【作者】 张斌；

【导师】 刘勇； 费林峰；

【作者基本信息】 南昌大学 ， 材料工程（专业学位）， 2022， 硕士

【摘要】 镁合金密度小（1.78 g/cm³）,约为铝的2/3,铁的1/4,具有比强度和比刚度高,阻尼性能好,电磁屏蔽性能强,易回收等优点,被誉为21世纪绿色工程材料。然而镁合金存在绝对强度和弹性模量低的缺点。在镁基体中添加增强体制备镁基复合材料能够有效提高镁合金强度和模量。但镁基复合材料强-韧性矛盾严重限制了其工业应用。通过石墨烯与镁基体的界面调控,借助中间桥梁氧化镁对镁基体异质形核作用,巧妙构筑GNPs/AZ91复合材料双峰结构（粗/细晶/石墨烯）,充分发挥粗/细晶/石墨烯协调变形能力,实现镁基复合材料强度和韧性协同提升。本工作研究球磨、烧结和热变形工艺对GNPs/AZ91复合材料双峰结构的影响规律;研究镁基复合材料双峰结构的力学行为,探讨粗晶/细晶/石墨烯之间的协调变形机制;研究GNPs/AZ91复合材料双峰结构的断裂机制,探究粗晶/细晶、GNPs/Mg O/α-Mg的界面结构,实现界面结构调控;揭示GNPs/AZ91复合材料双峰结构的强韧化机制。本文的主要研究结果如下:1.采用球磨-冷压-热压烧结-热挤压的工艺方式制备了具有双峰结构特征的GNPs/AZ91复合材料。结果表明,调控球磨工艺参数（高速+低速球磨）结合高致密化的烧结和热挤压工艺是实现GNPs/AZ91复合材料双峰结构构建的关键。通过对双峰结构GNPs/AZ91复合材料不同尺度下的微观组织表征,探明了双峰结构中粗/细晶区的组织特征、第二相的成分和分布特征以及GNPs/Mg O/α-Mg之间的界面形貌,并且还观察到GNPs与α-Mg界面处纳米第二相(Mg₁₇Al₁₂)的异常析出行为。结合各工艺流程后的材料成分和微观组织表征结果,获得了制备过程中GNPs/AZ91复合材料双峰结构形成机制。2.对双峰结构GNPs/AZ91复合材料进行力学性能表征。硬度测试结果表明双峰结构GNPs/AZ91复合材料的显微硬度值为85.8±4.1 HV,比实验组AZ91合金提高18.3%。通过纳米压痕测试结果表明,双峰结构GNPs/AZ91复合材料的粗/细晶区硬度值分别为105.51±5.82 HV和126.64±3.83 HV,弹性模量分别为45.45±0.84 GPa和49.43±2.00 GPa,粗/细晶区间表现出明显的力学性能差异。粗/细晶区间的性能差异与石墨烯分布、晶粒尺寸大小以及第二相的富集分布有密切关联;拉伸性能测试结果表明,双峰结构GNPs/AZ91复合材料的抗拉强度和延伸率分别为375±8.6 MPa和8.20±0.70%,与均质GNPs/AZ91复合材料相比,抗拉强度和延伸率分别提高8%和146%,塑性变形能力有显著的提升。最终结果表明,双峰结构构建对镁基复合材料的强-塑性有明显的协同提升作用,有效改善了GNPs/AZ91复合材料的塑性表现。依托原位SEM力学测试,对双峰结构GNPs/AZ91复合材料的变形行为进行讨论,探讨了粗细晶区在拉伸过程中的组织演变规律,揭示双峰结构GNPs/AZ91复合材料在裂纹萌生、扩展直至断裂的失效机制。此外,采用数字图像相关（Digital Image Correlation,DIC）方法对双峰结构GNPs/AZ91复合材料进行全局和不同倍率下的应变分析,建立宏观力学性能与其内部微结构参数之间的关系,追踪晶粒形状演化、第二相开裂的应变变化规律,获得了双峰结构GNPs/AZ91复合材料变形时协调变形过程的实验依据。3.对双峰结构GNPs/AZ91复合材料强韧化机理进行了定量的讨论。结果表明,镁基复合材料存在细晶强化和载荷转移强化两种主要的强化机制,二者对屈服强度的贡献占强化机制的66.3%;韧化机制采用加载-卸载-再加载的拉伸测试过程,获得双峰结构GNPs/AZ91复合材料迟滞回线,从而量化双峰结构在塑性变形过程中非均匀变形的额外硬化贡献,揭示双峰结构对镁基复合材料本征变形的协调机理。与此同时,结合原位拉伸实验数据,揭示了形变过程中双峰结构对裂纹阻碍的外在韧化行为。系统地阐明了双峰结构GNPs/AZ91复合材料强韧化机理。本文通过球磨-冷压-热压烧结-热挤压的工艺制备双峰结构GNPs/AZ91复合材料。探讨了双峰结构制备过程的组织演变规律,分析了双峰结构中粗晶/细晶/石墨烯之间的协调变形机制,揭示了石墨烯增强镁基复合材料双峰结构的强韧化机制。本工作将为高性能镁基复合材料结构设计与强韧化提供一种新思路。更多还原

