Preparation And Fracture Behaviour Of The Multi-layered 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-nanolayered 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 solve 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,the crack initiation and propagation and the fracture characteristics were studied by in-situ loading.And finally reveals the strengthening and fracture mechanism of layered CNTs/Ti composites.The results showed that the agglomeration of CNTs could be broken by the acidification treatment for 7 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,while the thickness of CNTs deposited and the distribution and the volume of CNTs could be controlled by adjusting the voltage and electrophoresis time of electrophoresis and the thickness of Ti foil respectively.Raman test showed that,before hot rolling,the CNTs will be damaged in the acidification treatment,electrophoresis and sintering process,but it will not affect the basic structure.SPS sintering(500℃/50MPa/10min)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(>55%)was conducive to improving the combination of layers;On its basis,rolling 10%-30% deformation per pass,and annealing at 500 oC for 5-10 min,a layered CNTs/Ti composite with a clean interface and good Interlayer combination could be obtained.TEM images show that CNTs/Ti has a certain damage after hot rolling,the atomic chaotic arrangement of the port were observed on the outer wall of the CNTs and the open tube end,which it’s easy to react with Ti to generate Ti C at the above location.The size of Ti C is 20-30 nm,corresponding to the diameter of the carbon tube;CNTs and Ti can not only be directly combined,but also rely on the birth of Ti C.The interface among CNTs,Ti,Ti C are flat straight with orderly arrangement of atoms which implied a good enhance effect of CNTs.The results of tensile test showed that the strength of CNTs/Ti composites was improved with the increase of CNTs content,only 0.08wt% of CNTs were added,the tensile strength of CNTs/Ti composites was increased to 723 MPa,which was 22% higher than that of pure Ti laminated materials(590 MPa),and its elongation was still as high as about 17%.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 0.08wt%,The reinforcing effect was 4.5 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)Orowan strengthening,the motion of dislocation is restrained by the CNTs and Ti C generated by interface reaction which lead to the strengthening of composite materail(3)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.In situ tensile combined with three-dimensional X ray tomography show that the microcracks in the mulit-layered CNTs/Ti composite first originated from the interface between CNTs and Ti.With the tensile test going on,microcracks continued to generate at the interface between CNTs and Ti in other regions,and the density of microcracks increased continuously,but the cracks was not merged into a large crack and immediately failed.When the tensile deformation reaches 11%,there is a crack penetrating through the interface in the local area.At the same time,the Ti layer obviously shrinks and transverse shear cracks are observed.It can be concluded that the multi-layered CNTs/Ti composite has good crack resistance.