The Fabrication Of Micro-Nanocrystalline Ti6Al4V5Cu Alloy And Study On Its Microstructure And Fatigue Properties

Ti6Al4V5Cu alloy is a new type of medical metal material with unique antibacterial property,excellent corrosion resistance,higher strength and many other advantages.However,due to the addition of Cu,it is easy to cause the precipitation of coarsened and brittle Ti2Cu phases along the grain boundaries,resulting in a significant reduction in plasticity of the material.In addition,as an implantable orthopedic loadbearing material,titanium alloys have many fatigue failure cases in clinic.Therefore,improving the fatigue life of Ti6Al4V5Cu alloy will increase its safety and contribute to wider clinical applications.In this thesis,both micro-nanocrystalline Ti6Al4V5Cu alloy with core-shell structure and metastable Ti6Al4V5Cu alloy with phase transformation-induced plasticity(TRIP)effect were designed and fabricated.Their strength,plasticity and fatigue properties were systematically studied by means of electron backscatter diffraction(EBSD),high-resolution transmission electron microscope(HRTEM),transmission electron microscope(TEM),scanning electron microscope(SEM),X-ray diffraction(XRD)and so on.The main results are as follows:First,micro-nanocrystalline Ti6Al4V5Cu alloy was successfully prepared.An ultrafine martensitic lath structure was obtained by quenching the alloy in the β phase region at 960℃.After the thermal simulation experiments at different deformation temperatures and different strain rates,the microstructures of these samples were observed.It was determined that a uniform micro-nanocrystalline Ti6Al4V5Cu alloy could be obtained after deformation at 740℃ with a strain rate of 2s-1,in which the grains with size below 100 nm accounted for more than 70%.In contrast,the hotcompressed alloy after furnace cooling from 960℃,which was taken as the control material,had a non-uniform microstructure with grain size of about 20 μm.Furthermore,bulk size micro-nanograined Ti6Al4V5Cu alloy was fabricated through industrial hydraulic machine,meanwhile,the commercial Ti6Al4V alloy was also fabricated as a comparison.Results showed that due to prismatic slip systems were activated during the hot deformation in the Ti6Al4V5Cu alloy,numerous transverse boundaries formed thus the grains were prominently refined.While in the Ti6Al4V alloy,due to basal slip systems were activated,the dynamic recovery and dynamic recrystallization were suppressed,the grains was not significantly refined.Then,the thermal stability of micro-nanocrystalline Ti6Al4V5Cu alloy was further studied.The aging treatments at 400℃,500℃ 600℃ and 700℃ showed that the microstructure of micro-nanocrystalline Ti6A14V5Cu alloy remained stable up to 600℃.This was mainly due to the unique "core-shell" structure in the micronanocrystalline Ti6A14V5Cu alloy,that was,a layer of conjugated β and Ti2Cu phases wrapped around the equiaxed a grains.According to three-dimensional atom probe tomography(3DAPT)analysis and transmission electron microscope(TEM)observation,the grain size of β and Ti2Cu phases was only about 10 nm.The existence of these two phases could greatly reduce the grain boundary energy and play the role of pinning grain boundaries,to improve the thermal stability of micro-nanocrystalline Ti6A14V5Cu alloy.Subsequently,micro-nanocrystalline Ti6Al4V5Cu alloy rods were fabricated by an industrial level hot rolling mill in the(quenching+hot deformation)process,and their tensile properties and high cycle fatigue properties were studied.The room temperature tensile strength of the micro-nanocrystalline Ti6Al4V5Cu alloy was more than 1280 MPa and the elongation was 18%,showing excellent comprehensive mechanical properties.Under the condition of R=-1,its fatigue strength was as high as 750MPa(the stress amplitude corresponding to 107 cycles),which was 15.4%and 36.3%higher than those of fine-grained Ti6Al4V5Cu alloy(the furnace cooled alloy)and conventional Ti6Al4V alloy,respectively.This was because the core-shell structure of the micro-nanocrystalline Ti6Al4V5Cu alloy presented good microstructural stability and inhibited grain coarsening and cyclic softening under fatigue loading,thus effectively suppressing the initiation of fatigue cracks.Moreover,the grains in the micro-nanocrystalline Ti6Al4V5Cu alloy could constantly deflect the crack in the process of crack propagation,and the flexible β phases in the core-shell structure were conducive to generating a larger plastic zone at the crack tip,thereby effectively reducing the crack propagation rate under fatigue loading.Finally,based on the above research,a new metastable Ti6Al4V5Cu alloy with TRIP effect was fabricated,and its low-cycle fatigue properties were studied.The alloy had nearly equal proportions of a phase and β phase after solution treatment,which could effectively inhibit the grain coarsening.Since RTi/RCu>1.07,the metastable βphase would transform into α’ phase with higher strength,instead of α" phase with lower strength.The room temperature tensile strength of the metastable Ti6Al4V5Cu was up to 1286 MPa and the elongation was as high as 22%.Under the same strain amplitude,the fatigue life of the metastable Ti6Al4V5Cu alloy was 2～5 times longer than that of the compared Ti6Al4V alloy.The main reasons were:on the one hand,the β→α’ phase transformation in the alloy reduced the plastic strain amplitude and the driving force for fatigue crack propagation;on the other hand,the large amount of metastable βphases in the alloy caused a continuous deflection to the crack during its propagation process,which increased the resistance to fatigue crack growth.In summary,by regulating the grain size and microstructure of the Ti6Al4V5Cu alloy,a micro-nanocrystalline Ti6Al4V5Cu alloy with excellent microstructure stability and high cycle fatigue properties can be fabricated,and a metastable Ti6Al4V5Cu alloy with excellent low cycle fatigue properties can also be fabricated,which lays a rich research foundation for application and future development of the Ti6Al4V5Cu alloy.