Regulation And Mechanical Behavior Of GNPs/AZ91 Composites With Bimodal Structure

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.