Quantitative evaluation of initial galvanic corrosion behavior of CNTs reinforced Mg–Al alloy

The macroscopic corrosion phenomenon of the CNTs reinforced Mg composites remarkably occurred in the moist environment, due to a large potential of the galvanic cells formed between α-Mg matrix and CNTs. Therefore, it is necessary to reduce the potential difference at their interfaces in order to obstruct the galvanic corrosion phenomenon. In this study, AZ61B alloy composites reinforced with CNTs (CNT/AZ61B) were fabricated by powder metallurgy method, and their potential differences between CNTs and the α-Mg matrix were reduced by concentration of Al atoms around CNTs via heat treatment. The potential distribution around CNTs was measured by using scanning Kelvin probe force microscopy (SKPFM). Heat treatment of CNT/AZ61B composites at 823 K for 10 h caused the obvious concentration of Al atoms around CNTs, and resulted in the remarkable decrease of the potential difference at the interface between the α-Mg matrix and CNTs. Additionally, the salt water immersion test results indicated that the corrosion rate of CNTs/AZ61B composite materials after heat treatment was obviously reduced to less than about 30% of the non-treated composite material. Thus, the changes of α-Mg matrix potential by concentrating Al atoms around CNTs was effective to improve initial galvanic corrosion resistance of CNTs reinforced Mg composites.     

碳纳米管和石墨烯纳米片复合增强AZ91镁基复合材料组织与力学性能

目的 解决纳米碳材料在镁基体中分散难的瓶颈问题,制备出力学性能优异的镁合金复合材料。方法 采用超声工艺将质量分数为3.0%的碳纳米管插入到质量分数为0.5%的石墨烯纳米片的片层之间,添加到AZ91镁合金基体中,借助粉末冶金技术+热挤压工艺制备了0.5%GNS+3.0%CNTs复合增强的镁基复合材料。采用光学显微镜和透射电子显微镜观察和分析了复合材料的显微组织和界面结合。测试了复合材料的力学性能,并利用扫描电子显微镜观察了复合材料的拉伸断口形貌。结果 复合材料的屈服强度、伸长率和显微硬度分别为（274±5.0）MPa,（8.4±0.2）%,HV（90.5±1.8）,与基体合金相比,分别提高了63.1%,20.0%,20.1%。结论 GNS+CNTs的加入有效细化了基体合金的晶粒组织,且与镁基体形成了较好的界面结合,促使细晶强化、应力转移强化等各种强化机制的共同作用,使复合材料力学性能显著提高。     

Effect of solidification on microstructures and mechanical properties of carbon nanotubes reinforced magnesium matrix composite

A novel approach was successfully developed to fabricate bulk carbon nanotubes (CNTs) reinforced Mg matrix composites. The distribution of CNTs in the composites depends on the solidification rate. When the solidification rate was low, CNTs were pushed ahead of the solidification front and will cluster along grain boundaries. When the solidification rate was high, CNTs were captured by the solidification front, so the CNTs remained inside the grain. Moreover, good interfacial bonding was achieved in the composite under high solidification rate. Meanwhile, compared with the matrix alloy, the ultimate tensile strength (UTS) and yield strength (YS) of the composite were significantly improved. The mechanical properties of the composite under higher solidification rate are better than composite under low solidification rate and the alloy. Moreover, most CNTs on the fracture surfaces were directly pulled out from the matrix. The Kelly–Tyson formula agreed well with the experimental tensile value in the composite under higher solidification rate, and the load-transfer efficiency is almost equal to 1.     

Aging behavior of the matrix of aluminum-magnesium-silicon alloy including carbon nanotubes

The mechanical behavior of the non-aged and the aged Al-Mg-Si alloy composites reinforced with CNTs was evaluated with tensile test and Vickers hardness test. The composites showed higher mechanical strength than the pristine Al alloy before the aging, although exhibiting lower mechanical strength after the aging. The aging treatment was ineffective to strengthen the Al-Mg-Si alloy composites reinforced with CNTs. EDS elemental mapping clarified the Mg and O concentration around CNTs. The reduction of Al oxide species formed around CNTs facilitated the Mg concentration. Due to the Mg consumption around CNTs, Mg elements in Al matrix decreased, and resulted in the incomplete matrix strengthening after the aging.     

