镁基复合材料热膨胀的研究进展和前景展望

简要介绍了镁合金及镁基复合材料热膨胀的研究进展,叙述了温度、增强体体积分数、增强体颗粒尺寸、增强体颗粒形状、增强体种类和热处理及其他对镁基复合材料热膨胀的影响。简要介绍了热膨胀的理论预测模型,并对今后的发展做了展望。     

不同涂层碳纳米管对增强镁基复合材料力学性能的影响

采用化学镀的方法在用化学气相沉积法(CVD)制备的一维多壁碳纳米管(MWNTs)上化学镀镍、镍-锌处理,然后在氩气的保护下,采用搅拌铸造的方法与镁复合,制得碳纳米管增强镁基复合材料.在MTS电液伺服机上对其抗拉强度和伸长率进行了测试,同时,采用TEM和SEM方法对复合材料的微观组织和机构进行分析.试验结果表明:随着碳纳米管含量的增加,复合材料的抗拉强度和伸长率逐渐提高,当镀镍碳纳米管的体积百分数达到1.0%时,抗拉强度和伸长率分别提高了105%和96%;相应的镀镍锌的碳纳米管复合材料则分别提高了124%和107%,但当碳纳米管的含量达到1.5%时,抗拉强度和伸长率都明显的降低.     

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.     

纳米碳管增强镁基材料的复合工艺及性能研究

采用正交试验方法研究了碳纳米管加入量、复合温度、搅拌时间等工艺参数对用铸造方法制备纳米碳管增强镁基复合材料过程的影响,并探讨了这些工艺参数对复合材料力学性能和显微组织的作用.试验结果表明:碳纳米管(CNTs)能明显细化复合材料的晶粒组织,提高复合材料的抗拉强度、伸长率.在3个工艺参数中,CNTs对材料的力学性能影响最大,其含量约为1%时对力学性能最为有利;其次是加热温度,温度取低值(680 ℃)较好;搅拌时间在3 min时,其综合性能较好.影响材料拉伸强度的3个因素最优组合为A 2B 1C 3,影响伸长率的3个因素的最优组合为A 2B 1C 3.     

纳米碳管增强铝基复合材料的工艺及性能研究

采用正交试验方法研究了碳纳米管加入量、加入碳纳米管时的温度、凝固速度、浇注温度等工艺参数,对用半固态复合铸造法制备纳米碳管增强铝基复合材料过程的影响,并探讨了这些工艺参数对复合材料力学性能的作用。结果表明:碳纳米管(CNTs)能细化复合材料的晶粒组织,明显提高复合材料的抗拉强度、硬度。得出实验的最佳工艺参数为:碳纳米管的加入量为2%,加入碳纳米管时的温度为600℃,采用金属型铸造,浇注温度为680℃时,其综合性能最好;在4个工艺参数中,CNTs的加入量对材料的力学性能影响最大。     

TiC含量对高温钛基复合材料组织与性能的影响

通过真空非自耗熔炼工艺制备了不同TiC含量(1,2.5,5,7.5,10,15vol.%)的近α高温钛合金基复合材料。采用X射线衍射仪(XRD)、金相显微镜(OM)、扫描电镜(SEM)和万能材料试验机,系统研究了TiC_p含量对近α高温钛合金基复合材料显微组织和力学性能的影响规律。研究结果表明,可以利用Ti与C之间的原位反应制备TiC/Ti复合材料,随着TiC含量的升高,TiC的形态逐渐由长条状向等轴状、枝晶状发展,其不同的形态主要是由其凝固路径决定的。室温压缩性能表明,随着TiC含量的升高,抗压强度和屈服强度明显升高,但达到一定值后强度有不同程度的降低,而压缩率随着TiC含量的升高明显降低。     

界面改性对SiCf/Cu基复合材料热膨胀性能的影响

研究了SiCf/Cu基复合材料分别在有无Ti6Al4V界面改性涂层两种情况下的纵向热膨胀行为,并采用扫描电镜对热循环后的试样进行显微形貌观察。结果表明,界面结合强度对纤维增强金属基复合材料的纵向热膨胀行为有很大影响。对于没有Ti6Al4V涂层的复合材料,其热膨胀行为不稳定,在经历连续两次热循环后,其纵向均表现为正的残余应变,原因是基体发生了严重的界面脱粘、滑移和膨胀;而对于有Ti6Al4V涂层的复合材料,其纵向热膨胀系数明显减小,两次热循环后其尺寸保持稳定,纤维/基体界面结合也保持稳定。     