【Abstract】 Magnesium alloys have a low density(1.78 g/cm3,which is about 2/3 of aluminum and 1/4 of iron),and possess the advantages of high specific strength/stiffness,high damping performance,good electromagnetic shielding capability,easy to be recycled,etc,making them one of the "greenest" engineering materials for the 21 st century.Simultaneously,magnesium alloys also have the disadvantages of low absolute strength and elastic modulus.One of the effective solution to improve the strength and modulus of magnesium alloys is to add reinforcement in magnesium matrix to prepare magnesium-based composites;however,the tradeoff between strength and ductility of magnesium-based composites still limits their practical applications.In this thesis,bimodal-structured GNPs/AZ91 composites(coarse grain/fine grain/graphene)are constructed through the interfacial engineering of graphene and magnesium,in which the coordinated deformation of coarse grain/fine grain/graphene is "bridged" by the interfacial magnesium oxide to realize the synergistic enhancement of strength and toughness.Specifically,we study the influence of ball milling,sintering and thermal deformation conditions on the bimodal structure of GNPs/AZ91 composites.We also investigate the mechanical behaviors of bimodalstructured magnesium-based composites to reveal the coordinated deformation mechanism between coarse grain/fine grain/graphene.Furthermore,we explore the fracture mechanism of the bimodal structure in GNPs/AZ91 composites and study the interfacial structure of coarse/fine grains and GNPs/Mg O/α-Mg.The main results of this paper include:1.A "ball milling-cold pressing-hot pressing sintering-hot extrusion" pathway is used to prepare GNPs/AZ91 composites with bimodal-structured characteristics.The results show that the choices of processing parameters for ball milling,sintering and hot extrusion are crucial to achieve the bimodal-structured GNPs/AZ91 composites.The systematical microstructure characterizations for the bimodal-structured GNPs/AZ91 composites further extract the organizational characteristics for the coarse/fine grain regions,the distribution of the second phase,together with the interfacial structure between GNPs/Mg O/α-Mg and the nanoscale precipitation of the second phase(Mg17Al12)at the interface between GNPs and α-Mg.Therefore,the formation mechanism of bimodal-structured GNPs/AZ91 composites is revealed by analyzing the material compositions and microstructures at all stages.2.The mechanical properties of the bimodal-structured GNPs/AZ91 composites are investigated.The hardness testing results show that the microhardness value is 85.8± 4.1 HV,which is 18.3% higher than that of AZ91 alloy.According to nanoindentation test,the hardness(elastic modulus)values for the coarse/fine regions in the bimodalstructured GNPs/AZ91 composites are determined to be 105.51 ± 5.82 HV(45.45 ±0.84 GPa)and 126.64 ± 3.83 HV(49.43 ± 2.00 GPa),respectively,showing significant differences in mechanical properties across coarse and fine regions;this performance differences are closely related to the graphene distribution,grain size and the distribution of second phase.Furthermore,the tensile test shows that the tensile strength and elongation of bimodal-structured GNPs/AZ91 composites are 375 ± 8.6MPa and 8.20 ± 0.70%,respectively;compared with homogeneous GNPs/AZ91 composites,a significant improvement in plastic deformation by 8% and 146%,respectively,is achieved.This finding suggests that the bimodal structure increases plasticity of GNPs/AZ91 composites by establishing a strong synergistic effect on the strength-plasticity enhancement.Additionally,based on in situ SEM mechanical experiments,the deformation behavior of the composites is discussed;the structural evolutions of coarse/fine grain regions during tensile process are investigated in order to uncover the failure mechanism of the composites(in crack initiation,expansion,and fracture).The digital image correlation(DIC)method is used to analyze the strain of bimodal-structured GNPs/AZ91 composites during tensile,which helps to establish the relationship between mechanical properties and microstructural characteristics,and to trace the strain change patterns including grain shape evolution and second phase cracking.3.The toughening mechanism of the bimodal-structured GNPs/AZ91 composites is quantitatively discussed.Two major strengthening mechanisms exist for the composites,which are fine grain strengthening and load transfer strengthening.The two mechanisms contribute 66.3% of the yield strength.The hysteresis loop of bimodal-structured GNPs/AZ91 composites is obtained using a "loading-unloadingreloading" testing procedure to quantify the additional hardening contribution of nonuniform deformation of the bimodal structure in the plastic deformation process,which reveals the role of bimodal structure on the intrinsic deformation of the composites.Meanwhile,by considering in-situ tensile experimental result together with the toughening behavior of bimodal structure on crack hindrance during deformation,the toughening mechanism of the GNPs/AZ91 composites with bimodal structure is thoroughly disclosed.In short,bimodal-structured GNPs/AZ91 composites are prepared by sequential ball milling,cold pressing,hot pressing sintering and hot extrusion,in which the microstructure evolution of bimodal structure during preparation is explored.Subsequently,the coordinated deformation mechanism between coarse grain/fine grain/graphene in bimodal structure is analyzed,and therefore the toughening mechanism of bimodal-structured graphene-reinforced magnesium-based composites is revealed.This work may shed light on the structural design and toughening of high performance Mg-based composites.更多还原