| Crystallization of amorphous phase is a unique way to fabricate nanostructured and ultrafine-grained materials. Mechanical properties of porous materials can be tailored in greater scope by introducing porous structure. The aim of the present study is to prepare ultrafine-grained (UFG) biomedical titanium alloys with low modulus (close to or match that of human bone) by powder consolidation and crystallization of amorphous phase. This can provide a way for preparation of high-performance biomedical materials with excellent biocompatibility and biomechanical compatibility.Firstly, from the perspective of improving the biocompatibility, biomechanical compatibility and glass-forming ability (GFA), the chemical compositions were designed as Ti70.0Nb23.33Zr5.0Ta1.67, Ti68.0Nb23.33Zr5.0Ta1.67Si2.0 and Ti65.0Nb23.33Zr5.0Ta1.67Si5.0 (at.%, Si-containing system alloys), Ti72.19Nb16.69Zr5.58Fe5.54, Ti71.08Nb16.69Zr5.58Ta1.11Fe5.54 and Ti65.0Nb23.33Zr5.0Ta1.67Fe5.0 (Fe-containing system alloys) according to d-electron alloy design theory, Inoue’s three principles, thermodynamic Miedema model, Mo equivalent, electron concentration (e/a) and cluster theory. The GFA and elastic modulus of the designed alloys were predicted theoretically. This can provide sufficient theoretical support for the designation of high-performance biomedical titanium alloys.Then, Ti65.oNb23.33Zr5.oTa1.67Si5.0 was selected preferentially and mechanically alloyed. The formation mechanism of bulk UFG titanium alloy was investigated under non-isothermal and isothermal crystallization conditions based on crystallization kinetics. Results are as follows. The mechanically alloyed amorphous powder has two crystallization steps, precipitating β-Ti and (Ti,Zr)2Si phases successively. The lower activation energy of the first step indicates the easier precipitation of p-Ti matrix. Non-isothermal and isothermal crystallization mechanisms have little difference. The first step is dominated by diffusion-controlled 3D and 2D growth of nuclei, while the second one is governed by diffusion-controlled 3D growth of nuclei. Both nucleation rates increase firstly and then reduce. Compared with isothermal crystallization, the bulk alloy fabricated by non-isothermal crystallization has smaller grain size and higher yield strength due to shorter holding time and lower growth rate. Under isothermal crystallization condition, the bulk alloy fabricated by high isothermal crystallization temperature has smaller grain size and higher yield strength due to higher nucleation rate. The intrinsic relationship between crystallization mechanisms and microstructure of the bulk UFG alloys was revealed. The results obtained provide theoretical and practical guidance for preparation of high-performance UFG biomedical titanium alloys by powder consolidation and crystallization of amorphous phase.Thirdly, the six kinds of amorphous/nanocrystalline alloy powders were synthesized by mechanical alloying. Results are as follows. The GFA of the Si-containing system alloys increases with the increasing of Si content. The synthesized Si-containing alloy powders transform from full nanocrystalline structure for Si=0 to full amorphous structure for Si=5at.%. The as-milled Fe-containing alloy powders all have nanocomposite structure possessing GFA due to the minor addition of Fe, while Nb and Ta content has no obvious influence on GFA. The GFA of Ti65.0Nb23.33Zr5.0Ta1.67Si5.0 alloy is the highest among the six kinds of alloys.Afterwards, the bulk UFG Si-and Fe-containing titanium alloys with near full density (NFD) were fabricated by spark plasma sintering and crystallization of amorphous phase. Results are as follows. Si content affects microstructure and mechanical property of the as-fabricated Si-containing bulk alloys to some extent. The microstructure consists of full β-Ti phase for Si=0 and a two-phase region with (Ti,Zr)2Si phase surrounded by β-Ti matrix for Si=2 and 5at.%, respectively. The elastic modulus and yield strength increase gradually with increasing Si content due to higher content of (Ti,Zr)2Si phase. The bulk UFG Si-containing (Si=2 and 5 at.