预制体对C／C复合材料热膨胀系数的影响

以天然气为前驱气体,整体碳毡和2D针刺碳毡为预制体,采用热悌度化学气相渗积技术制备了两种C／C复合材料,其表观密度均为1．74g／cm^3。借助光学显微镜和扫描电子显微镜观察了热解碳基体的生长特征和微观形貌,采用热膨胀仪测量了两种材料的热膨胀系数（CTE）,研究了由不同预制体增强C／C复合材料的CTE,解释了造成材料不同方向CTE差异的主要原因。结果表明,随着温度升高,材料A和B的CTE是逐渐升高的,且Z向CTE值均大于XY向。当两种材料在Z向的纤维体积分数接近时,随着XY方向纤维体积分数增大,材料在Z向的CTE增大,在XY向的CTE降低,两种材料存XY和Z向的CTE旱如下分布：αB—z〉dAz〉αA—xy〉αBxY-C／C复合材料的CTE主要取决于纤维体积分数和排市、碳基体及材料中的孔隙分布情况,前者起决定作用。     

碳纳米管-银复合材料力学性能研究

采用粉末冶金方法制备碳纳米管-银复合材料，研究了碳纳米管的含量对碳纳米管-银复合材料的硬度、抗弯强度影响。实验表明：当碳纳米管的含量小于11％时，复合材料的密度、硬度较好，碳纳米管起到了增强作用；当碳纳米管的含量大于11％时，由于碳纳米管的团聚，导致复合材料密度、硬度迅速下降；由于碳纳米管和银的弱界面结合，以及碳纳米管在拉应力条件下载荷传递的效力比在压应力时低，使得碳纳米管对复合材料抗弯强度的增加不明显。     

碳纳米管/ZM5镁合金复合材料的制备与力学性能研究

采用搅拌铸造法制备了碳纳米管/ZM5镁合金复合材料,研究了搅拌法加入碳纳米管的工艺特点,测试了复合材料的力学性能,并利用扫描电子显微镜和能谱分析对复合材料断口形貌进行了观察和分析.研究结果表明：当搅拌温度接近于ZM5镁合金液相线温度时,碳纳米管能较好的加入到镁合金熔体中.与基体合金相比,复合材料的抗拉强度、弹性模量、显微硬度显著增加,伸长率最大可提高110%,但是碳纳米管加入量过多会导致偏聚,使力学性能下降.碳纳米管能细化复合材料的晶粒组织,并且起搭接晶粒和承载变形抗力的作用.     

Effect of TiB2 Particles on the Tribological Properties of Stainless Steel Matrix Composites

The AISI316L stainless steel composites reinforced with 2,4,6,and 8 vol%titanium diboride（TiB₂） particles were sintered by the high pressure-high temperature method.Ball-on-disk method was carried out to study wear behavior of the composites.Tests were carried out at room temperature.The TiB₂ particles improved the hardness and tribological properties of the composites.The friction coefficient of the composites decreased with the increasing content of TiB₂.The reduction of the wear rate with the increasing of the content of TiB₂ particles in the steel matrix was also observed.It is demonstrated that the friction coefficient of composites with the same content of TiB₂ particles depend on the sintering conditions.     

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.     

Microstructure and Mechanical Properties of CNT-Reinforced AZ91D Composites Fabricated by Ultrasonic Processing

Carbon nanotube(CNT)-reinforced AZ91 D alloy composite was fabricated by ultrasonic processing.The microstructure and mechanical properties of the CNTs/AZ91 D composites were investigated.Obvious grain refinement was achieved with the addition of 0.5 wt%CNTs.The SEM observation indicated that CNTs were distributed near the grain boundary or around the inter-grain β-Mg_(17)Al_(12) phase.No evident reaction product was found at the interface between CNTs and AZ91 D matrix.Compared to the monolithic AZ91 D alloy,the yield strength,ultimate tensile strength,and elongation of the 0.5 wt%CNTs/AZ91 D composite were improved significantly.However,the poor interface bonding between CNTs and AZ91 D matrix restricted further improvement in mechanical properties.     

Influence of Multi-pass Friction Stir Processing on Microstructure and Mechanical Properties of Die Cast Al–7Si–3Cu Aluminum Alloy

The influence of overlap multi-pass friction stir processing on the microstructure and the mechanical properties, in particular, strength, ductility and hardness of die cast Al–7Si–3Cu aluminum alloy was investigated.It was observed that increase in the number of overlap passes friction stir processing resulted in significant refinement and redistribution of aluminum silicon eutectic phase and elimination of casting porosities. The microstructural refinement by the friction stir processing not only increases the ultimate tensile strength from 121 to273 MPa, but also increases the ductility as observed by the increase in fracture strain from 1.8% to 10%. Analysis of the fractured surface reveals that the microstructural refinement obtained by friction stir processing plays a vital role in transforming the fracture mode from completely mixed mode to the ductile mode of the fracture with increasing number of passes. The change in the size, shape, morphology and distribution of eutectic silicon particles and elimination of the porosities are the main reasons for the increases in tensile strength and ductility due to friction stir processing.     

