Materials Science Forum Vols. 475-479

Abstract: In order to replace Ti-Ni shape memory alloys, new biomedical shape memory alloys have been developed which are composed of beta titanium and nontoxic elements only. In this paper, experimental results of mechanical and shape memory properties are reported for the Ni-free Ti-18mol%Nb shape memory alloys containing 3mol% of 13-group and 14-group ternary elements in the periodic table. The ternary elements selected are Al, Ga, In, Ge and Sn. It was found that the solution treated alloys exhibit good shape memory effect but almost no pseudoelasticity at room temperature. Ultimate tensile strength and elongation to failure at room temperature are ranged from 250 to 710MPa and from 13 to 21%, respectively, depending on the kind of ternary elements. Effect of solution hardening on strength is discussed.

Abstract: Wear behavior of titanium materials such as C.P.Ti, Ti-6Al-4V and Ti-6Al-7Nb, was studied in simulated body fluids by means of the pin-on-disk type wear testing. The mass loss in wear testing increased with increasing the sliding distance. The a+b type titanium alloys, Ti-6Al-4V and Ti-6Al-7Nb, exhibited higher wear resistance than the C.P.Ti. Average size of the wear debris was 4.5 µm and 3.7 µm for C.P.Ti and the a+b type titanium alloys, respectively. The elution of metallic constituents into 1 mass% lactic acid solution was detected after the wear test.

Abstract: In an attempt to optimize the shape recovery temperature, the effect of Zr addition on phase constitution and heat treatment behavior is investigated by electrical resistivity and Vickers hardness (HV) measurements, X-ray diffractometry (XRD) and shape recovery tests. Ti-25mass%Nb-0, 2, 7 and 12mass%Zr alloys (abbreviated as 0Zr, 2Zr, 7Zr and 12Zr, respectively) were prepared using an arc-furnace. Specimens were solution-treated at 1273 K for 3.6 ks and then quenched by iced water (STQ). STQed specimens were isochronally heat-treated. In 0Zr and 2Zr, only the orthorhombic martensite phase a” was identified by XRD, while the two-phase alloys a” and b were identified in 7Zr and 12Zr. In 7Zr, resistivity at liquid nitrogen and room temperature (rLN and rRT, respectively) and resistivity ratio (rLN/rRT) drastically increased at 523 K because of the reverse-transformation of a” into b phase. Thereafter, resistivity and resistivity ratio decreased with increasing heat treatment temperature due to isothermal w precipitation. Starting temperature of shape recovery is 623 K in 7Zr and 523 K in 12Zr. In 7Zr, shape recovery ratio is about 80% at 723 K, which is the maximum obtained in this study.

Abstract: An intensive amplitude arrangement for reciprocal tribocontact of biomedical alloys, Ag-20.0Pd-14.5Cu-12.0Au, Au-5.0Pt-2.0Pd-9.2Ag-15.0Cu and Ti-29Nb-13Ta-4.6Zr was conducted in order to check the details of friction and wear alterations around the transition from fretting to sliding. It is shown that the friction force exhibits stagnation for the Ag alloy and decrease for the Au alloy and the Ti alloy in certain domain of oscillating amplitude. Beyond the domain the friction force increases with the amplitude, and below the lower threshold and above the upper threshold triboevent is complete fretting and sliding respectively. Observation of friction-force waveform and SEM topography found, a hybrid mechanism of fretting and sliding is dominant in the amplitude domain, and the strong adhesion between the self-mated alloys is responsible to this exhibition.

Abstract: Ti-50.6 at.% Ni shape memory alloy was coated with tantalum using multi arc ion-plating technique with the aim to increase its radiopacity and biocompatibility. The surface characteristics were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results of XPS survey spectra show that a thin oxide film are formed inside tantalum coating as a result of natural passivation of Ta in atmosphere. The hemocompatibility was evaluated in vitro by clotting time and platelet adhesion measurement. The results of our study showed that the clotting time of tantalum was higher than that of the TiNi alloys and no sign of accumulation and only slight pseudopodium was observed on the tantalum coatings, suggesting that the tantalum coatings can improve the biocompatibility of TiNi alloy.

Abstract: b-tricalcium phosphate(TCP, Ca3(PO4)2) ceramics are preferred as a bioceramics because of its chemical stability and reasonable degradation rate in vivo, but it is difficult to obtain b-TCP ceramics with high compressive strength at lower temperature than that of phase transition to a-TCP. In this study, the sintering behavior of TCP, Ca2P2O7-doped TCP, and CaCO3-doped TCP in the range of 2wt%~5wt% were investigated respectively. Phase transition of pure TCP took place between 1100°C to 1150°C, and pure b-TCP ceramics could achieve a compressive strength of only 3MPa. However, calcium pyrophosphate (CPP, Ca2P2O7) additive prevented the transformation of b-TCP to a-TCP, but the second phase of CPP was observed in the resultant ceramics. Phase transition of TCP ceramics by addition of both CPP and calcium carbonate (CC, CaCO3) took place between 1200°C to 1250°C and the resultant TCP ceramics had few impurity of CPP. By adding CPP and CC to TCP, final ceramics with compressive strength over 12MPa could be obtained when sintered at 1200°C for 2hrs.

Abstract: The osteoinductivity of calcium phosphate ceramics has been studied extensively, but the mechanism is still unclear and few reports about the molecular mechanism in the osteoinductive process. In this study the osteoblast related gene expressions induced by biomaterials were investigated by isolating the RNA from the tissue grown in porous hydroxyapatite/tricalcium phosphate (HA/TCP) ceramics implanted in rat femur muscle on day 7, 15, 30, 60, 90,120, and analyzed by RT-PCR technique. RNA extracted from muscle without implant was used as control at the same time. The results showed that osteopontin and osteocalcin genes, the important osteoblastic markers, expressed in early stage, on day 7 after implantation, and were detected at any period. Collagen type I gene expressed on day 60, 90 and 120. It revealed that osteoblast differentiation occurred very early before collagen type I expression after implanting HA/TCP ceramics in vivo.

Abstract: Porous blocks of bioactive ceramics are a more preferable tissue substitute to replace and reconstruct the defected tissues in that their interconnective pores are beneficial to new tissue ingrowth. The most important specification requires the controlled interconnective pores with required mechanical strength. A novel chitin-gel-cast approach is utilized to prepare porous bioceramic blocks. In order to improve their interconnection of pores, several acids are selected to etch the windows between porous. Light microscope (LM) and Scanning electronic microscope (SEM) are used to investigate the etching effect on the windows and walls of porous blocks. The results show that the acid etching can improve the interconnection of porous blocks with slight decrease of compressive strength under carefully controlled process conditions.