| Because of their good biocompatibility, excellent mechanical properties and corrosionresistance, Ti alloys are widely used as biomedical materials. Though the third generation βtype Ti alloys prepared by casting method have good mechanical properties andbiocompatibility, they also possess shortcomings such as coarse grain, component segregationbrought by refractory metal, higher elastic modulus compared with human bone and poorwear resistance. Moreover, preparation of fine-grained biomaterials also becomes a researchfocus because of their better biocompatibility. The aim of the present study is to prepareultrafine-grained (UFG) biomedical titanium alloys with high strength and low modulus bypowder metallurgy (mechanical alloying (MA) and spark plasma sintering (SPS)). It canprovide a new method for preparation of biomedical materials. Therefore, this project hasimportant academic and application significance.As a first step, the alloying elements were selected preliminary according to elementsbiocompatibility. The chemical composition was selected as (Ti-35Nb-7Zr-5Ta)100-xFex(x=0、2、6、10) according to “d electronical theory”and “Inoue’s three principles”. Then,“MiedemaModel” was used to calculate glass forming ability (GFA) of designed alloys and “clustertheory” was used to predict chemical composition in order to find the alloy composition withlowest elastic modulus.During MA, the effect of Fe content on GFA, thermal stability and crystallizationmechanism was investigated. The result shows that the GFA of the alloys increases with theincreasing of Fe content. With the increased Fe content, the synthesized alloy powders afterthe steady state milling transform from full nanocrystalline structure for x=0to fullamorphous structure for x=10. The synthesized amorphous/nanocrystalline alloy powderspossess high thermal stability because they all show wide supercooled liquid region Tx.Moreover, the crystallization activation energy E is affected by Fe content, and thus theirthermal stability is affected. The crystallization activation energy E for x=10is bigger thanthat for x=6. The crystallization mechanism of milled powder is also affected by Fe content.The average Avrami exponent n are2.5and2.0for x=6and10, respectively. Thecrystallization mechanisms are typical volume diffusion-controlled three and two-dimensionalgrowth of nuclei for x=6and10, respectively. The different crystallization mechanisms ofmilled powder affect the species of crystallization precipitation phase. For x=6, thecrystallization precipitation phases are composed of β-Ti and FeTi. For x=10, the crystallization precipitation phase contain α-Ti, β-Ti and FeTi.Subsequently, biomedical UFG Ti-based composites with different Fe contents werefabricated by SPS method. The effect of Fe content on microstructure and mechanicalproperties of UFG bulk composites was investigated. The result shows Fe content affectsmechanical properties of consolidated bulk composites obviously. Only for x=6, the UFG bulkcomposites possess obvious plasticity. For x=0,2and10, the UFG bulk composites have noplasticity. The difference in mechanical properties of UFG bulk alloys is decided by species ofcrystallization precipitation phase and their microstructures. The microstructure is atwo-phase region consisting of FeTi phase surrounded by β-Ti matrix for x=6, and the fracturemechanism can be explained by the “soft-hard model”. The biomedical UFG (TNZT)94Fe6composite exhibits high strength and low modulus. The UFG bulk composite possessesexcellent mechanical properties fabricated by heating to1243K at290K/min and holding for5min. Its yield strength σy, fracture strength σmax, fracture strain and ultrasonic elasticmodulus are2292MPa,2531MPa,9.16%and53GPa, respectively. The value of elasticmodulus is relative small in the reported Ti-Nb-Zr alloy system.The wear resistance of the fabricated UFG (TNZT)94Fe6composite was investigated bycompared with conventional biomedical Ti-6Al-4V (TAV) and Ti-13Nb-13Zr (TNZ) alloys.The result shows that the UFG (TNZT)94Fe6alloy possess the best wear resistance while theTNZ alloy possesses the worst. Besides, the effect of grain size on wear resistance of the UFG(TNZT)94Fe6composite was that the smaller the grain size, the better of its wear resistance.The corrosion resistance of the fabricated UFG (TNZT)94Fe6composite was investigatedby compared with conventional biomedical TAV and TNZ alloys. The result shows that thestability for three kinds of Ti alloys in corrosive media is (TNZT)94Fe6>TNZ>TAV. Moreover,the corrosion resistance of the UFG (TNZT)94Fe6alloy is affected by grain size. The smallerthe grain size, the greater its corrosion tendency, and the slower the speed of the corrosionreaction. The cytotoxicity test for UFG (TNZT)100-xFex(x=0,6and10) alloys shows thatbiological toxicity of three kinds of titanium alloy are all class one and suitable as... |
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