Preparation And Mechanical Characteristics Of Micro-nano Laminated CNTs/Ti Composites

Titanium matrix composites(TMCs)can meet the dual requirements of lightweight and high strength for engineering structural components in aerospace due to its high specific strength,high specific stiffness,high temperature performance.However,the problem of strength-toughness(plasticity)inversion of traditional TMCs greatly limits their practical application.To solve this bottleneck problem,a novel micro-nano laminated CNTs/Ti composite was developed in this paper.By optimizing preparation parameters,the key issues such as the uniform dispersion of CNTs in Ti matrix and the interface control between CNTs and Ti were successfully solved.The microstructure and interface characteristics of CNTs/Ti composite were studied by scanning electron microscope(SEM)transmission electron microscope(TEM)electron back scattering diffraction(EBSD).And the mechanical properties of CNTs/Ti composite were evaluated.Local strain distribution and fracture characteristics of micro-nano laminated CNTs/Ti composite were characterized by in-situ tension combined with digital image correlation(DIC),SEM and 3D X-ray microscope(3d-xrm).Finally,fracture characteristics and strengthening mechanism of CNTs/Ti composite were preliminarily revealed.Micro-nano laminated CNTs/Ti composite was prepared in the following three steps:(1)construction of CNTs/Ti element layer: the electrophoresis solution with uniform dispersion of CNTs was obtained through 7h acidification treatment combined with 8h ultrasonic dispersion.CNTs/Ti element layer with the uniform dispersion of CNTs on the surface of Ti foil were prepared by combining with optimized electrophoretic deposition(EPD)process(30V,15s/30s);(2)spark plasma sintering(SPS)of CNTs/Ti laminated materials: CNTs/Ti laminated materials were obtained after stacking several CNTs/Ti element layers by SPS(the optimal process was 520℃/50MPa/17min);(3)Temperaturecontrolled rolling of CNTs/Ti laminated materials: micro-nano laminated CNTs/Ti composite was prepared by low-temperature large deformation rolling of CNTs/Ti laminates with 520℃ and 90% of total deformation.Microscopic studies show that: the interface reaction between CNTs and Ti matrix was effectively controlled and a good metallurgical bonding interface was formed.Most CNTs were completely retained and showed obvious layered distribution characteristics,that is to say,micro-nano laminated CNTs/Ti composite were formed by alternating arrangement of nano-level CNTs layer and micron-level Ti layer.Among them,the spacing between CNTs layers was 5-8 microns,and CNTs basically showed an ideal single dispersed,which was very beneficial to maximize the ultra-high strengthening efficiency of CNTs.In addition,a very small number of CNTs with defects,such as the open tip and radially unzipped on the side wall,would react with Ti matrix to generate dispersed nano-TiC particles.Mechanical tests show that: the strength of micro-nano laminated CNTs/Ti composite was significantly increased with the increase of the content of CNTs when the content of CNTs was between 0 and 0.08 wt.%.The yield strength of the 0.08 wt.% CNTs/Ti composites reached 616 MPa,27.5% higher than the strength(483MPa)of pure Ti rolled plates,and it still had a high elongation of 17%.In addition,compared with the published literature,the CNTs in micro-nano laminated CNTs/Ti composites have the highest enhancement efficiency.The main strengthening mechanism of micro-nano laminated CNTs/Ti composites are as follows: fine grain strengthening,load transfer strengthening,heat mismatch strengthening and dislocation blocking effect strengthening caused by layered structure.The initiation and propagation of cracks were studied by X-ray microscope,and the results show that: the crack evolution process in this composite is divided into two stages.Stage I: stage of crack initiation and uniform growth,the crack volume increases linearly with the increase of macroscopic strain;Stage II: stage of nonuniform propagation and instability merge of cracks,the crack volume increases exponentially with the increase of macroscopic strain.The fracture process of micro-nano laminated CNTs/Ti composite was as follows: Cracks are easy to form at interlaminar interfaces and gradually propagation evenly across interfaces.When the macroscopic strain continues to increase,the interlaminar crack accelerates and causes local necking.When the macroscopic strain increases further,the transverse crack will occur due to severe local necking.The formation and propagation of transverse cracks make the cracks run through completely,resulting in the ultimate failure fracture of the material.