Fabrication,Microstructure And Mechanical Properties Of SiC/Al-Zn-Mg-Cu Composites

As a strategic material for the lightweight development of aerospace equipment,the study of SiCp/Al composites has been paid much attention by scholars.Although the study of composites based on Al-Cu-Mg and Al-Mg-Si alloys was relatively mature,but their strength was still unsatisfactory,which was difficult to meet the requirements of mechanical properties for new equipment structure design.There-fore,light structural materials with higher strength and higher modulus need to be developed urgently.In this study,SiCp/Al-Zn-Mg-Cu composites with different chemical composition and corresponding unreinforced alloys were fabricated by powder metallurgy technology.The key scientific issues in the design and prepara-tion of the composites were discussed through detailed microstructure characteriza-tion and mechanical properties testing.To clarify the negative effect of SiC particle on the mechanical properties of the composites,SiCp/7085Al composite and 7085A1 alloy were fabricated by the same processes.The differences in grain size,element distribution and precipitated phase morphology of the two materials were compared in detail.The effect of SiC particle on various strengthening mechanisms of the composites was analyzed.The results showed that the obvious interface reaction layer could be observed in the SiCp/7085Al composite,and the interfacial products were face-centered cubic MgO.The interfacial reaction caused Mg segregation around SiC particles,and Mg content in the far-particle matrix region decreased obviously,which would lead to the de-crease of MgZn2 content.According to the theoretical calculation,the precipitation strengthening value of SiCp/7085Al composite was 163 MPa lower than that of 7085A1 alloy,while the sum of load transfer and dislocation strengthening contribu-tion of SiC particles was only 109 MPa.The weakening of precipitation strengthen-ing ability was the primary reason for the lower strength of SiCp/7085Al composite.By adding additional 1.0wt.%Mg to the SiCp/7085Al composite,the aging harden-ing ability of the composite could be improved obviously,the size of the precipitated phase was reduced and the density was increased.By optimizing the preparation process parameters,the degree of interfacial reaction could be obviously reduced,and the strength of the composite could be improved.To optimize the chemical composition of SiC/Al-Zn-Mg-Cu composites,the ef-fects of various alloying elements and Zn/Mg ratio on the microstructure and prop-erties of the composites were studied by adjusting the contents of Zn,Mg and Cu in the composites.The results showed that Zn,Mg and Cu could improve the tensile strength of the composites.However,with the increase of alloy element content,the strength enhancement was accompanied by the plasticity decrease.When the content of(Zn+Mg+Cu)was fixed,the proper reduction of Zn/Mg ratio was beneficial to the precipitation stren gthening of the composites.However,too low Zn/Mg ratio would mean excessive Mg,which leaded to the sharp deterioration of plasticity.15vol.%SiCp/Al-6.5Zn-2.8Mg-1.7Cu composite showed the best strength and plas-tic matching.It's tensile strength was 686 MPa,which was 133 MPa higher than that of 15vol.%SiCp/7085Al composite,while their elongation was very close.To enhance mechanical properties of SiCp/Al-Zn-Mg-Cu composites,SiC/Al interface structure was designed.The surface pretreatment of SiC particle was car-ried out by sol-gel and hydrogen reduction techniques to introduce tungsten(W)nanoparticles with uniform dispersion and controllable size into the SiC/Al interface Through extrusion deformation,W particles could be separated from the interface,so as to strengthen the matrix near the interface and delay the failure of SiC particle.The results showed that 100 nm W particles were ellipsoid and tightly attached to the surface of SiC particles.The synergistic strengthening effect could not be re-flected.200 nm W particles were spherical,and the contact area with SiC particles was small.In the process of extrusion deformation,some W particles could be transferred to the matrix near the interface.Unfortunately,W particles and the matrix alloy reacted to form Al18Mg3W2 on the surface of W particles,resulting in weaken-ing of W/Al interface.Many debonded W particles were observed in the tensile fracture,and both strength and plasticity decreased.However,tensile strength and yield strength of the composites could be improved effectively by introducing ap-propriate amount and uniform distribution of carbon nanotubes on the surface of SiC particles by solution-assisted method.Nanocrystalline composites with SiC particles were fabricated by stirring ball milling.According to the calculation results of grain boundary strengthening,the introduction of nanocrystals should greatly improve the strength of the composites.However,due to the large surface area of the ball-milling nano-Al powders,the amount of Al2O3 increased significantly,resulting in serious consumption and seg-regation of Mg.The existence of nanocrystals and MgO at the grain boundary sig-nificantly reduced the deformation ability of the matrix alloys.Therefore,the me-chanical properties of nanocomposites were not ideal.Combined with the mechani-cal mixing method,50%of microcrystals were introduced into the nanocrystalline composites to form the micro/nano crystal dual scale structure SiCp/Al-Zn-Mg-Cu composites.Nanocrystals could play the role of grain boundary strengthening,and the introduction of oxide reduced obviously.The dual scale structure composites ex-hibited higher strength and better plasticity than the pure nanocrystalline or micro-crystalline composites.The peak aging heat treatment technology of composites with medium alloy content and high alloy content was optimized by adjusting aging temperature and aging time.The results showed that the introduction of SiC particle could not change the aging precipitation sequence of the matrix alloys,but could accelerate aging process obviously.Compared with the composites with medium alloy content,the optimal peak aging temperature of the composites with high alloy content was lower,and the aging time was shorter.The tensile strength of the T6-treated composites with high alloy content could reach 781 MPa,which was close to the level of tita-nium alloys.However,due to the high degree of supersaturation,the precipitated phases were prone to preferential nucleation and abnormal growth at the high-angle grain boundaries and SiC/Al interfaces.The growth of grain boundary phases was accompanied by the formation of precipitation-free zones,which could reduce the grain boundary strength.The solute atoms and various compounds at the SiC/Al in-terfaces weakened the SiC/Al interfaces.The cracks would propagate along the weak grain boundaries and the SiC/Al interfaces.Intergranular fracture and interfa-cial debonding were the main fracture mechanisms of the composites with high alloy content.