Preparation And Mechanical Properties Of The Laminated CNTs/Ti Composites

Titanium matrix composites have the characteristics of high specific strength,high specific rigidity and good high temperature performance,which allows it to fulfill the increasing demand for engineering materials with light weight and high strength in airplane and automotive industries.Through adding a large number of reinforcements to improve the strength,the plasticity of the traditional titanium matrix composites with uniform distribution of reinforcement will be greatly reduced,that is the problem of strength-toughness inversion.In response to this problem,based on the shell micro-nano-layered structure,carbon nanotubes(CNTs)were prepared on the pure Ti foil by electrophoretic deposition.Followed by stacking CNTs/Ti monolayer material,the CNSs/Ti composites with layered structure were prepared by spark plasma sintering(SPS)combined with rolling technology.Controling the process parameters of the preparation to solves the common problem that CNTs are difficult to disperse uniformly in the titanium matrix and the interface reaction between CNTs and titanium matrix.The microstructures and interface features of the composites were characterized by SEM and TEM.The mechanical properties of layered CNTs/Ti composites with different CNTs content were studied by tensile test.The effects of macroscopic deformation and local strain distribution were studied by in-situ loading and digital image correlation.The fracture characteristics were studied by X-ray tomography,and finally reveals the strengthening and toughening mechanism of layered CNTs/Ti composites.The results showed that the agglomeration of CNTs could be broken by the acidification treatment for 6 hours combined with ultrasound for 8 hours,and the CNTs were dispersed uniformly.The uniform deposition of CNTs could be obtained on the surface of pure Ti foil by electrophoretic deposition,and the thickness of CNTs deposited could be controlled by adjusting the voltage and electrophoresis time of electrophoresis.Raman test showed that,compared with hot pressing sintering,SPS sintering(500 ?/70 MPa/10 min)could obtain CNTs/Ti interlayer with good interface,and the interface reaction of CNTs with titanium matrix was obviously inhibited.No Ti C formation was found.The CNTs/Ti stack was then subjected to hot rolling;The study found that the first rolling pass of large reduction(>45%)was conducive to improving the combination of layers;On its basis,rolling 14 passes(10% deformation per pass),and annealing at 475 oC for 10 min,a layered CNTs/Ti composite with a clean interface and good Interlayer combination could be obtained.The results of tensile test showed that the strength of CNTs/Ti composites was improved with the increase of CNTs content,only 0.02 wt.% of CNTs were added,the tensile strength of CNTs/Ti composites was increased to 542 MPa,which was 16% higher than that of pure Ti laminated materials(468 MPa),and its elongation was still as high as about 27%.It could be seen that the layered CNTs/Ti composites prepared by electrophoretic deposition,SPS sintering and rolling techniques showed a significant strengthening effect,when the content of CNTs was only 0.02 wt.%,The reinforcing effect was 13 times higher than that of CNTs/Ti composites prepared by powder metallurgy method.The reason for high strength might be attributed to:(1)CNTs were uniformly dispersed,and CNTs do not react with the titanium matrix,forming a good metallurgical interface;Every single CNTs had played a good enhancement effect;(2)grain refinement strengthening effect.The layered CNTs/Ti composites had good plasticity due to the fact that the grain size of the titanium matrix was submicron,and the dislocations had considerable mobility.And the content of CNTs was very low,the titanium matrix was favorable for its plasticity,so CNTs/Ti composites showed good plasticity.DIC analysis showed that the strain localization of the layered CNTs/Ti composites was earlier than that of the Ti laminates,but still within the acceptable range.Fortunately,there was not a sharp decrease in uniform deformation ability.XRM results showed that as the shrinkage of the cross section increases,the porosity increased,the pore size increased,and the crack and the pore were distributed along the bedding.It could be seen that the micropores were sprouted into the interlayer interface and were merged into microcracks in the interlayer interface,and the microcracks continued to grow along the interlayer interface.As the deformation increased,the titanium matrix in the layers shrinked,eventually forming a transverse and through crack and fractured.