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.     

压力对挤压铸造Mg-Zn-Y准晶增强AZ91D镁基复合材料显微组织和力学性能的影响(英文)

采用挤压铸造工艺制备Mg-Zn-Y准晶增强AZ91D镁基复合材料,研究挤压压力对此复合材料显微组织和力学性能的影响。研究结果表明:挤压铸造工艺是细化晶粒的有效方法,复合材料由α-Mg基体、β-Mg17Al12相以及二十面体Mg3Zn6Y准晶相(I相)组成,且随着挤压压力的增大,β-Mg17Al12相以及Mg3Zn6Y准晶颗粒含量增加,基体晶粒进一步细化,α-Mg树枝晶向等轴晶转变;当挤压压力为100 MPa时,极限抗拉强度和断后伸长率达到最大值,分别为194.3 MPa和9.2%,拉伸断口出现大量韧窝;准晶增强AZ91D镁基复合材料的强化机制主要为细晶强化和准晶颗粒强化。     

高应变速率下AZ31镁合金的各向异性及拉压不对称性

采用分离式霍普金森拉杆及压杆装置,研究挤压态AZ31镁合金高速变形下的各向异性及拉压不对称性,并从微观变形机制的角度探讨具有强烈初始基面织构的挤压态镁合金各向异性及拉压不对称性产生的原因。结果表明:在高速变形条件下,依据加载方向及应力状态挤压态AZ31镁合金的拉伸行为表现出很强的各向异性,但压缩行为的各向异性不明显;在挤压方向表现出很强的拉压不对称性,而在垂直于挤压方向的拉压不对称性很低。挤压态AZ31镁合金宏观上的各向异性及拉压不对称性是由于不同的微观变形机制所引起的。沿挤压方向拉伸的主要变形机制为柱面滑移,沿垂直于挤压方向拉伸及压缩的主要变形机制为锥面滑移;沿挤压方向压缩时初始变形机制为拉伸孪晶,当变形量为0.08(8%)左右时由于孪晶消耗殆尽,变形变而以滑移的方式进行。     

过渡型界面对粗晶/超细晶异质结构铜屈服和断裂行为的影响（英文）

为了研究界面约束对异质结构材料屈服和断裂力学性能的影响,采用搅拌摩擦加工方法制备具有过渡型界面的粗晶/超细晶纯铜式样。拉伸测试结果表明:由粗晶和超细晶力学不协调单元弹-塑性交互作用所产生的协同强化效应随约束面积的增加而增加。使用数字散斑相关方法揭示异质结构材料颈缩过程中界面区域的应变集中分布。使用扫描电子显微镜系统分析断口形貌特征。结果发现,界面区域分布有大量大尺寸韧窝,这可能是因为粗晶与超细晶的交互约束作用在界面处产生高三轴内应力和塑性应变累积。此外,还发现粗晶区域通过剪切方式断裂,而不是拉伸应力所引起的韧窝生长机制。     

铝镁双金属复合棒材反向等温挤压微观组织演化机理

针对铝镁双金属复合挤压工艺,探究棒材挤压过程中材料流动特性及微观组织演变规律。通过热模拟压缩试验得到不同温度下纯铝和镁合金的流变曲线,建立纯铝和AZ31镁合金的组织演化热力学模型并进行计算。研究发现双金属挤压过程中铝镁界面处材料流动速度相差较大,铝表层局部应变在进入模具工作带前达到临界值,摩擦热促使晶粒严重长大,产生粗晶:通过EBSD扫描结果可知交界面层从Al至Mg的相组成依次为Al、Al3Mg2、Al12Mg17、Mg,沿速度方向层状分布,与数值计算结果相符。通过与试验对比验证,晶粒尺寸计算的平均误差为17%,计算发现在温度满足热变形的条件下,材料应变对挤压复合材料的晶粒度影响较大,应变的增大促进再结晶致使晶粒尺寸增大。     

