Plastic Deformation Mechanism And Strength-toughness Mechanism Of Magnesium Matrix Nanocomposites At Room Temperature

As one of the lightest commercial engineering metal materials,magnesium alloys were widely used in the field of aerospace and automotive.However,due to its HCP crystal structure,the number of slip systems that are easy to operate is small at room temperature,leading to poor plasticity of magnesium alloys.As a kind of composite material with nanoparticles or fibers added into the matrix of magnesium alloys,magnesium matrix nanocomposites can improve the strength and toughness on the basis of magnesium alloys without damaging the advantages of light weight,so it is a lightweight material with great potential.There are many kinds of plastic deformation mechanisms such as basal slip,prismatic slip,pyramidal slip,extension twinning and contraction twinning in magnesium crystal with HCP structure.There are various precipitates in magnesium alloys and reinforcements in magnesium matrix nanocomposites.These reasons make the plastic deformation mechanism of magnesium matrix nanocomposites very complex.The plastic deformation mechanism is inalienable from the strength and toughness of the material,so the internal mechanism needs to be perfected.n-SiC_p/AZ91D magnesium matrix nanocomposites with uniform dispersion of SiC nanoparticles were prepared by high energy ultrasonic dispersion method and semi-solid stirring assisted gravity casting process.After T4 solution heat treatment and T6 aging heat treatment,the mechanical properties of the nanocomposites were further improved.The yield strength,tensile strength and elongation of the as-cast magnesium matrix nanocomposites are116.3MPa,186.3MPa and 3.5%,respectively.The yield strength,tensile strength and elongation of the magnesium matrix nanocomposites after T4 solution treatment are 132.3MPa,296.0MPa and 17.3%,respectively,showing good toughness.The yield strength,tensile strength and elongation of the magnesium matrix nanocomposites after T6 aging treatment are150MPa,293MPa and 6.3%,respectively,showing high strength.In this paper,microstructure characterization and crystal plasticity finite element modeling(CPFEM)were used to explain the mechanism of plastic deformation and strengthening and toughening of the magnesium matrix nanocomposites after the heat treatment.After T4 solution heat treatment,Mg₁₇Al₁₂ precipitates disappear in the matrix of magnesium matrix nanocomposites.A large number of twins and slip appear in n-SiC_p/AZ91D magnesium matrix nanocomposites after the tensile deformation at room temperature.Twinning and slip are the main mechanism of the plastic deformation of SiC_p/AZ91D magnesium matrix nanocomposites.High strain zones are formed around SiC nanoparticles in the?-Mg matrix during the tensile process at room temperature,and a lot of dislocations and stacking faults are formed in the high strain zones.These dislocations and stacking faults evolve into a large number of slip bands and twins under the action of tensile strain,which is the plastic deformation mechanism of n-SiC_p/AZ91D magnesium matrix nanocomposites with high plasticity after T4 solution heat treatment at room temperature.After T6 aging treatment,cellular Mg₁₇Al₁₂ phases was precipitated near the grain boundaries,and fine Mg₁₇Al₁₂ laths are precipitated within the grains.SiC nanoparticles have a certain influence on the morphology,size and distribution of Mg₁₇Al₁₂ precipitates,which make the precipitates finer and more disperse,thus giving full play to the precipitation strengthening effect of the secondary precipitation phases.A large number of dislocations occur around the nanoparticles and the precipitates in the plastic deformation process.The dislocations tangle around the precipitates when the dislocations with different orientations encounter,and some dislocations leave the dislocation sinks after cutting through the Mg/Mg₁₇Al₁₂interfaces.Refinement of the grains and Mg₁₇Al₁₂precipitates,entanglement of the dislocations and formation of the dislocation sinks are the main reasons for the high strength and good plasticity of n-SiC_p/AZ91D nanocomposites after T6 ageing treatment.CPFEM is used to simulate the effect of the high strain zones around SiC nanoparticles of n-SiC_p/AZ91D magnesium matrix nanocomposites under the tensile plastic deformation.Compared with magnesium alloys without particles,the strength of magnesium matrix composites containing particles is improved to a certain extent,and the activation of the slip systems and twinning systems is more complicated.Under the same grain orientation,the activated slip systems or twinning systems of the single crystal magnesium alloy under uniaxial tension is less than that of the single crystal magnesium matrix composites containing particles.Some slip systems or twinning systems that cannot be activated in single crystal Mg alloy are activated in single crystal magnesium matrix composites.When the volume of the reinforcement is constant,the smaller the particle size is,the better the strengthening effect is.Therefore,n-SiC_p/AZ91D magnesium matrix nanocomposites with added nanoparticles have good strengthening effect and better fit Mises criterion,thus improving the strength and toughness.