碳纳米管-Ti3AlC2/AZ91D复合材料的微观组织及力学性能

采用化学镀铜的方法对增强相碳纳米管(CNTs)和Ti₃AlC₂进行表面改性,热压烧结制备了CNTs-Ti₃AlC₂/AZ91D复合材料,研究了其微观组织和力学性能的变化及增强机制。结果表明:CNTs-Ti₃AlC₂/AZ91D复合材料内部主要物相为CNTs、Ti₃AlC₂、Mg和Al₁₂Mg₁₇,增强相均匀分布在基体内,在增强相与基体的界面处存在U相(MgAlCu),使二者界面结合良好。当增强相CNTs 和Ti₃AlC₂含量分别为1wt%和25wt%时,较镁合金AZ91D,CNTs-Ti₃AlC₂/AZ91D复合材料的弹性模量、拉伸强度、屈服强度和延伸率分别提高了120.30%、25.72%、126.50%和36.84%,弯曲强度和压缩强度分别为337.92 MPa和436.27 MPa。CNTs-Ti₃AlC₂/AZ91D复合材料的断裂方式表现为脆性断裂,其强化机制主要为热配错强化、Orowan强化和细晶强化机制。      

高能球磨法制备高性能均一分散CNTs/Al5083复合材料

采用卧式高能球磨法制备0%~2%CNTs/Al5083(质量分数)复合材料,研究球磨时间和CNTs含量对复合材料性能的影响。采用扫描电镜(SEM)和透射电镜(TEM)对复合材料的形貌进行表征,测试复合材料的抗拉强度及硬度。结果表明:当球磨时间为1.5h时,CNTs可均匀分散在Al5083基体中;CNTs质量分数为1.5%时,CNTs/Al5083界面结合力最好,复合材料的抗拉强度和硬度分别为188.8MPa和136HV,比未加CNTs的Al5083合金基体分别提高了32.2%和36%。     

Influence of nano-sized carbon nanotube reinforcements on tensile deformation,cyclic fatigue,and final fracture behavior of a magnesium alloy

Magnesium alloy (AZ31) based metal matrix composite reinforced with carbon nanotubes (CNTs) was fabricated using the technique of disintegrated melt deposition followed by hot extrusion. In this research paper, the microstructure, hardness, tensile properties, tensile fracture, high cycle fatigue characteristics, and final fracture behavior of CNTs-reinforced magnesium alloy composite (denoted as AZ31/1.0 vol.% CNT or AZ31/CNT) is presented, discussed, and compared with the unreinforced counterpart (AZ31). The elastic modulus, yield strength, tensile strength of the reinforced magnesium alloy was noticeably higher compared to the unreinforced counterpart. The ductility, quantified both by elongation-to-failure and reduction in cross-section area of the composite was higher than the monolithic counterpart. A comparison of the CNT-reinforced magnesium alloy with the unreinforced counterpart revealed a noticeable improvement in cyclic fatigue life at the load ratios tested. At all values of maximum stress, both the reinforced and unreinforced magnesium alloy was found to degrade the cyclic fatigue life at a lower ratio, i.e., under conditions of fully reversed loading. The viable mechanisms responsible for the enhanced cyclic fatigue life and tensile behavior of the composite are rationalized in light of macroscopic fracture mode and intrinsic microscopic mechanisms governing fracture.     

Processing and Characterization of In Situ (TiC–TiB2)p/AZ91D Magnesium Matrix Composites

In this paper, a practical and cost‐effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB₂ particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B₄C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Ti_p + B₄C_p) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X‐ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four‐point‐bending test at room temperature. Microstructural characterization of the (TiB₂–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB₂ and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH₂ powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

SPS制备TiNi增强镁合金复合材料的微观结构及力学性能

以AZ31镁合金为基体,TiNi形状记忆合金丝为增强体,利用放电等离子烧结法（SPS）制备了TiNi/Mg复合材料,用OM、SEM、EDS对其微观形貌进行表征,并用XRD及DSC研究TiNi丝的相变,同时对该复合材料进行准静态拉伸实验,对其室温及高温力学性能进行研究。结果表明,所制备的TiNi/Mg复合材料中界面处存在Mg、Ti、Ni元素的互扩散现象,并形成宽度约为2 μm的互扩散层;所制备的TiNi/Mg复合材料的高温力学性能高于室温,其中其屈服强度、抗拉强度及弹性模量在100℃时（分别为157 MPa,292 MPa,22 GPa）较室温分别提高了12%、33%和29%,150℃时（分别为143 MPa,251 MPa,20 GPa）较室温分别提高了2%、14%和18%。     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.     

