Study On The Relationship Between Microstructure And Mechanical Properties Of Graphene-reinforced Magnesium Matrix Composites

In order to obtain higher performance magnesium matrix composites,GNPs-reinforced AZ31 magnesium matrix composites（GNPs/AZ31）with uniform distribution of GNPs were prepared using graphene nanoplatelets（GNPs）as reinforcement and aqueous ultrasonic dispersion first to avoid agglomeration,and AZ31 magnesium alloy as matrix.The effects of GNPs addition and T6 heat treatment on the microstructure and mechanical properties of GNPs/AZ31 composites were investigated,and the relationship between microstructure evolution and mechanical property changes was further revealed.The main findings of this paper are summarized as follows:（1）To improve the dispersion of GNPs in GNPs/AZ31 composites,aqueous ultrasonic dispersion of GNPs powder was performed,and GNPs were first ultrasonically oscillated in deionized aqueous solution of sodium dodecyl sulfate（SDS）to prepare GNPs-AZ31 composite flakes with turned AZ31 magnesium flakes as GNPs carriers.avoid the agglomeration of GNPs in the subsequent preparation of magnesium matrix composites.The results showed that the I_G/I_2D values in the Raman spectra of GNPs on the composite sheets were reduced from 2.42 to 1.12,indicating that the aqueous ultrasonic dispersion helped to reduce the number of layers of GNPs.（2）GNPs/AZ31 composites with GNPs additions（mass fraction,wt%）of0.1wt%,0.3wt%and 0.6wt%were prepared by a semi-continuous casting process combined with hot extrusion tests,and uniaxial tensile and microstructural characterization tests were carried out.The uniaxial tensile test results showed that the strength and elongation of GNPs/AZ31 composites were enhanced with the addition of GNPs.The tensile strength,yield strength,elongation and fracture work reached360 MPa,253 MPa,23.7%and 72 J·m^-3at 0.6wt%GNPs addition,which were 6.2%,4.1%,16.2%and 24.1%higher than those of GNPs/AZ31 composites with 0.1wt%GNPs addition,respectively.Microstructural analysis showed that the GNPs were The GNPs/AZ31 composites showed a uniform distribution within the composites,and with the increase of GNPs addition,the grain size of GNPs/AZ31 composites gradually decreased,the dislocation density gradually increased,the（0002）basal weave strength appeared to be weakened,the brittle fracture features such as deconstruction platform,tearing ribs and steps in the fracture gradually decreased,and the number of tough nests gradually increased.（3）Uniaxial tensile and microstructural characterization tests were carried out on the T6 heat-treated 0.3 wt%GNPs/AZ31 composites.The uniaxial tensile tests showed that the strength and toughness of the GNPs/AZ31 composites after T6 heat treatment were significantly improved.Among them,the best mechanical properties were obtained by solid solution at 300℃for 1 h and aging at 200℃for 20 h.The tensile strength、yield strength、elongation and fracture work reached 381 MPa,258MPa,23.7%and 76 J·m^-3,respectively,which were improved by 9.5%、4.5%、13.9%and 24.6%,respectively,compared with the untreated state.The Vickers hardness also reached its maximum value at 20 h of aging,with an increase of 34.4%compared to the untreated state.Microstructural analysis showed that the number of precipitated second phase(Mg₁₇Al₁₂)showed a trend of increasing and then decreasing with the increase of aging time,dominated by the precipitation of small-sized particles with particle size≤1.0μm,reaching a peak at 20 h of aging,which was consistent with the trend of the material strength change after heat treatment;the second phase in the range of 2θof 35.8°-36.1°and 40°-45°(Mg₁₇Al₁₂)diffraction peak intensity was improved with the increase of aging time;after T6 heat treatment,（0002）basal weave strength were all improved,which in turn helped to enhance the tensile strength;meanwhile,the number of steps in the fracture became more and the morphology looked rougher,which was consistent with the enhancement of material strength in tensile test.（4）Tensile cyclic fatigue tests and microstructural characterization tests were conducted on the GNPs/AZ31 composites in the initial extruded state.The results of the fatigue test showed that the fatigue life of GNPs/AZ31 composites increased significantly with the increase of GNPs addition when the strain amplitude was 0.5%,but the fatigue life of GNPs/AZ31 composites did not change significantly when the strain amplitude increased to 1.0%and 1.5%;with the increase of the strain amplitude,the fatigue life of GNPs/AZ31 composites did not change significantly.The fatigue life（cycle cycles）of GNPs/AZ31 composites decreased with the increase of strain amplitude,and the changes in the curves of stress amplitude,average stress,maximum stress and minimum stress versus loading cycles showed an increasing trend.Microstructural analysis showed that the number of twins within the grains increased with the increase of the stress amplitude,accompanied by cross-twins;the crack sources nucleated on the surface of the specimen and expanded inward,the surface roughness of the fatigue fracture gradually increased,the area of the crack extension zone gradually decreased,the width of both the slip zone and the microcrack decreases gradually.