Study On Pore Characteristics And Properties Of Porous Ti-24Nb-4Zr Alloy

The purpose of this paper was to design and study biomedical porous titaniumalloy. The elements composition of this porous titanium alloy was designed by meansof Mo equivalent amount and it was prepared by high vacuum sintering furnace andargon t ube f urnace. The por osity, m orphology, m echanical pr operties a ndelectrochemical properties of this porous alloy were analyzed by optical microscope,SEM, XRD, tri-point bending test, compression test and electrochemical tests.The porous Ti-24Nb-4Zr alloy prepared by metal powder metallurgy method wasmetastable β titanium alloy. With the addition of more NH₄HCO₃powder, the αpercentage increased a ccordingly. The a ddition of f orming a gent c ould effectivelyincrease the porosity and pore diameter. With the addition of more forming agent, theporosity increased linearly an d t he av erage p ore d iameter i ncreased su bstantiallywhile little influence was observed to pore roundness and maximum diameter. Theinner wall of pores possessed micro pores which were far smaller than average poreswhich means those pores were not completely close. The addition of NH₄HCO₃couldinfluence mechanical properties of the porous alloy by changing its porosity and porediameter. A s t he porosity increases, t he Y oung’s m odulus and be nding s trengthdecreases linearly. This means that porous titanium alloys with different mechanicalproperties could be prepared by addition of different amount of forming agent. PorousTi-24Nb-4Zr alloy was prepared by adding20wt.%NH₄HCO₃, and it possessed anelastic m odulus of3.3G Pa a nd a yi eld s trength of172M Pa w hich m eet t herequirements of bi omedical i mplant m aterials. T he f ractography of por ousTi-24Nb-4Zr a lloy exhibited b rittle f racture w ith tra nsgranular c leavagecharacteristics. Ju dging f rom t he m acro f ractography a nd s tress-strain c urve ofimpression test, the samples showed evidently slip failure with45°orientation and itwas different from ordinary brittle facture which means that the fracture mechanismwas a mixture of brittle fracture and plastic fracture.The phase composition of final sintering products was metastable β phase whichshowed no distinct different when the samples were sintered with different sinteringtemperature and time as w ell as pressing pressure. When the sintering temperatureincreases, the density of samples and pore diameter decreases distinctively while theporosity and pore roundness increases sharply. Meanwhile as the pore characteristics changing, the c ompression m echanical p roperties c hange a ccordingly: t hecompression elastic modulus and compression strength increases. When the sinteringtime increases, the density of samples and pore diameter decreases slowly while theporosity and pore roundness increases, and the compression elastic modulus showsthe trend of increasing. Compression stress shows no obvious influence to the samples.The tests show that cold press with pressure of90M Pa to130M Pa exhibits littleinfluence to the final sintering samples.The four materials exhibits similar corrosion potential and that of the poroustitanium a lloy w as lo wer th an th e o ther th ree m aterials which means t he p oroustitanium alloy possesses lower corrosion tendency. The anodic polarization curves ofporous TNZ alloys showed that passivation occurred during the tests. The passivecurrent density was3.4μA and breakdown potential was1.2V in NaCl solution withpH value of7.4which was t he same with r eference material; the passive currentdensity was4.4μA and breakdown potential was0.6V in Hanks solution with pHvalue of7.4which was lower than reference material and means it is easier to overdeactivated. H owever, por ous T NZ a lloy s howed no s econdary pa ssivation w hichmeans that the corrosion velocity under high voltage is much faster than referencematerial. According to electrochemical impedance spectroscopy and fitting analysis,the equivalent circuits of corrosion systems of porous TNZ alloy, pure Ti and TC4areall R（QR） types. The impedance of porous TNZ alloy oxidization film was a littlelower than the other three materials. In sum, corrosion resistant evaluation by relevantelectrochemical tests shows that porous TNZ alloy possesses high corrosion resistancewhich could meet the requirements of biomedical applications.