Study On The Mechanical Properties And Microstructure Evolution Mechanism Of Single Crystal/polycrystalline Aluminum

Aluminum alloys are widely used in aircraft fuselages and automotive components by virtue of their better specific strength,but they face complex loading environments during service,which requires research on the service performance of aluminum alloy materials.It provides theoretical support for reliability design of aluminum alloy components and has important theoretical guiding significance.Studying the microstructure deformation mechanism of aluminum alloy materials is a key way to understand their microstructure deformation behavior.In this paper,the mechanical response and microstructure evolution of single/polycrystalline aluminum under tensile and compressive loading are investigated by molecular dynamics methods to reveal the microstructure evolution mechanism of the difference in mechanical response under the two loads.Secondly,the Bauschinger effect caused by preloading and then load reversal of single/polycrystalline aluminum is studied to reveal the microstructure evolution mechanism of mechanical properties degradation after load reversal.Also,the influence of temperature on the tension-compression asymmetry and Bauschinger effect of single/polycrystalline aluminum was investigated.The sensitivity of microstructure to temperature,which leads to mechanical properties difference and degradation,is emphatically discussed.Subsequently,graphene-reinforced aluminum models were constructed to analyze the tension-compression asymmetry and Bauschinger effect.The strengthening mechanism of graphene interface on aluminum matrix was revealed to provide theoretical guidance for the design of high strength and high toughness graphene/aluminum composites.Finally,the fatigue properties of single/polycrystalline Al and graphene/Al composites are investigated,and the initiation of fatigue damage,and the microstructure evolution that promotes and resists local damage accumulation are discussed.This is an important theoretical guidance for the failure mechanism of aluminum alloys under complex service behavior.The following research conclusions were obtained in this paper based on the above research ideas:(1)The tension-compression asymmetry and microstructure evolution behavior of single/polycrystalline aluminum were studied by molecular dynamics method.Therefore,the reason leading to the tension-compression asymmetry of aluminum materials is the difference of the microstructure deformation mechanism under the two loads.The microstructural evolution behavior of single crystal aluminum under different loads eventually leads to a positive tension-compression asymmetry.The plastic deformation of single crystal aluminum under tensile load is dominated by Shockley dislocation slip.However,dislocations are blocked under compressive loading and the plastic deformation is dominated by twinning.Polycrystalline aluminum exhibits inverse tension-compression asymmetry,and its plastic deformation is dominated by grain boundary motion.However,it is mainly manifested that the formation of twins under the action of compressive load hinders the movement of grain boundaries,resulting in a high compressive yield stress.The sensitivity of each microstructure to temperature varies and has a significant effect on the mechanical response.The yield behavior under tensile and compressive loads decreases gradually with increasing temperature,and the compressive load was more sensitive to temperature.(2)The Bauschinger effect and microstructural evolutionary behavior of single/polycrystalline aluminum were investigated.The Bauschinger effect of single crystal aluminum is most obvious under tension-compression load.The strength softening of polycrystalline aluminum is not obvious under tension-compression loading,and the Bauschinger effect is weaker than that of single crystal aluminum.The Bauschinger effect is caused by the defects introduced by preloading,and the asymmetry of the Bauschinger effect is caused by different microstructure defects.The elevation of temperature has an effect on both pre-dislocation and residual stresses.The Bauschinger effect under tension-compression loading is significantly weakened with the increase in temperature.The increase in temperature activates the motion of pre-dislocations,which decreases the density of pre-dislocations within the crystal.During reverse loading,the possibility of pre-locations is reduced for nuclear reproliferation.Thermal motion of atoms removes residual stress within the 600 K crystal.(3)A model of graphene-reinforced aluminum composites with different graphene distributions and chirality was developed.The mechanical response and microstructural evolution behavior of the composites under tensile and compressive loading were investigated,with emphasis on the strengthening mechanism of the graphene interface.The composites strengthen about 10% of Young’s modulus and 5% of tensile yield strength compared with aluminum.The results show that the distribution mode and chirality of graphene in the composites affect its mechanical properties.The graphene interface can effectively limit dislocation motion even if the graphene breaks under tensile loading.Under compressive loading,the twins can only extend around the graphene.The results of both tensile and compressive tests showed that the uniform distribution of graphene was beneficial to enhance the mechanical properties of the composites.Bauschinger effect exists in graphene/aluminum composites.The weakening effect of pre-dislocation movement and re-nucleation are obviously stronger than the strengthening effect of graphene,which is the main reason for the Bauschinger effect of composites.Graphene hinders the motion of pre-dislocations more than pinning dislocations.Therefore,the Bauschinger effect of composite is lower than that of pure aluminum under tension-compression loading.Subsequently,the fatigue properties of single/polycrystalline aluminum and graphene/aluminum composites were investigated by strain-controlled fatigue method.Local dislocation structure is the origin of fatigue characteristics of single crystal aluminum and composite materials.The local characteristics of polycrystalline aluminum fatigue are the movement of grain boundaries and the formation of twins.Pinning dislocations in single crystal aluminum fatigue loading are microstructures that resist local plastic accumulation.The local dislocation structure reduces the condition of dislocation nucleation is the direct cause of compression cycle softening.Large cycle amplitude will destroy the pinned dislocation and lead to tensile cycle softening.The dynamic equilibrium between mobile and pinned dislocations is a condition for cycle saturation.The grain boundary movement of polycrystalline aluminum makes the grains merge and grow larger under cyclic loading.The grain boundary movement of polycrystalline aluminum under cyclic loading makes the grains merge and grow.Since the grain size is in the inverse Hall-Patch interval,the growth of a single grain will enhance the mechanical properties,while the growth of multiple grains will reduce the conditions for dislocation nucleation.The twins in polycrystalline aluminum are microstructures that resist local plastic accumulation,and large cyclic amplitude will destroy the twin structure and reduce the fatigue performance.The fatigue performance of the scatter composite is stable,which is due to dislocation entanglement caused by graphene obstruction.The graphene interface is a microstructure in a composite that resists local plastic deformation.