Study On The Preparation,Structure And Properties Of Novel Ti-based Bulk Metallic Glasses And Composites

Ti-based bulk metallic glasses?BMGs?have drawn great attention from researchers due to their low density,high strength,high elastic limit,good corrosion resistance and excellent biocompatibility.They have shown a variety of potential applications for biomedical materials,aeronautics and astronautics,and ocean engineering.In order to obtain high glass forming ability?GFA?,the existing Ti-based BMGs alloy systems usually contain a lot of toxic elements Be and Ni,and noble metal Pd.In consideration of the attractive biomedical applications of Ti-based BMGs,it is necessary to develop novel Ti-based BMGs free of Ni,Be and Pd elements and possessing antibacterial function.On the other hand,metallic glasses usually present a degree of room-temperature brittleness due to their unique atomic structure,which severely handicaps their application for structural engineering.This thesis is aimed to develop novel Ti-based BMGs and their composites with potential biomedical applications.In this first part,the liquid-solid transition and atomic configuration of Ti₆₀Cu₄₀,Ti56Cu44,and Ti56Cu40Ag4 alloy were calculated and analyzed systemically by using ab initio molecular dynamics method so as to provide theoretical guidance for the composition design of novel Ti-based BMGs.It is shown that the mobility of atoms in these liquid alloys is correlated to the GFA very well.The atomic mobility of the three liquid alloy systems from high to low is in order of Ti₆₀Cu₄₀,Ti56Cu14 and Ti56Cu40Ag4.The diffusivity of Ti and Cu atoms is decreased effectively with the addition of Ag into Ti-Cu alloy,which is beneficial to the enhancement of GFA in the viewpoint of dynamics.Compared with Ti₆₀Cu₄₀ alloy,The icosahedral clusters in the atomic configuration of amorphous Ti56Cu44 alloy are more than those of amorphous Ti₆₀Cu₄₀ alloy.The addition of Ag into Ti56COu44 alloy decreases slightly the amount of the perfect icosahedron of<0,0,12,0>,but increases the diversity of icosahedral-like clusters.It was also found that in Ti56Cu40Ag4 glassy structure,MROs are built out of icosahedral-like clusters through edge-,face-and intercrossed connection,resulting in the raising of atomic packing density so as to be in favor of the enhancement of GFA.Based on the above AIMD calculation results,the effects of Zr and Si addition on the GFA of?Ti,Co,Sn?56Cu44 alloy system were studied.Bulk metallic glasses of Ti46Zr12.5-xCu31.5Co7Sn3Six?x = 0,0.5,1,1.5?with critical diameter of 2-3 mm were successfully prepared by water cooled copper mold casting.These novel Ti-based BMGs are free of toxic elements Be,Ni and noble metal Pd,indicating a promising prospect for biomedical application.When the addition of Zr reaches 12.5 at.%,the Ti46Zr12,5Cu31,5Co7Sn3 alloy rod with the diameter of 2 mm is of full amorphous structure.The reason for the improvement of GFA induced by the addition of Zr may be due to the increase in the atomic size difference,electronegativity difference and absolute values of the enthalpy of mixing.With the addition of 1 at.%Si,the GFA of Ti46Zr11.5Cu31.5Co7Sn3Si1 alloy is further enhanced and the critical diameter is increased to 3 mm.The compression yield and ultimate strengths of Ti46Zr11.5Cu31,5Co7Sn3Si,alloy reach 2478 MPa and 2623 MPa,respectively.The addition of Si promotes the initiation and multiplication of shear bands,resulting in the plastic strain up to 0.83%.In order to further develop Ti-based BMGs with antibacterial function,a comprehensive study was performed on the microstructural evolution,thermal stability,mechanical properties,corrosion behavior and antibacterial property of Ti46Zr11.5Cu31.5-xCo7Sn3Si1Agx?x=0,1,2,3,4,5?alloy system.It is shown that the critical diameter of Ti46Zr11.5Cu31.5Co7Sn3Si1 alloy was increased from 3 mm to 5 mm through the proper addition of Ag.The addition of Ag decreases the melting temperature and increases the activation energy of crystallization.A yield strength of 2604 MPa as well as yield strain of 2.55%were measured with addition of 1 at.%Ag.It is found that the shear fracture mechanism of this kind of BMGs basically agrees with the Tresca criterion.The electrochemical polarization experiments show that Ti46Zr11.5Cu27.5Co7Sn3Si1Ag4 BMG exhibits excellent corrosion resistance properties.These BMGs have higher corrosion potentials and lower corrosion current densities than those of Ti-6A1-4V alloy in four different kinds of corrosion solutions.The antibacterial experiment shows that this kind of Ti-based BMGs has obvious antiseptic effect on staphylococcus aureus.The antibacterial rate of Ti46Zr11.5Cu27,5Co7Sn3Si1Ag4 BMG is higher by 96.81%than that of Ti-6A1-4V alloy.In this thesis,the effects of Nb on the GFA and mechanical properties of Ti46Zr11.5Cu31.5Co7Sn3Si,BMG were also studied.With the addition of 1 at.%Nb,the microstructure of Ti46Zr10.5Cu31.5Co7Sn3Si1Nb1 alloy is composed of amorphous matrix and uniformly distributed crystalline phases with the size of 5-30 ?m.Ti46Zr10.5Cu31.5Co7Sn3Si1Nb1 alloy shows excellent compressive properties with yielding strength of 2362 MPa,plastic strain of 6.8%,and total strain of 10.1%,respectively.The addition of high content of Nb promotes the formation of ?-Ti dendrites in the microstructure.With the increase of Nb content,the plastic strain of alloy is increased from 6.74%for Ti57,5Cu25.5Co7Sn3Si1Nb6 alloy to 23.21%for Ti57.5Cu17.5Co7Sn3Si1Nb14 alloy.For Ti60Cu14Ni12Sn4Ta10 nanostructure-dendrites composite,stress induced phase transformation?SIPT?of B2 NiTi phase in the eutectic matrix region takes place during the elastic deformation process.Large elastic strain up to 1.92%has been observed,which may be attributed to the SIPT together with the elastic deformation of both ?-Ti and CuTi₂ phases.