A Novel Melt Processing for Mg Matrix Composites Reinforced by Multiwalled Carbon Nanotubes

Carbon nanotubes(CNTs) reinforced Mg matrix composites were fabricated by a novel melt processing.The novel processing consisted of two courses:CNTs pre-dispersion and ultrasonic melt processing.Mechanical ball-milling was employed to pre-disperse CNTs on Zinc(Zn) flakes.Serious CNT entanglements were well dispersed to single CNT or tiny clusters on Zn flakes.The ultrasonic melt processing further dispersed CNTs in the Mg melt,especially tiny CNT clusters.Thus,a uniform dispersion of CNTs was achieved in the as-cast composites.Hot extrusion further improved the distribution of CNTs.CNTs increased both the strength and elongation of the matrix alloy.Notably,the elongation of the matrix alloy was enhanced by 40%.Grain refinement and the pulling-out of CNTs resulted in the evident improvement of ductility for the composites.     

Improved processing of carbon nanotube/magnesium alloy composites

Carbon nanotubes (CNTs) are promising reinforcements for light weight and high strength composites due to their exceptional properties. However, until now, the main obstacle is to obtain a homogenous dispersion of the CNTs in the desired material matrix. Quite a few methods have been studied to help improving the dispersion of CNTs in a polymer matrix. But not much research has been conducted on how to disperse CNTs in metal matrices. In this study, a two-step process was applied. In the first stage, a block copolymer was used as a dispersion agent to pre-disperse multiwall carbon nanotubes (MWNTs) on Mg alloy chips. Then the chips with the well dispersed MWNTs on their surface were melted and at the same time vigorously stirred. The molten MWNT Mg alloy composites were poured into a cylindrical mould to solidify quickly. For the pre-dispersion step, the microstructures of the Mg alloy chips were studied under SEM. MWNTs were quite successfully dispersed on the surfaces of the Mg alloy chips. The mechanical properties of the MWNT/Mg composites were measured by compression testing. The compression at failure, the compressive yield strength and ultimate compressive strength have all been improved significantly up to 36% by only adding 0.1 wt% MWNTs to the Mg alloy. In order to predict the potential yield strengths of the MWNT reinforced Mg alloy composites, the contributions by load transfer, Orowan strengthening and thermal mismatch were added up.     

碳纳米管对Fe-P非晶的力学性能和晶化行为影响的研究

采用快速凝固技术制备了碳纳米管／铁碳非晶复合材料,并对其组织、力学性能和热稳定性进行了研究检测结果表明,碳纳米管在非晶基体中的弥散存在,使得非晶抗拉强度提高,晶化激活能增加,晶化特征温度明显提高,加入２Ｗ／％碳纳米管,使铁磷非晶的室温抗拉强度提高了１２０％,晶化激活能增加了约４０％,晶化开始温度提高了约１００Ｋ,此外,在温度高于其晶化温度约２００Ｋ时,碳纳米管和非晶基体界面间发生了固相反应。     

Compressive behaviour of SiC/ncsc reinforced Mg composite processed through powder metallurgy route

In this present work nano coconut shell charcoal (ncsc) and silicon carbide (SiC) particulates were reinforced with AZ31B Mg alloy and suitable magnesium composite was developed by using the powder metallurgy technique followed by hot extrusion. Density measurement of the Mg composites revealed that the addition of ncsc significantly improved the density of the composites and porosity measurement showed minimal porosity. The microstructure of the composites showed even distribution of the ncsc in the AZ31B/3SiC Mg composite. The compressive and impact behaviour of the samples were characterized, the results showed that on increasing the weight percentage of ncsc in AZ31B/3SiC/0.5ncsc Mg composites the mechanical properties such as ultimate compressive strength, 0.2% yield strength, ductility and impact strength decreased. The scanning electron microscope (SEM) analysis of fractured surface of AZ31B Mg alloy and AZ31B/3SiC/0.5ncsc Mg composites showed quasi-cleavage fracture. The presence of ncsc above 0.5 wt% composites revealed mixture of quasi cleavage planes and some dimples.     

