Study On Mechanical Behavior And Strengthening Mechanism Of Magnesium-based Composites Reinforced With Nanomaterials

Magnesium(Mg)alloy has a wild application value in aerospace,automotive,military and other fields due to its excellent properties such as low density,high specific strength and stiffness,and vibration damping et al.However,the low strength,poor high temperature performance and plastic formability could make Mg fracture easily,which limits its wide application in industry.In recent years,the research on the mechanical properties of different nanomaterials reinforced Mg-based composites has been increasing.The experiments are mainly focusing on the static and dynamic mechanical performance behavior,but there is a lack of in-depth theoretical research on the dynamic mechanical behavior,the microscopic strengthening mechanism and the deformation failure principles.In this paper,molecular dynamics(MD)simulation method is used to study the dynamic tensile properties and shock compression properties of carbon nanotubes(CNTs),graphene plates(GNPs),and β-SiC nanoparticles reinforced Mg matrix composites.The dynamic mechanical properties,the failure principles and strengthening mechanisms of composites are systematically analyzed.The specific research topics and main results of this paper are as follows:(1)The effects of different factors on the pull-out behavior and interface strength of CNTs and nickel-coated CNTs(Ni-CNTs)pull-out from the Mg matrix were simulated and analyzed.The effect of chiral parameters,number of walls,percentage of Ni-coated atoms and the pull-out direction on the pull-out force and the interface shear strength(ISS)were obtained.The wetting ability of Mg atoms on the surface of CNT and Ni-CNT is calculated and proved.The numerical results revealed the physical mechanism of the Ni atoms how to increase the interfacial loading transfer.At the same time,the exponential cohesive zone model(CZM)parameters based on the oblique pull-out fitting of CNT and Ni-CNT from the matrix are obtained and the accuracy are verified.The results show that the pull-out force of CNT is proportional to its diameter,but the chiral parameters and length of CNT have little effect on the pull-out force.The Ni atoms can enhance the bonding strength between the reinforcement and the matrix,and the obliqued pull-out of Ni-CNTs further improves the pull-out force.The comparison analysis of the consistency of simulation results and experimental data proves the feasibility of the CZM parameters obtained by MD simulation.(2)Atomic-scale models of different carbon nanomaterials(CNTs,GNPs)reinforced single crystal Mg-based composites have been established,and their tensile mechanical behaviors under different temperatures,strain rates,and temperature-strain rate coupling effects have been studied.The influence of the geometric parameters of carbon nanomaterials and the surface Ni-coating on the mechanical properties of composite materials are studied.The strengthening mechanisms of composites are discussed based on the stress-strain relationship and microstructure evolution of different carbon nanomaterials reinforced pure Mg matrix composites during the tensile deformation process.The numerical results based on the atomic structure model show that the Ni-CNTs/Mg composites have a greater improvement in tensile properties than the single crystal Mg matrix.The simulated elastic modulus are in good agreement with the theoretical values,which proves the models’effectiveness.In addition,the mechanical properties of CNT/Mg composites also exhibit temperature softening effects and strain rate hardening effects.High strain rates at low temperatures or low strain rates at high temperatures have a greater impact on the tensile properties of composites.For Ni-GNPs-Ni/Mg composites,the enhanced tensile properties mainly depend on the number of GNPs’ layers and the percentage of Ni-coating on the surface.The reinforcement mechanisms of composite material are mainly manifested in the enhanced load transfer and dislocation reinforcement at the interface.(3)The tensile mechanical behaviors of polycrystalline Mg(polyMg),β-SiC reinforced polyMg composites(β-SiC/polyMg)and nano-hybrid reinforcements(Ni-CNTs+β-SiC)reinforced polyMg((Ni-CNTs+β-SiC)/polyMg)composites have been studied and discussed.The synergistic enhancement effect of the hybrid reinforcements are explored.The results showed that under the condition of the positive Hall-Petch effect of matrix mechanical properties(grain size>10nm),the elastic modulus and flow stress of composite materials increase as the volume fraction of β-SiC nanoparticle(11～17vol.%)increases.After 2vol.%Ni-CNTs randomly distributed in 11vol.%β-SiC/polyMg composite,the mechanical properties of(Ni-CNTs+β-SiC)/polyMg composite are improved to varying degrees compared with β-SiC/polyMg.When the mixed volume ratio of β-SiC and Ni-CNTs is 5.5:1,the modulus,strength and elongation of(Ni-CNTs+β-SiC)/polyMg increase by 35.92%.37.93%and 19.27%respectively compared with the matrix.These improvements are mainly attributed to the synergistic effect of the loading transfer,the relatively uniform dispersion of the nano-reinforcements in the matrix,the strengthening of the interface,and the interaction between the reinforcement phases and the dislocations.The research results can provide a new research idea for the hybrid nano-reinforced Mg-based composites.(4)The impact compression behavior of polyMg and randomly distributed Ni-GNPs-Ni reinforced polyMg(Ni-GNPs-Ni/polyMg)composites are systematically simulated and studied by MD method.The impact velocity,initial temperature,the volume fractions and distribution of Ni-GNPs-Ni on the impact of microstructure evolution,plastic deformation mechanism and shock wave front structure under shock compression of polyMg were discussed.The research results show that the main plastic deformation mechanisms of polyMg under impact compression are characterized by grain boundary slip and crystal grain rotation.Dislocations and twins formed in the crystal grains dominate the later plastic deformation.The microstructure of the crystal grain is mainly HCP structure atoms.Twins and stacking faults will slip from the grain boundary through the entire grain to the corresponding grain boundary.At the same time,it is found that the impact pressure and shock wave front width of the Ni-GNPs-Ni/polyMg composites material are larger than the matrix,and the mechanical properties increase with the increase of the volume fraction of the reinforcements and decrease of the aggregates.This is related to the increase of the critical slitting stress of the composite material emitting dislocations and the plastic deformation related to the grain boundary.In addition,the shock pressure and shock wave front width of Ni-GNPs-Ni/Mg increase with the increase of shock speed and shock time,the shock pressure and Hugonoit Elastic Limit(HEL)decrease when the initial temperature of the system increases.The research results are helpful to understand the dynamic response mechanism of nanocomposites.