The Investigation On Fabrication Of Multi-scale Hybrid Reinforced(SiC+CNTs)/AZ61 Composites And Their Microstructure And Mechanical Properties

With the increasing demand for lightweighting in the aerospace,military,and transportation industries,magnesium matrix composites are favored in various fields for their significant advantages in low density and high strength.Traditional micron ceramic particle reinforced magnesium matrix composites often add high content of micron reinforcement to obtain high strength and high elastic modulus,but it will lead to a sharp reduction in plasticity and affect service safety.Nano-reinforcements represented by carbon nanotubes(CNTs),graphene(GN),etc.can increase the strength of magnesium matrix composites while maintaining good ductility.However,the van der Waals force effect and its poor compatibility with the magnesium matrix lead to a very limited content of nano-reinforcements that can be uniformly dispersed,so it is difficult to obtain an ideal strengthening effect.The above-mentioned"trade-off"of strength and plasticity has restricted the wide application of magnesium-based composites to a certain extent.In order to solve the above problems,based on the idea of integrating the respective advantages of micro and nano reinforcements to exert their synergistic strengthening effect,a design of multi-scale hybrid reinforcement containing micro Si C particles and CNTs is proposed.AZ61-5Si C-x CNTs magnesium matrix composites were successfully fabricated by multi-stage dispersion methods including hydrogen bonding-assisted pre-dispersion,shift-speed high-energy ball milling dispersion,and hot extrusion.And then we has systematically studied:1)the effects of components and content of the reinforcements on the microstructure,macrotexture,room temperature mechanical properties and fracture characteristics of the composites;2)hot deformation behavior of AZ61-5Si C-0.5CNTs magnesium matrix composite within the temperature range of 250??400?and the strain rate range of 10-4s^-1?0.1 s^-1;3)The effects of ECAP process on the microstructure and mechanical properties of AZ61-5Si C-0.5CNTs magnesium matrix composite.And the main results are shown as follows:Based on hydrogen-bonding adsorption,CNTs grafted with-COOH groups were uniformly adsorbed on the surface of micron Si C particles grafted with-OH groups,and then a shift-speed high-energy ball milling at 200 rpm-4 h+300 rpm-1 h was used to disperse the hybrid reinforcements uniformly in AZ61 matrix powder,and then the multi-scale hybrid reinforced AZ61-5Si C-x CNTs magnesium-based composites were successfully prepared after cold pressing,sintering,hot pressing and hot extrusion.The microstructure characterization results showed that the fabrication method could effectively maintain the structural integrity of CNTs.The addition of hybrid reinforcement can significantly refine the grains of the composites and weaken their basal texture.With the increase of CNTs content,the grains become finer and the basal texture become weaker;interface characterization results exhibited that the submicron/nano Mg O formed by in-situ reaction of Mg atoms with oxygen-containing groups such as-COOH and-OH can enhance the interface bonding between the hybrid reinforcements and the matrix.When the content of CNTs is 0.5 vol.%,the yield strength and ultimate tensile strength of the composite can reach 345 MPa and 412 MPa(compared to the AZ61 matrix by about 66%and 45%,respectively),respectively,while the elongation can still keep at 8.0%,indicating that the combination of ultra-high strength and good ductility was successfully achieved.The strength improvement could be mainly attributed to the combined effects of Hall-Petch strengthening,load transfer strengthening,thermal-mismatch strengthening,and Orowan strengthening.Meanwhile,the basal surface texture,the weakening of the basal texture,and the"pull-out mechanism"and"bridge mechanism"of CNTs maintained its good ductility.Based on the above-mentioned strengthening mechanisms,a prediction model for yield strength of multi-scale hybrid reinforced Mg matrix composites was proposed.Moreover,the model checking demonstrated that the prediction model can predict the yield strength of AZ61-5Si C-x CNTs magnesium matrix composites well.The addition of the multi-scale hybrid reinforcements could improve the high temperature deformation resistance of the composite,but at the same time,it also reduced its hot workability.Both AZ61 alloy and AZ61-5Si C-0.5CNTs composite exhibited typical hardening,softening,and steady-state flow characteristics during hot compression.The flow stress of both AZ61 and its composite increased with the decrease in the hot compression temperature and the increase in the strain rate.In addition,and the constitutive equations of both materials during hot compression could be described by Arrhenius model.The average stress exponent of AZ61 alloy and AZ61-5Si C-0.5CNTs composite are 3.7 and 4.9,respectively,and the average apparent activation energy are 149 k J/mol and 174 k J/mol,respectively,indicating that the dominant deformation mechanism under the experimental deformation conditions is dislocation climbing.This also means that adding hybrid reinforcements can not change the dominant deformation mechanism.Based on the dynamic material model and the plastic instability criterion,the thermal processing diagrams of AZ61 alloy and AZ61-5Si C-0.5CNTs composite are proposed,according to comprehensive consideration,the best thermal processing parameters of AZ61 alloy and AZ61-5Si C-0.5CNTs composite are:250??340?,0.001 s^-1?0.01 s^-1 and 250??330?,0.001 s^-1?0.006 s^-1.Equal channel angular pressing(ECAP)could effectively reduce the porosity and refine the grains of AZ61-5Si C-0.5CNTs magnesium matrix composite,and is conducive to the weakening of the basal texture and its pole differentiation.Besides,ECAP could also promote the further dispersion of CNTs.However,the severe shearing effect of ECAP could also damage the structural integrity of CNTs and reduce its strengthening effect.Due to the combined effects of grain refinement,porosity change,texture evolution,residual dislocation density change,and structural damage of CNTs,the composite reached highest yield strength and tensile strength(383 MPa and 445 MPa,respectively)after 2 passes of deformation at250?,and a slight reduction in elongation occurred,but it could still maintain at about 6%,indicated a optimization of mechanical properties.In summary,this work systematically investigated and explored the fabrication process of multi-scale hybrid reinforced magnesium matrix composites,strengthening and toughening mechanisms and the prediction model,hot deformation behavior,and optimization of mechanical performance by ECAP.And the results can provide a certain theoretical and experimental basis to design and prepare multi-scale hybrid reinforced magnesium matrix composites with high performance,and further optimize their microstructure and mechanical properties.Thus,this work is beneficial to further broaden the application range of magnesium matrix composites.