石墨烯纳米片/AZ91镁基复合材料的显微组织与力学性能

采用铸造工艺制备了石墨烯纳米片(GNPs)增强的AZ91镁基复合材料,测试了复合材料的力学性能,并利用光学显微镜、X射线衍射仪、透射电子显微镜、扫描电子显微镜和能谱仪对复合材料的微观组织、界面结合和断口形貌进行了表征和分析,讨论了复合材料的强化机理。结果表明:石墨烯纳米片可有效细化镁基体的晶粒组织,在添加少量石墨烯纳米片时(0.1%),复合材料的屈服强度、延伸率和显微硬度分别为(164±5)MPa、(7.7±0.1)%和(74.2±2)HV,比基体分别提高了37.8%、13.2%和24.7%。GNPs与镁基体形成了强界面结合,这更有利于发挥应力转移强化、细晶强化等作用,提高镁合金强度、塑性等力学性能。     

Eigenspannung im mit gasdruckunterstütztem Feingussverfahren hergestellten langfaserverstärkten Aluminium‐Matrix‐Verbundwerkstoff

Residual Stress in Continuous Fibre Reinforced Aluminium Matrix Composites Prepared by Modified Investment Casting The residual stresses between matrix und fibres in the continuous γ‐Al₂O₃ fibre reinforced aluminium alloy (AlZn6Mg1Ag1) matrix composites prepared by modified investment casting were measured with x‐ray diffraction as well as simulated with FEM. It was indicated as expected that tensile residual stress exists in the Matrix und compressive residual in the fibre. The average value of the residual stress in both matrix and fibre in the composites is not very significant. However it is distributed very unevenly. Next to the interface between matrix and fibre there is a small zone in the matrix with relative great tensile residual stress. The effect of fibre volume percentage on the residual stress in the composite was also analysed. With increase of the fibre volume percentage the tensile residual stress in the matrix increases while the compressive residual stress in the fibre decreases. If the fibre volume percentage in the composite exceeds 65 %, the maximal tensile residual stress will reach the yield stress of the matrix alloy and local plastic deformation will occur.     

Alumina powder assisted carbon nanotubes reinforced Mg matrix composites

Mg matrix composites reinforced by carbon nanotubes (CNTs)-Al₂O₃ mixture, which was synthesized by in situ growing CNTs over Al₂O₃ particles through chemical vapor deposition (CVD) using Ni catalyst, were fabricated by means of powder metallurgy process, followed by hot-extrusion. By controlling synthesis conditions, the as-grown CNTs over Al₂O₃ particles possessed high degree of graphitization, ideal morphology, higher purity and homogeneous dispersion. Due to the ‘vehicle’ carrying effect of micrometer-level A₂O₃, CNTs were easy to be homogeneously dispersed in Mg matrix under moderate ball milling. Meanwhile, Al₂O₃ particles as catalyst carriers, together with CNTs, play the roles of synergistic reinforcements in Mg matrix. Consequently, the Mg matrix composites reinforced by CNTs-Al₂O₃ mixture exhibited remarkable mechanical properties.     

New alignment procedure of magnetite–CNT hybrid nanofillers on epoxy bulk resin with permanent magnets

The main properties of epoxy composites reinforced with aligned carbon nanotubes (CNTs) have been studied. The alignment was carried out in a specific designed device applying a weak magnetic field (0.3 T) with permanent magnets. CNTs were modified with magnetite nanoparticles (Fe₃O₄) functionalized, in a one-stage-process which does not require use of strong acids or aggressive treatments which could affect the structural integrity of CNTs. The study by transmission electron microscopy confirmed that the Fe₃O₄ nanoparticles were closely bonded over CNT surfaces. The thermo-mechanical and tensile properties of composites measured were higher than neat epoxy resin and were similar for both composites: reinforced with neat CNTs and magnetite–CNT hybrid nanofillers. The electrical behaviour indicates a high anisotropy for aligned composites, showing an increase of one order of magnitude for the electrical conductivity in the direction of aligned nanotubes.     

Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites

Carbon nanotubes (CNTs) treated by using a mixed solution of H₂SO₄ and HNO₃ were uniformly dispersed into cement paste by means of ultrasonic energy. Electrical resistivity and pressure-sensitive properties under cyclic compressive loading of this composite were analyzed and compared to that of untreated-CNT reinforced cement paste. Results show that the addition of treated or untreated CNTs to cement paste leads to a notable decrease in volume electrical resistivity and a distinct enhancement in compressive sensitivity. The microstructures of these cement composites were analyzed by using scanning electron microscope. The microscopic observation reveals that both treated and untreated CNTs were dispersed homogenously in the cement matrix. For untreated CNT-reinforced cement composites, the CNTs with glossy surface were zigzag and cling to cement matrix; the bridging of cracks and a well three-dimensional meshwork were also observed. For treated-CNT reinforced cement composites, the surface of CNTs was covered by C–S–H, which leads to a higher mechanical strength. The contact points of the treated-CNTs in composites were much fewer than that of the untreated-CNTs in cement matrix composites, which leads to a higher compressive sensitive properties and a lower electrical conductivity.