the samples from 50 lm Ni foils(Ni50)when treated at the same process. The tensile strength of Ni100 samples increases with the temperature increasing at the expense of ductility. Dissimilarly, Ni50 composites treated at higher temperatures exhibit enhanced strength and ductility. Both Ni50 and Ni100 laminate fracture in a similar mechanism. Cracking first occurs in the brittle intermetallic layers. These original cracks result in shear bands in Ni layers emitted from the crack tips, and thus producing local stress concentration, which initiates new cracks in adjacent intermetallic layers. The multiplication of cracks and shear bands leads to the failure of the laminates.