%) alloys exhibit better comprehensive mechanical properties with elastic modulus, fracture strain, yield and fracture strength of 37-40GPa,58-66%,1143-1347MPa and 2793-3267MPa, respectively. The value of elastic modulus, which is close to that of human bone, is relatively small in the reported new generation β type biomedical titanium alloys.Besides, Nb content affects microstructure and mechanical property of the as-fabricated Fe-containing bulk alloys obviously. The bulk alloy possess obvious plasticity only for Nb=23.33at.%, while the bulk alloys have poor plasticity for Nb=16.69at.%. The difference in mechanical property is decided by their microstructures. The microstructure consists of a three-phase region with FeTi2 and minor α-Ti surrounded by β-Ti matrix for Nb=23.33at.% and β-Ti matrix surrounded by a-Ti and minor FeTi2 for Nb=16.69at.%, respectively. The β-Ti content increases with increasing Nb content. The higher the β-Ti content, the better the plasticity. The bulk UFG Fe-containing alloy for Nb=23.33at.% exhibits better comprehensive mechanical properties with elastic modulus, fracture strain, yield and fracture strength of 65GPa,23.4%,2872MPa and 2247MPa, respectively. Its elastic modulus is higher than that of Si-containing bulk alloys, which is decided by their microstructures.To further reduce elastic modulus of the as-fabricated NFD bulk titanium alloys, Ti65.0Nb23.33Zr5.0Ta1.67Si5.0 (TNZTS) and Ti65.0Nb23.33Zr5.0Ta1.67Fe5.0 (TNZTF) with high strength and low modulus were selected. The bulk porous UFG TNZTS and TNZTF alloys were fabricated by traditional powder metallurgy and crystallization of amorphous phase. Results are as follows. The space-holder content has no significant effect on the phase constituent, but exerts an obvious influence on the pore structure and mechanical property of the as-fabricated bulk porous alloys. The microstructure consists of β-Ti and (Ti,Zr)2Si for TNZTS and β-Ti, a-Ti and FeTi2 for TNZTF, respectively. With increasing space-holder content, porosity increases linearly, average pore diameter and pore irregular degree also increase, while the elastic modulus and compressive strength decrease linearly. The bulk porous TNZTF alloy possesses higher strength than TNZTS because of its special microstructure. Its porosity, elastic modulus and compressive strength are 50-56%, 9.7-12.2GPa and 197-327MPa, respectively. Bulk porous UFG biomedical titanium alloys with such a high porosity and high strength are few reported so far.Among the preparation of porous titanium materials, elemental powders are mostly used. For comparison, the bulk porous Ti72.19Nb16.69Zr5.58Fe5.54 (TNZF) alloys were fabricated using the same space-holder content and sintering process parameters with respective elemental powder and amorphous alloy powder. Results are as follows. The bulk porous TNZF alloy fabricated by crystallization of amorphous phase possesses uniform phase distribution, smaller scale of phase region and higher compressive strength. This highlights the unique advantages of crystallization of amorphous phase in the fabrication of bulk porous UFG titanium biomaterials with high strength. This can provide a new method for preparation of high-performance bulk porous UFG titanium alloys for biomedical applications.The wear and corrosion resistance and of the as-fabricated NFD high-performance bulk UFG TNZTS and TNZTF alloys were investigated compared with conventional biomedical Ti-6A1-4V ELI (TAV ELI) alloy and commercially pure titanium (CP Ti) in Hank’s balanced salt solution. Results are as follows. The NFD bulk UFG TNZTF and TNZTS alloys possess the better wear resistance while the CP Ti possesses the worst. Wear volume loss of the four kinds of titanium materials is TNZTF<TNZTS<TAV ELKCP Ti. The NFD bulk UFG TNZTS and TNZTF alloys possess the better corrosion resistance while the CP Ti possesses the worst. Corrosion resistance of the four kinds of titanium materials is TNZTS>TNZTF>TAV ELI>CP Ti. |
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