Microstructure And Properties Of In-situ（ZrB₂+Al₂O₃）_np/AA6111 Composites Synthesized By External Field Synthesis

As a new kind of material,in-situ nanoparticle-reinforced aluminum matrix composite has the characteristics of high specific strength,high specific stiffness,and good plastic toughness and corrosion resistance.In recent years,it has been widely used in automobile body industry.However,in general,the introduction of reinforced particles into the composites can improve the mechanical properties,but at the same time,due to the serious particle clusters and uneven particle distribution,its corrosion resistance is reduced.Therefore,it is of great significance to develop a new type of lightweight body material with high strength and excellent corrosion resistance.In this paper,the high-performance in-situ（ZrB₂+Al₂O₃）nanoparticle-reinforced AA6111 matrix composite was successfully prepared from an AA6111-Na₂B₄O₇-K₂ZrF₆ reaction system,through the application of a magnetic field in the in-situ reaction stage and the application of the ultrasonic field in the solidification stage to solve the problems of particle clusters and particle size control during the preparation of the composites.The effect of different（ZrB₂+Al₂O₃）particle contents on the microstructure,mechanical properties and corrosion resistance of composites was studied to determine the optimal particle volume fraction.In addition,the study focused on the influence of the combination of magnetic field frequency/ultrasonic field power regulation on the microstructure and mechanical properties of composites,and determined the optimal magnetic field frequency/ultrasonic field power combination regulation parameters.Besides,the relationship between the microstructure evolution,tensile properties and corrosion resistance of composite materials under the control of magnetic field/ultrasonic field combination were investigated,and the corrosion mechanism was also discussed.The microstructure analysis shows that when the particle content is 3 vol.%,the synthesized ZrB₂ and Al₂O₃ reinforced particles are mostly distributed in the form of clusters near the grain boundaries,and a small part are distributed in the grains.The average grain size of the obtained composite material is 125.7 μm.Compared with the 6111 matrix aluminum alloy,coarse dendrites are transformed into regular equiaxed crystals.When a magnetic field（10 Hz）is applied in the in-situ reaction stage and a ultrasonic vibration（1 kW）is applied in the solidification stage,the particle clusters of in-situ 3 vol.%（ZrB₂+Al₂O₃）np / AA6111 composite is almost disappeared,and the particles are dispersed near the grain boundary in the matrix.The shapes of most ZrB₂ particles are tetragonal and hexagonal,and the Al₂O₃ particles are nearly round.The particle size is 10 nm 80 nm.The average grain size is refined to 50.4 μm,which is about 75 μm less than when no external field is applied.The analysis of mechanical properties shows that when the magnetic frequency is 10 Hz and the ultrasonic power is 1 kW,the mechanical properties of 3 vol.%（ZrB₂+Al₂O₃）np / AA6111 nanoparticle reinforced aluminum matrix composites are the best.The tensile strength is 355.4 MPa,the yield strength is 259.4 MPa and the elongation is 22.4%,which is 20.6%,12.7% and 29.5% higher than that of the composites prepared without external field,and 36.6%,22.9% and 33.3% higher than that of the AA6111 alloy.A large number of small and deep dimples are distributed in the fracture,and the fracture mode is a typical dimple fracture.This result is consistent with the microstructure analysis results.The strengthening mechanism of the composites is the result of the combination of Orowan strengthening,fine grain strengthening and CTE strengthening.The analysis of corrosion resistance shows that the self-corrosion potential of the composites with（ZrB₂+Al₂O₃）particles is higher than that of the AA6111 matrix,and the arc radius of the composite is larger than that of the AA6111 alloy.When the particle content is 3 vol.%,the corrosion potential of the composite is the highest and the corrosion current density is the lowest.The polarization resistance obtained by equivalent analog circuit is 4 times higher than that of AA6111 alloy.Besides,the selfcorrosion current density of 3vol.%（ZrB₂+Al₂O₃）np / AA6111 composites was further reduced under the optimal combination of magnetic field frequency and ultrasonic power,and the simulated resistance obtained by the simulation circuit fitting was twice as high as that of the composite without external field,and the corrosion resistance was further improved.On the one hand,the improvement of corrosion resistance can be attributed to the grain refinement caused by magnetic field/ultrasonic field.On the other hand,the addition of magnetic field/ultrasonic field promotes the uniform dispersion of ZrB₂ and Al₂O₃ particles in the matrix,which avoids the formation of more galvanic corrosion due to the large particle cluster splitting the matrix.