挤压铸造准晶增强AZ91D镁基复合材料组织与性能

为了改善AZ91D镁合金的性能,采用挤压铸造法制备了Mg-Zn-Y准晶中间合金增强AZ91D镁基复合材料,研究了准晶中间合金含量对复合材料组织和性能的影响。结果表明挤压铸造工艺可以有效细化晶粒,复合材料的显微组织主要由α-Mg基体、晶界上分布的β-Mg17Al12相以及Mg3Zn6Y准晶颗粒组成,准晶颗粒和α-Mg基体之间形成稳定结合。当准晶中间合金含量为5%时,抗拉强度和断后伸长率达到最大值,分别为194.3MPa和9.2%。复合材料的强化机制为细晶强化和准晶颗粒强化。     

激光焊接铜与不锈钢接头之间焊缝区显微组织的非均匀性（英文）

研究激光焊接(LBW)工艺对铜(Cu)和不锈钢(SS)异种焊缝显微组织与力学性能关系的影响。采用背散射(BSE)扫描电子显微镜(SEM)成像来表征LBW (Cu-SS)焊缝的高度非均匀组织特征。BSE分析充分证明在异种焊接界面和熔合区产生复杂的显微组织。沿铜/焊缝界面存在明显的晶粒生长差异:从粗柱状晶到等轴状超细晶。高分辨电子背散射衍射(EBSD)分析证实铜/焊缝界面存在晶粒细化机制。拉伸和冲击试验结果表明,异种焊缝的拉伸和冲击性能与铜基体的基本一致。在熔合区和铜/焊接界面区等非均匀材料成分区,存在明显的显微硬度梯度。为了全面了解局部焊缝亚区的组织与性能的关系,确定出焊缝亚区的异质形核点,并与显微硬度测试结果相互关联。     

Al2O3/Al纳米复合材料的强化机制

将含氢等离子蒸发法制备的Al2O3/Al纳米复合粉体冷压成直径为25mm，厚度为2mm的块材，并通过620℃，40min热烧结和变形量为55％的冷轧形变处理使样品的相对密度达到99％。对官致密Al2O3/Al纳米复合材料的拉伸实验表明：其屈服强度σ0.2和断裂强度σb分别为粗晶Al的12－16倍和5－6倍，延伸率δ比同质冷轧粗晶Al约高28％。表征了Al2O3/Al纳米复合材料的结构和热稳定性，研究了晶粒细化的强化效应、非晶Al2O3弥散增强和冷变形加工硬化等对材料强度的影响。探讨了Al2O3/Al纳米复合材料的强化机制。     

等通道转角挤压制备超细晶Mg15Al双相合金组织与性能

对高铝双相合金Mg15Al在553K以Bc路线进行了不同道次的等通道挤压(ECAP),获得了超细晶高铝镁合金。通过OM,SEM,TEM分析了ECAP前后合金的微观组织结构及断口形貌,并测试了不同挤压道次后合金的硬度和室温拉伸性能,分析了ECAP细化晶粒机理及其性能改善原因。结果表明,随挤压道次增加,累计形变增强,网状硬脆相β-Mg17Al12破碎,合金晶粒显著细化,但对单相区和两相混合区细化效果不同。在α、β两相共存区内,4道次ECAP后形成100nm～200nm的细晶粒;在α单相区,4道次ECAP后晶粒为1μm以下,且在初晶α-Mg内析出弥散细小的β相,起到细晶强化和弥散强化作用。8道次ECAP后,晶粒略有长大。ECAP使合金的硬度、抗拉强度和延伸率同时得到提高,尤其是4道次ECAP后,硬度提高了32.04%,抗拉强度σb从150MPa提高到269.3MPa,延伸率δ由0.05%提高到7.4%;8道次ECAP后,硬度、抗拉强度略有下降,延伸率略有上升。SEM断口观察显示ECAP使合金拉伸断口形貌由铸态的解理断裂特征转变为延性韧窝断裂特征。     

粒度和界面原子调控对SiC/Al复合材料强度的影响

SiC/Al复合材料作为一种轻质高强材料,因其优异的物理化学性能被外界广泛关注。本研究利用分子动力学方法,构建了不同SiC粒径的SiC/Al复合材料模型,根据拉伸变形模拟结果得出更小的SiC粒径有利于材料获得更高的抗拉强度。随着拉伸形变的逐渐增加,SiC颗粒在沿拉伸方向的两侧与Al基体发生分离从而产生孔隙,再从孔隙缺陷处产生位错形核并扩展至Al基体内形成塑性形变。在调节SiC/Al界面上C、Si的占位情况后,界面富Si的条件下结合更强,孔隙产生的难度增大从而对SiC/Al复合材料产生强化作用。     

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.     

