Microstructure And Properties Of CNTS/Mg-6Zn Magnesium Matrix Composites Fabricated By The Stirring Casting Assisted With Ultrasonic Vibration

Magnesium matrix composites reinforced with carbon nanotubes(CNTs) were fabricated by the stirring casting assisted with ultration vibration. The stirring casting assisted with ultration vibration included pre-dispersion, semisolid stirring and ultrasonic vibration. The effect of process parameter on microstructures of as-cast composites was studied systematically. And then the CNTs/Mg-6Zn composites were extruded under different extrusion temperatures and ratios. Process parameters of extrusion for CNTs/Mg-6Zn composites were optimizated. OM, SEM, TEM and Raman were used to study the microstructures of composites. Systematic study was made to indicate the mechanical properties and evolution of textures of as-extruded composites. The mechanism of strengthening was obtained, meanwhile, the interface and fracture surface were studied.The raw CNTs were agglomerated in large clusters. Moreover, CNTs were seriously entangled together in the clusters. Pre-dispersion is necessary to fabricate CNTs/Mg-6Zn composites with uniformly dispersed CNTs. The chemical dispersant TNADIS was used to disperse the CNTs on the Mg chips. The mass ratio of the chemical dispersant to CNTs is 15%. The time for ultrasonic vibration is 3 h.The CNTs/Mg-6Zn composites were fabricated under different melt condition, stirring time and ultrasonic time. And the best process parameters for 1.0vol.% CNTs/Mg-6Zn composite was obtained while the semisolid stirring time was 10 min and then ultrasonically processed for 20 min. The grain sizes decreased with the increase of the volume fraction of CNTs. The ultimate tensile strength(UTS) and yield strength(YS) of 1.0vol.% CNTs/Mg-6Zn composite was the highest.Hot extrusion can eliminate CNT segregation in as-cast composites and improve CNT distribution. Higher extrusion temperatures and larger extrusion ratios are favorable for improving CNT distribution. The grain size of matrix in the composite was smaller in CNT bands. This indicates that dynamic recrystallization(DRX) occurs during extrusion. The CNTs can fine the matrix grains because of the pinning effect of CNTs on grain boundary during extrusion process. The addition of CNTs could not change a new texture type of matrix but could change the texture intensity. The basal planes are directed parallel to extrusion direction, and the intensity of the basal plane texture decreased with increasing extrusion temperatures and decreasing extrusion ratios and increasing volume fraction.The ratio of the intensity of D band to the intensity of G band( ID/IG) in the Raman spectra has been widely used to study the quality of the C NTs. The ID/IG was 0.90 for the raw CNTs, 0.91 for the pre-dispersed CNTs, 0.97 for the as-cast composite and 1.02 for the as-extruded composite. This result implied that our process did not significantly increase the defects in the CNTs, and further damag e to the CNTs did not occur during deformation of the composite by extrusion. And no intermediate compound or nanopore was detected by HRTEM at the interface, it can be concluded that a good interfacial bonding was also achieved. The interfaces between Mg matrix and CNTs were clean without other materials, and good interfacial bonding was achieved.The mechanical properties of 350R20 composite were the best. YS and UTS of 1.0vol.% CNTs/Mg-6Zn composite were the best, and the elongation to fracture of 0.5vol.% CNTs/Mg-6Zn composite were the best, and Young’s modulus of 1.5vol.% CNTs/Mg-6Zn composite were the best. Hall-Petch strengthening, load transfer and Orowan strengthening mechanism had important effect on the mechanical properties of CNTs/Mg-6Zn composite. A formula was concluded on account of the strengthening mechanism above, and the values calculated by the formula agreed well with the experimental YS. The CNTs added into the matrix can play pulling-out and bridging effects in crack which can increase the resistance of crack propagation. The UTS can be calculated by the Kelly–Tyson formula, and the formula agreed well with the experimental UTS. Young’s modulus can be calculated by the Halpin-Tsai formula, and the formula agreed well with the experimen tal Young’s modulus. The composites fabricated by the stirring casting assisted with ultration vibration showed the best strengthening efficiency. This means that the stirring casting assisted with ultration vibration has broad application prospects.