【作者】 Muhammad Rashad；

【导师】 潘复生；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2014， 博士

【摘要】 随着人们对材料强度和节能降耗要求的不断提高，轻金属元素镁及镁基复合材料在航空航天和汽车行业的应用引起研究人员的极大关。纳米颗粒（如SiC、Cu、Al2O3、CNTs）及其混合物增强镁基复合材料得到广泛研究以期获得最优的强度，但对其高比强度的研究还不深入，需进一步研究。本实验，采用石墨烯纳米片做增强相颗粒来提高镁基复合材料的机械强度，此外，也探索了金属-石墨烯、金属-金属复合强化颗粒的添加对纯镁显微结构及力学性能的影响。实验发现石墨烯纳米片与少量铝颗粒复合添加可使石墨烯和镁基体界面结合良好。主要内容总结如下：首先研究了单独添加石墨烯纳米片对纯镁的影响，试验表明，石墨烯纳米片的加入可提升纯镁的抗拉强度，但塑性会有所降低。虽然石墨烯纳米片可强化镁基体，但强度提升幅度不大，其原因在于石墨烯纳米片与镁基体的界面结合效果不佳。然而少量铝的加入可有效改善石墨烯与镁基体的结合，当铝含量保持在1.0wt%，石墨烯纳米片含量为0.09~0.3wt%，0.3%，复合材料的抗拉强度和塑性均增加，这是因为越来越多的石墨烯纳米片位向与挤压方向趋于一致，从而达到阻碍基体断裂的效果。另一方面，当石墨烯纳米片含量维持在0.18wt%,铝颗粒含量为0.5%~1.0wt%时，复合材料的抗拉强度和塑性也同时增加。复合材料机械强度的提高，可用基本的强化机制解释，即强化相与基体热膨胀系数的不同，Orowan环及载荷转移机制。第二，研究了符合添加石墨烯和碳纳米管对镁基体的强化作用。对于只添加石墨烯纳米片或多壁碳纳米管强化复合材料，石墨烯纳米片-碳纳米管-铝颗粒混合添加强化复合材料具有更高的拉伸失效应变，拉伸失效应变(%)的显著增加说明石墨烯纳米片和多壁碳纳米管对镁基体的协同强化作用效果明显，原因如下：(a)一维多壁碳纳米管的引入阻碍了二维石墨烯纳米片的快速聚集。(b)基体中即软又长的多壁碳纳米管桥接相邻的石墨烯纳米片形成三维复合结构，从而阻挡了它们的聚集，使得碳纳米管+石墨烯纳米片复合结构与基体的接触面积增大。第三，研究了添加石墨烯纳米片对Mg-10Ti合金力学性能的影响，室温拉伸结果表明，钛和钛-石墨烯纳米片复合加入镁基体，复合材料的抗拉强度和失效应变均增加。此外，还研究了铜-石墨烯纳米片复合添加对纯镁力学性能的影响，试验发现铜含量保持在1.0wt%，石墨烯纳米片含量为0.18%~0.36wt%,材料强度和失效应变随之增加，然而，由于GNPs的团聚，当GNPs的含量从0.36%增加到0.54%时，失效应变逐渐下降。当继续增加其含量时，由于石墨烯纳米片的团聚，失效应变逐渐下降，直到石墨烯纳米片含量增加到0.54wt%。第四，采用粉末冶金法，研究了石墨烯纳米片对Mg-1Al-Sn合金拉伸强度的影响，试验表明，向Mg-1Al-Sn合金中加入0.18wt%的石墨烯纳米片，合金抗拉强度提高塑性降低。，复合材料强度的提升可由基本的强化机制解释。第五，研究了不同金属混合颗粒的添加对纯镁的影响，试验发现当复合添加10wt%Ti和10%Ti-1%Al颗粒时，Al元素对Ti颗粒与Mg基体界面结合的改善，Mg基复合材料的强度和塑形均得到提高。此外，也研究了复合添加Al、Cu颗粒对镁基体的影响，发现Cu颗粒在Mg基体中弥散分布，因此，当1.0wt.%Al-0.6wt.%Cu颗粒混合添加时，可同时提升材料强度和塑性。另一方面，当Cu含量保持不变，Al含量从1%增加到9%，复合材料的硬度、抗拉和抗压强度均有所提高。当Al含量在3wt.%以下时，随Al含量的增加，材料拉伸失效应变相应增大。当复合材料中Al含量从6wt%变化到9wt%时，由于脆性中间化合物Mg17Al12的产生，拉伸失效应变具有下降趋势。金属颗粒强化镁基复合材料机械强度的提升，在于基体与强化颗粒的热膨胀系数不同，造成界面处位错钉扎，同时，Orowan环和载荷也由软基体向硬质相或第二相转移。更多还原