Effect of Spark-Plasma-Sintering Conditions on Tensile Properties of Aluminum Matrix Composites Reinforced with Multiwalled Carbon Nanotubes (MWCNTs)

In this study, aluminum (Al) matrix composites containing 2 wt.% multiwalled carbon nanotubes (CNTs) were fabricated by powder metallurgy using high-energy ball milling (HEBM), spark plasma sintering (SPS), and subsequent hot extrusion. The effect of SPS conditions on the tensile properties of CNT/Al composites was investigated. The results showed that composites with well-dispersed CNTs and nearly full-density CNT/Al can be obtained. During HEBM, CNTs were shortened, inserted into welded Al powder particles, bonded to Al, and still stable without CNT-Al reaction. After consolidation, Al₄C₃ phases formed in composites under different sintering conditions. With the increase of sintering temperature and holding time, the strength decreased. Conversely, the ductility and toughness noticeably increased. As a result, a good balance between strength (367 MPa in ultimate tensile strength) and ductility (13% in elongation) was achieved in the as-extruded CNT/Al composite sintered at 630°C with a holding time of 300 min.     

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.     

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.     

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.     

Fatigue strength and microstructure evaluation of Al 7050 alloy wires recycled by spray forming,extrusion and rotary swaging

Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated (solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 °C, imparting higher tensile (586 MPa) and fatigue (198 MPa) strengths.     

Fabrication of aluminum matrix composite reinforced with carbon nanotubes

1.0wt.% carbon nanotube （CNT） reinforced 2024Al matrix composite was fabricated by cold isostatic press and subsequent hot extrusion techniques. The mechanical properties of the composite were measured by a tensile test. Mean-while,the fracture surfaces were examined using field emission scanning electron microscopy. The experimental results show that CNTs are dispersed homogeneously in the composite and that the interfaces of the Al matrix and the CNT bond well. Although the tensile strength and the Young’s modulus of the composite are enhanced markedly,the elongation does not decrease when compared with the matrix material fabricated under the same process. The reasons for the increments may be the extraordinary mechanical properties of CNTs,and the bridging and pulling-out role of CNTs in the Al matrix composite.     

A comparative study on microstructural evolution and surface properties of graphene/CNT reinforced Al6061−SiC hybrid surface composite fabricated via friction stir processing

A comparative study on the surface properties of Al−SiC−multi walled carbon nanotubes (CNT) and Al−SiC−graphene nanoplatelets (GNP) hybrid composites fabricated via friction stir processing (FSP) was documented. Microstructural characterization reveals a more homogeneous dispersion of GNPs in the Al matrix as compared to CNTs. Dislocation blockade by SiC and GNP particles along with the defect-free interface between the matrix and reinforcements is also observed. Nanoindentation study reveals a remarkable ∼207% and ∼27% increment in surface nano-hardness of Al−SiC−GNP and Al−SiC−CNT hybrid composite compared to as-received Al6061 alloy, respectively. On the other hand, the microhardness values of Al−SiC−GNP and Al−SiC−CNT are increased by ∼36% and ∼17% relative to as-received Al6061 alloy, respectively. Tribological assessment reveals ∼56% decrease in the specific wear rate of Al−SiC−GNP hybrid composite, whereas it is increased by ∼122% in Al−SiC−CNT composite. The higher strength of Al−SiC−GNP composite is attributed to the mechanical exfoliation of GNPs to few layered graphene (FLG) in the presence of SiC. Also, various mechanisms such as thermal mismatch, grain refinement, and Orowan looping contribute significantly towards the strengthening of composites. Moreover, the formation of tribolayer by the squeezed-out GNP on the surface is responsible for the improved tribological performance of the composites. Raman spectroscopy and various other characterization methods corroborate the results.     

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。