| Considering performance advantages of excellent biocompatibility,mechanical properties,and corrosion resistance,etc.,titanium alloys are widely used as biomedical implant materials.Compared with coarse-grained metallic alloys,ultrafine-grained alloys have not only high strength resulted from the scale and interface effects,but also excellent biocompatibility originated from high grain-boundary energy.Among all materials processing approaches,powder metallurgy has specific advantages in fabricating ultrafine-grained alloys.On this account,selecting Ti28Nb2 Zr X(X=In and Sn)alloys as research object and comparing b-Ti isomorphous element Sn,this dissertation systematically investigated the effects of b-Ti eutectoid element In on microstructural evolution of Ti28Nb2 Zr In biomedical b-Ti alloys during mechanical alloying,and powder densification mechanism and microstructureproperties correlation during spark plasma sintering(SPS),and finally evaluated electrochemical properties and biocompatibility of sintered Ti28Nb2 Zr X biomedical b-Ti alloys.As for physical properties of Ti28Nb2Zr8 Sn alloy powder,pure ?-Ti solid solution is formed after 60 h milling.Correspondingly,?-Ti average grain size is 0.3 ?m,energy stored by crystalline defects is 1437 J/mol,and average particle size of the milled powder is 1214 ?m.Sintered bulk Ti28Nb2Zr8 Sn ? titanium alloy has equiaxed and ultrafine-grained structure with high-angle grain boundary and grain size of 1.5-2.5 ?m.Increasing heating rate or decreasing holding time would cause the decrease in grain size and consequently the increase in the strength of bulk titanium alloys.Under the heating rate of 150 ?/min and the holding time of 0 min,Ti28Nb2Zr8 Sn bulk titanium alloys has the minimum grain size of 1.770 ?m,the highest fracture strength of 2675 MPa and fracture strain of 0.54.These properties parameters meet the requirement of biomedical implant metallic alloys.In terms of physical properties of Ti28Nb2 Zrx In(x=2,4,8 and 12)alloy powders,60 h milling causes the formation of pure ?-Ti solid solution;with the increased In content,physical properties of ?-Ti alloy powders change at the critical point of x=8,at which grain size achieves the minimum value of 10 nm,and other physical properties parameters have the maximum values,such as the lattice constant of 3.3017 ?,exothermic heat enthalpy of 1500 J/mol,and average particle size of 1088 mm.This may be attributed to the preferential occupation of b-Ti eutectoid element In at body-center position of bcc lattice;the 8 wt.% In content may completely occupy body-center positions,and the further increased In content may occupy lattice point positions.Powder densification mechanism reveals that compared with the 60 hmilled Ti28Nb2Zr8Sn alloy powder,the milled Ti28Nb2Zr8 In alloy powder has significant different physical properties of the higher content crystalline defects and smaller particle size.This decides higher comprehensive impact factor f(=3g /(4D?0))and lower viscous flow activation energy Q,and thus results in higher relative density during powder densification process.As far as the microstructure and mechanical properties of Ti28Nb2 Zrx In(x=2,4,8 and 12)bulk alloys,they have high-angle grain boundary and equiaxed and ultrafine-grained structure with grain size of 1.3-1.5 mm.With the increased In content,bulk titanium alloys fabricated by the same conditions have firstly decreased and then increased grain size,among which the Ti28Nb2Zr8 In alloy has the minimum value of 1.318 mm.Correspondingly,compressive mechanical properties increase firstly and then decrease,and especially,the Ti28Nb2Zr8 In bulk alloy has yield strength of 1172 MPa,fracture strength of 2647 MPa,and fracture strain of 0.56,respectively.Noted that,compared with the Ti28Nb2Zr8 Sn bulk alloy fabricated under the same condition,the Ti28Nb2Zr8 In bulk alloy possesses the lower yield strength,and the higher fracture strength and strain.This is due to the different uniformity in texture distribution of the two kinds of alloys.Because of the preferential occupation on the body-center positions for b-Ti eutectoid element In and the lattice positions for b-Ti isomorphous element Sn inside bcc lattice,the In atoms is more beneficial to the random uniform distribution of easy slip surface {110} of bcc b-Ti.This is more conducive to the multiplication and movement of dislocation and slip during deformation process,and thus the Ti28Nb2Zr8 In bulk alloy has the higher fracture strength and strain.In contrast,the nonuniform distribution of easy slip surface {110} gives rise to the hindered trend of easy slip surface {110} by other difficult slip surfaces during deformation process.This leads to the higher yield strength of the Ti28Nb2Zr8 Sn bulk alloy.Electrochemical corrosion properties and biocompatibility studies show that the corrosion current density of Ti28Nb2Zr8 In and Ti28Nb2Zr8 Sn bulk alloys is 341 n A/cm~2 and 407 n A/cm~2 in Hank's simulated body fluid,smaller than those in 3.5 % Na Cl solution,391 n A/cm~2 and 540 n A/cm~2,respectively.This indicates that the two kinds of alloys have better corrosion resistance in Hank's simulated body fluid.Meanwhile,Ti28Nb2Zr8 In alloy(555300 ?·cm~2)has greater charge transfer resistance compared with Ti28Nb2Zr8 Sn alloy(293530 ?·cm~2)in Hank's simulated body fluid.These results imply that the Ti28Nb2Zr8 In alloy has better corrosion resistance.In addition,cell adhesion and cytotoxicity tests show that the Ti28Nb2Zr8 In alloy has better biocompatibility owing to its higher content grain boundary induced by the smaller grain size.In summary,this dissertation proves that the type of addition elements In and Sn exerts significant influences on microstructure,texture evolution,mechanical properties,and biocompatibility of biomedical ? titanium alloys.The resulted obtained herein can provide useful reference into composition design and microstructure-properties tailoring for novel biomedical ? titanium alloys. |
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