【Abstract】 Due to ever-increasing demand for structural strength and energy efficientmaterials, the light weight magnesium metal and its composites have attractedsignificant research interest in aerospace and automotive industry. Magnesium basedcomposites reinforced with nano-size (i.e, SiC, Cu, Al2O3and CNTs) and hybridparticles have been explored extensively to achieve the optimum strength. However,further research is required to achieve high strength to weight ratio for these composites.In present work, carbonaceous material graphene nanoplatelets (GNPs) have beenused as reinforcement particles to enhance the mechanical strength of magnesiummatrix composites. Along with individual GNPs particles, the effects of metal-grapheneand metal-metal hybrid reinforcement particles addition on microstructure andmechanical behavior of pure magnesium were also investigated. Addition of smallamount of aluminum metal particle along with the graphene nanoplatelets isadvantageous to obtain good compatibility between graphene and matrix magnesium.The main contents can be summarized below:Firstly, the effect of individual GNPs addition to pure magnesium was investigated.Experimental results revealed improvement in tensile strength of pure magnesium butductility of the composite was adversely affected. It was observed that improvement intensile strength is low which may be attributed to the poor compatibility of GNPs withmagnesium matrix. In order to improve the compatibility between graphene andmagnesium matrix, small amount of aluminum was added in the composite.Experimental results showed that when aluminum content is kept constant at1.0wt.%and GNPs content was varied from0.09to0.3wt.%, both tensile strength and ductilitywas increased simultaneously. This is because more and more GNPs align along theextrusion direction and resist composite rupture. On the other hand when GNPs contentwas kept constant at0.18wt.%and aluminum particle contents were varied from0.5to1.5wt.%, then tensile strength and ductility of resulting composite increase till thethreshold of1.0wt.%. Improved mechanical strength of the composites is attributed tothe basic strengthening mechanisms, mismatch in coefficient of thermal expansionbetween reinforcement and matrix, Orowan looping and load transfer mechanism.Secondly, synergetic effect of graphene and carbon nanotubes was investigated inthe magnesium matrix. Mechanical characterization revealed that composite reinforced with hybrid (GNPs+CNTs)-Aluminum particles exhibited higher tensile failure strainrelative to those reinforced with individual GNPs and MW-CNTs. The impressiveincrease in tensile failure strain (%) confirmed the significant synergetic effect betweenGNPs and MW-CNTs. This improvement in failure strain can be attributed to:(a) therapid aggregation of two-dimensional GNPs can be inhibited by intercalating one-dimensional multi-walled carbon nanotubes (MW-CNTs) and (b) long and flexibleMW-CNTs bridge adjacent GNPs to form three dimensional hybrid structures whichprevent their aggregation, thus resulting in a high contact area between CNTs+GNPshybrid structure and the matrix.Thirdly, the effect of GNPs addition on mechanical properties of magnesium-10wt.%Titanium alloy was investigated. Room temperature tensile results revealed thataddition of Ti and Ti+GNPs into monolithic Mg lead to increase in both tensile strengthand failure strain. In addition, effect of1Cu-GNPs hybrids on mechanical strength ofpure magnesium was investigated. Experimental results shows that strength and failurestrain increase with increase in GNPs contents from0.18to0.36wt.%. However, whenGNPs contents increase from0.36to0.54wt.%then failure strain start to decrease,which is due to GNPs clustering.Fourthly, effect of graphene nanoplatelets addition on tensile strength of Mg-1Al-1Sn alloy was also investigated using powder metallurgy method. Tensile resultsrevealed that addition of0.18wt.%GNPs to the Mg-1Al-1Sn alloy matrix leads toincrease in tensile strength. However, the ductility of the resulting composite wasadversely affected. Increased tensile strength of the composite is attributed to the basicstrengthening mechanisms.Fifthly, effect of different metallic hybrid particles addition into pure magnesiumwas investigated. Pure Mg reinforced with10%Ti and10%Ti-1%Al hybrid particulatesrevealed improvement in both tensile strength and ductility. This can be attributed to thebetter compatibility between Ti particle and matrix due to presence of Al as alloyingelement. Furthermore, effect of Al-Cu particulates hybrids addition into pure Mg wasalso examined. The synthesized composites exhibited homogeneous dispersion of Cuparticles in the matrix, therefore leading to enhancement in tensile strength and ductilityby addition of1.0wt.%Al-0.6wt.%Cu hybrid particles. On the other hand, when Cucontent is kept constant and Al content was varied from1to9wt.%, experimentalresults showed improvement in hardness, tensile and compressive strength of the hybridcomposites. Increase in aluminum content led to increase in Vickers harness, strength (both in tension and compression). However, tensile failure strain of compositesincreases till the threshold value of3wt.%Al is reached. Decreasing trend of tensilefailure strain for the composites with6and9wt.%Al contents can be attributed to theformation of brittle intermetallic phases Mg17Al12. The increased mechanical strength ofmetallic particulate reinforced magnesium composites is caused by mismatch incoefficient of thermal expansion (between matrix and reinforcement particles) whichresults in punching of dislocations at the interface, Orowan looping and load transferfrom soft matrix to hard reinforcements or second phases.更多还原