Study On Microstructure And Properties Of TiC/TC4 Titanium Matrix Nanocomposites Fabricated In Situ By Selective Laser Melting

TC4 titanium alloy,as a structural material,has been widely used in military,aerospace,vehicles,medical and other fields because of its excellent corrosion resistance and high specific strength.However,with the development of aerospace technology,more stringent requirements are put forward for TC4 alloy.Material compounding can effectively improve the overall performance of TC4 titanium alloy,and enhance its strength,toughness and corrosion resistance because of its good toughness of titanium alloy and high specific strength and stiffness of ceramic particles.Therefore,it has a broader application prospect.In order to expand its application field,the TiC reinforced phase of nano-ceramic particles was introduced in situ to prepare titanium nanocomposites.In view of the difficulty of traditional processes to meet the rapid preparation requirements of complex TC4 alloy precision components,this dissertation uses selective laser melting（SLM）to prepare TiC/TC4 titanium matrix nanocomposites.Based on this,the relationship between the microstructure,stress-strain evolution,deformation behavior at atomic scale and its macro properties was established to reveal the deformation strengthening mechanism and corrosion mechanism of TiC/TC4 titanium matrix nanocomposites.Deriving from the above studies,through comparative analysis,the deformation evolution and fracture mechanism of porous TC4 alloy are explored,and the action mechanism of in situ nano ceramic particles TiC on the mechanical properties of porous TiC/TC4 titanium matrix nanocomposites is clarified.The main research contents and conclusions are as follows:（1）The mechanical properties of TC4 alloy and TiC/TC4 titanium matrix nanocomposites were studied.The results show that the stress-strain concentration point at the shearing part during the compression deformation process causes the generation of microcracks in the TC4alloy,and the large area stress-strain concentration during the tensile deformation process causes the TC4 alloy to fail to plastically deform and cause brittle fracture.In the process of SLM forming,the solid solution of C element and the in situ reaction between Ti atom and C atom triggered the nucleation of acicular martensite in thermodynamics and dynamics;these increased the grain boundaries and enhanced the movement resistance of dislocations,which had the effects of fine grain strengthening and dislocation strengthening.Meanwhile,the in-situ self-generated nano-ceramic particles make the fracture of microscopic pores（the precursor of dimple morphology）change from the originalα/βinterface to the interface between TiC and the Ti matrix,resulting in the uneven distribution of V element in the dimple fracture（TC4dimple fracture）changes to uniform distribution（TiC/TC4 titanium matrix nanocomposite dimple fracture）.（2）The deformation mechanism of TiC/TC4 titanium matrix nanocomposites at atomic scale was conducted.The self-grown nanoscale-TiC reinforcement not only strengthened its mechanical properties through the load transfer,but also provided stress concentration in its vicinity,which was beneficial to the activation oftwinning in theα-Ti matrix;these caused the orderly arrangement of atoms in the TiC/TC4 alloy nanocomposite during the deformation process,which improved the plastic deformation ability.Meanwhile,the interface area formed by the twin interface of the TiC/TC4 composite alloy sample and theα-Ti with different orientations in the titanium alloy matrix increased the lattice friction stress when the dislocation passed through this area,yeild an effect of dislocation pinning strengthening.（3）The effects of in-situ TiC from SLM on the texture and corrosion behavior of tic/tc4titanium matrix nanocomposites were studied.The results show that,compared with the conventional TC4 alloy,the corrosion surface of the composite material has a smoother surface,showing a better corrosion performance;TiC/TC4 titanium matrix nanocomposites prepared in situ by SLM have dense texture,few defects and the regeneration of Ti O₂ passive film,which improve their corrosion resistance.（4）The mechanical properties of TC4 alloy and TiC/TC4 titanium matrix nanocomposite alloy with different porous structures were studied.The results showed that the stress-strain evolution of diamond-type porous TC4 alloy mainly diffused symmetrically to both sides along the shear band,and the stress-strain of BCC-type porous TC4 alloy was transmitted along the lower half of its shear band.The fine acicular martensite with different sizes and tightly combined is the fundamental reason for the improvement of its mechanical properties.More importantly,SLM in situ TiC could be used as the nucleation site of dimple,which promoted porous TiC/TC4 titanium matrix nanocomposites into a ductile fracture mode.