| 其他摘要 | Biomedical porous NiTi shape memory alloy (SMA) will be widely used for the human body as implants for hard tissue due to its three-dimension connection pore and good biocompatibility. The biomedical interrelated properties, such as pore morphology, micro-deformation, phase transformation behavior, damping characteristics and Ni release behavior, of porous NiTi SMA fabricated by combustion synthesis (CS) have been investigated systemically in this paper. The research on the addition of Mo into porous TiNi (Mo) SMA has been carried out initially.
Synthesis process and powder characteristics have a strong influence on the pore structure of porous NiTi SMA fabricated by CS method. Only under the appropriate preheating temperature, the porous NiTi alloy owns uniform pore structure and its mechanical properties could meet the demand for hard tissue implants. The combustion wave will not propagate steadily at a relative low preheating temperature and the anisotropic pore structure will create in porous NiTi alloy. However, if the preheating temperature is too high, the liquid phase will appear in the sample. As a result, the solid NiTi will exist in some region, and uniform porous structure cannot be obtained. The suitable size and content of Ti powder are also necessary factors in fabricating porous NiTi alloy with uniform pore structure. Besides the predominated phase (NiTi phase), there are several other intermetallic compounds in porous NiTi alloy, such as Ni4Ti3 phase and NiTi2 phase. Preheating temperature and content of Ni have no notable effect on the transformation temperatures of B2↔B19’ in the porous NiTi alloy. Transformation temperatures of porous NiTi alloy decrease slightly with an increasing number of thermal cycles. Low temperature annealing will induce R transformation in porous NiTi SMA. The results confirm that M phase transformation occur independently prior to R phase transformation on cooling. After high temperature annealing, porous NiTi SMAs exhibit one-step transformation, but the phase transformation temperature and latent heat are decreased due to the precipitation of Ti-rich phase.
The micro-deformation characteristics of porous NiTi SMA under compression have been investigated using SEM moiré method. During the procedure of loading, the margin and center of pore wall endure both shear stress and compressive stress. The stress concentration is formed at the sharp-angled area of pore and induces the crack propagation from the margin of pore wall to the center along 45° angle with the loading direction. During the deformation, sliding deformation occurs at the margin of pore wall, however, the compressive and shear strain are slight at the center of pore wall.
The internal friction of porous NiTi SMA can be divided into strain amplitude independence internal friction, rapid-increasing strain amplitude dependence internal friction and slow-increasing amplitude dependence internal friction, which mainly comes from pinning and unpinning of dislocation, micro-plasticity deformation and mobility of martensite interface. The cycles of strain amplitude increase the internal friction of porous NiTi SMA at low amplitude strain part (<1×10-3), however, decrease that of porous NiTi SMA at high amplitude strain part (>1×10-3). An increasing of porosity decreases the internal friction of the porous NiTi alloy. The internal friction of porous NiTi alloy is not sensitive to low frequency. However, the porous NiTi alloy fatigue fracture easily at high frequency. One internal friction peak associated with the B2↔B19’ transformation appears on as-received porous NiTi shape memory alloy. There are three peaks both on heating and on cooling for the porous NiTi SMA with low temperature annealing, which is corresponding to M transformation, R transformation and R phase induced relaxation peak respectively.
Due to the irregular pore structure, the Ni release amount of porous NiTi alloy is much higher than that of the solid one with the same dimension. The Ni release amount increases obviously under the influence of the static bending strain. Two kinds of surface modification, high temperature oxidation and hydroxyapatite coating, are employed to restrain Ni out-diffusion from the porous NiTi alloy. High temperature oxidation can partially restrain Ni release. After a serials of chemical treatments and subsequent sample immersion in SBF for 5 days, a uniform hydroxyapatite layer forms, not only on the surface of the porous NiTi alloy, but also on the inside of the pores. This hydroxyapatite layer greatly decreases the amount of nickel release and improves the biocompatibility of the porous NiTi shape memory alloy.
Porous TiNi(Mo) ternary SMA has been fabricated by the CS method with the mixture Ni, Ti and Mo powder. A small amount of Mo addition strengthens compressive properties of porous TiNiMo alloy due to solution strengthening. However, the compressive strength and compressive strain of porous TiNiMo alloy with excessive Mo decrease sharply. Mo addition induces the R-phase during the cooling and decreases the transformation temperatures of porous NiTi alloy and there is a near-linear relationship between Mo content and Af temperature of porous TiNi(Mo) alloy. The porous TiNiMo alloy with about 1.1at.%Mo content will be more suitable to be a hard tissue implant than the porous NiTi binary alloy according to the transformation temperature. |
修改评论