Friction Stir Welding of Discontinuously Reinforced Aluminum Matrix Composites:A Review

Friction stir welding(FSW) is considered a promising welding technique for joining the aluminum matrix composites(AMCs) to avoid the drawbacks of the fusion welding. High joint efficiencies of 60%–100% could be obtained in the FSW joints of AMCs. However, due to the existence of hard reinforcing particles in the AMCs, the wearing of welding tool during FSW is an unavoidable problem. Moreover, the low ductility of the AMCs limits the welding process window. As the hard materials such as Ferro-Titanit alloy, cermet, and WC/Co were applied to produce the welding tools,the wearing of the tools was significantly reduced and the sound joints could be achieved at high welding speed for the AMCs with low reinforcement volume fraction. In this article, current state of understanding and development of welding tool wearing and FSW parameters of AMCs are viewed. Furthermore, the factors affecting the microstructure and mechanical properties of the joints are evaluated in detail.     

Microstructural Characteristics and Mechanical Behavior of Spark Plasma-Sintered Cu–Cr–rGO Copper Matrix Composites

Copper matrix composites were prepared through spark plasma sintering(SPS) process, mixing fixed amount of reduced graphene oxide(rGO) with the different amounts of Cr. In the sintered bulk composites, the layered rGO network and uniform Cr particles distributed in the Cu matrix. Both of mechanical blending and freeze-drying stages of the wet-mixing process obtained the Cu/Cr/rGO mixture powders, and then SPS solid-phase sintering realized the faster densification of these mixture powders. The hardness and compressive yield strength of the Cu–Cr–rGO composites depicted the higher values than those of pure Cu and single rGO-added composite, and they were gradually increased with increasing Cr. The rGO/Cr hybrid second-phases are believed to be beneficial to strengthening Cu matrix. The relevant formation and strengthening mechanisms involved in Cu–Cr–rGO composites were discussed.     

Effect of Particle Size on Mechanical Properties and Fracture Behaviors of Age-Hardening SiC/Al-Zn-Mg-Cu Composites

15 vol.％ SiC/Al-6.5Zn-2.8 Mg-l.7Cu (wt％) composites with varying particle sizes (3.5,7.0,14 and 20 μm),i.e.,C-3.5,C-7.0,C-14,and C-20,respectively,were fabricated by powder metallurgy (PM) method and subjected to microstructural examination.The effect of particle size on mechanical properties and fracture behaviors of the T6-treated composites was revealed and analyzed in detail.Element distribution and precipitates variations in the composites with varying particle sizes were emphatically considered.Results indicated that both tensile strength and plasticity of the T6-treated composites increased first and then decreased with particle size decreasing.The C-7.0 composite simultaneously exhibited the highest ultimate tensile strength (UTS) of 686 MPa and best elongation (El.) of 3.1％.The smaller-sized SiC particle would intro-duce more oxide impurities,which would react with the alloying element in the matrix to cause Mg segregation and deple-tion.According to strengthening mechanism analysis,the weakening of precipitation strengthening in the T6-treated C-3.5 composite was the main cause of the lower tensile strength.Additionally,the larger SiC particle,the more likely to fracture,especially in the composites with high yield strength.For the T6-treated C-20 composites,more than 75％ SiC particles were broken up,resulting in the lowest plasticity.As decreasing particle size,the fracture behaviors of the T6-treated composites would change from particle fracture to matrix alloy fracture gradually.     

Microstructural Characterization and Tensile Behavior of Rutile(TiO_2)-Reinforced AA6063 Aluminum Matrix Composites Prepared by Friction Stir Processing

Rutile(TiO_2) particle-reinforced aluminum matrix composites were prepared by friction stir processing. The microstructure was studied using conventional and advanced characterization techniques. TiO_2 particles were found to be dispersed uniformly in the composite. Clusters of TiO_2 particles were observed at a higher particle content of 18 vol%. The interface between the TiO_2 particle and the aluminum matrix was characterized by the absence of pores and reactive layer.Sub-grain boundaries, ultra-fine grains and dislocation density were observed in the composites. TiO_2 particles improved the mechanical properties of the composites. However, a drop in tensile strength was observed at a higher particle content due to cluster formation. All the prepared composites exhibited ductile mode of fracture.     

Effect of Submillimeter Variation in Plunge Depth on Microstructure and Mechanical Properties of FSLW 2A12 Aluminum Alloy Joints

Friction stir lap welding was conducted on 2 mm?+?2 mm sheets of aluminum alloy 2A12-T4. The plunge depth (PLD) was designed as 2.45-2.58 mm, which was varied in submillimeters as 2.45, 2.50, 2.53, 2.55, and 2.58 mm, and the axial force was recorded in the welding process. The results show that the PLD fluctuation in submillimeters causes significant variation in the axial force and affects the voids (i.e., Void I and Void II), hook, and effective sheet thickness (EST), among which Void I is the main factor that affects the EST. The fracture load-PLD function in the tensile shear test of the joints follows the rule of the EST-PLD function. An optimized PLD is approximately 2.55 mm, at which the EST reaches 1.71 mm, corresponding to a peak fracture load of 11.03 kN. Thus, a PLD of 2.55 mm is suggested with a tolerance of 0.02 mm, corresponding to a fracture load of 9.6-11.0 kN, i.e., within a fluctuation of 12%.