| Recently,βtype titanium alloys are getting much attention for biomedical materials because they have excellent strength and corrosion resistance, no allergic problems and the best biocompatibility among metallic biomedical materials. Ti-Mo-Zr-Fe alloy system especially got more attention for their excellent combined strength, ductility and elastic modulus.The effects of alloying elements on elastic modulus of binary bcc titanium have been simulated through the first principle method. The lattices parameters, elastic modulus and electronic structures of a 16 atoms super cell of bcc titanium with the centre titanium atom replaced by another elements, such as Zr, Hf, Ta, Nb, V, Cr, Mn, Fe and Mo, were calculated. The results show that all these elements mentioned above increase the elastic modulus of bcc titanium. Mo and Fe have much more effect on solution strengthening than other elements. The chemical composition optimization of Ti-Mo-Zr-Fe alloy system has been computed in the presence of virtual crystal approximation concept and mixture atoms method. In the specified scope of alloy element fraction (9.61~13.70wt.%Mo ,3.71~7.59wt.%Zr和0.91~2.74wt.%Fe), Mo and Zr with medium content and Fe with maximum content would give the alloy the lowest elastic modulus.The effect of melting parameters on chemical composition homogeneity of titanium alloys has been investigated by means of non-consumable arc-melting process and energy spectrum analysis method. The experiment results indicate that the homogenous distribution of alloying elements in melting bath is the precondition of getting ingots with homogenous chemical composition. The time of keeping melt state need to ensure chemical composition homogeneous, has been estimated by diffusion equation and forced convection model. The result has significance on determining melting times. Through simulation computation and experimental verification, the optimal composition combination has been confirmed. The scope of alloying element contents has been enlarged, two new alloys have been developed. The mechanical properties of these two alloys all meet the desired target. Using principally Ti-11.5Mo-6Zr-2Fe and Ti-7.5Mo-6Zr-2.5Fe alloys, the deformation processes, relationship between heat treatment and phase transformation, phase precipitating regularity in dynamic heating and cooling course, have been investigated in detail. Proposal has been suggested by the author that the activation energy of grain growth can be computed by changing heating temperature while holding time is fixed. Then the activation energy ofβgrain growth of Ti-11.5Mo-6Zr-2Fe and Ti-7.5Mo-6Zr-2.5Fe alloys is calculated. Theωphase would precipitate fromβmatrix when aged at 400℃, while theαphase will precipitate fromβmatrix when aged at 480℃and 560℃. In order to maintain low elastic modulus, the secondary phase, especiallyωphase, should be avoided in bio-medicalβtitanium alloys.The fracture surfaces of tensile testing specimen with various average grain sizes were analyzed by electronfractography. Fracture mechanism changed from plastic dimple to inter-granular crack or mixture of inter-granular crack and cleavage fracture with average grain size growth. When the average grain size is small, the macro fracture surface is typical cup-cone fracture, micro-pattern is homogenous equiaxial dimples, and there are nucleation particles or hollows in the dimples. Accompanying grain size increasing, it changed from dimple to mixture of dimple and cleavage, and to mixture of cleavage and inter-granular crack and little dimple sequentially.The relationship of indent depth and modulus, hardness of Ti-Mo-Zr-Fe alloy has been investigated by means of modern nano-indentation analysis technology. It indicates that there is an indent size effect with maximum indent depth variation. The effect of heat treatment to indent modulus and hardness suggest that modulus and hardness will increase notably withωphase existing. With increasing heat treatment temperature, theωphase disappeared, and the fraction ofαphase decreased, so the hardness and modulus decreased. |
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