Preparation And In Situ Characterization Of Mechanical Characteristics Of Biomimetic Laminated CNTs/Ti Composites

Titanium-based composite materials with high specific strength,high-temperature performance,and high specific modulus satisfy the requirements of lightweight and highstrength materials in many fields and present great application prospects.However,there is a bottleneck problem of strength-plastic(toughness)inversion in the titanium matrix composites with uniform distribution of reinforcing phase,which seriously limits its application.Based on the above problems,CNTs/Ti bionic laminated composites with excellent comprehensive properties were successfully prepared by using electrophoretic deposition combined with SPS sintering and rolling.Scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to characterize the microstructure,interlayer interface,and interface of CNTs/Ti bionic laminated composites.The mechanical properties of the materials obtained under different preparation processes were compared and analyzed.The digital image correlation and SEM in-situ tension technology were used to characterize the strain evolution of the material.SEM and threedimensional X-ray microscope(XRM)combined with in-situ tension were used to study the crack propagation and fracture behavior of the laminated composite during the deformation.By optimizing the electrophoretic deposition parameters(voltage 30V/ deposition time 15s),single dispersed CNTs were successfully deposited on the Ti foil surface to obtain the CNTs/Ti composite foils.Subsequently,a number of single-layer CNTs/Ti composite foils were stacked and sintered through SPS(SPS/600?/50MPa/15min)to obtain CNTs/Ti buik materials,and then the process of temperature-controlled rolling was used to CNTs/Ti bulk material.The rolling process selected 600?(15min)for the first pass,between the passes,all kept for 5 minutes at 550?,and the total deformation was 90%.Finally,the CNTs/Ti laminated composite materials were successfully prepared.Microstructure studies have shown CNTs were distributed discontinuously in the Ti matrix,and the CNTs were well dispersed and basically single dispersed,and the Ti layer thickness was 5?8?m.In other words,CNTs/Ti bionic laminated composites were composed of discontinuously CNTs and Ti layers arranged alternately.The interface reaction between CNTs and Ti is controlled,and only nano-scale dispersed Ti C particles are formed at the interface,which improves the interface bonding strength between the CNTs and Ti.Comparison of the tensile properties showed that the tensile strength and elongation of the prepared CNTs/Ti bionic lamination composites were 637 MPa and 31% respectively when the sintering temperature of SPS was 600?,which was similar to that of the sintered CNTs/Ti bionic lamination composites at 550?,but the elongation increased by 34.8%.Comparative analysis of the fracture morphology showed that CNTs/Ti layered composites prepared by SPS at 600? had higher interfacial bonding strength,that is,the phenomenon of interlayer cracking almost disappeared.More notably,compared with the pure Ti layered materials prepared by SPS at 550?,the strength and plasticity of CNTs/Ti laminated composites prepared by SPS at 600? increased simultaneously,that is,the tensile strength increased by 11.2% and the plasticity increased by about 14.8% simultaneously.The in-situ SEM-DIC results show at the stage of uniform plastic deformation(strain ? 4.5%),the local strain of CNTs/Ti bionic laminated composite increased with the increase of macroscopic strain,but the distribution of the local strain was basically uniform.However,in the non-uniform deformation stage(strain > 4.5%),with the increase of macroscopic strain,local concentration of strain occurs.Local strain concentration is not manifested as a sharp increase of strain in a single region,but a concentrated distribution of strain in multiple regions.It is proved that the deformation of the composite material is shared by multiple regions.The coordinated deformation of multiple regions prevented the overconcentration of stress in a single region,which delayed the initiation of cracks and improved the uniform deformation capacity of CNTs/Ti bionic laminated composites.This is one of the reasons for the improvement of the plasticity of CNTs/Ti biomimetic laminated compositesThe in-situ characterization results of cracks based on SEM show that the cracks in CNTs/Ti layered materials are divided into cracks along the layer and through-layer cracks according to the initiation and propagation position.The cracks along the layer exist in the whole process of plastic deformation of the material.Layer cracks appear at a later stage of material deformation.There are three main modes of crack propagation,namely passivation,re-cracking after passivation,and continuous expansion.Based on the in-situ characterization of SEM and XRM,the fracture behavior of CNTs/Ti bionic laminated composites is as follows: when cracks preferentially initiate at the weakly bonded interlayer interface,the pores and cracks were small in volume and uniformly distributed,and their volume increased linearly with the increase of strain variables.As the deformation increases,the cracks continue to propagate along with the layer,and the volume of the pores and cracks increases exponentially with the dependent variable.At the later stage of plastic deformation,there were laminar and translaminar propagation cracks in the material,and the growth rate of holes and crack volume was very high.Finally,the combination of translaminar cracks led to the failure and fracture of CNTs/Ti biomimetic laminated composites.