NiTi形状记忆合金在生物医用领域的研究进展

NiTi形状记忆合金由于具有良好的力学相容性、耐蚀性和生物相容性而在生物医用材料领域得到广泛应用。本文总结了医用NiTi形状记忆合金的研究现状,评述了NiTi形状记忆合金的耐蚀性和生物相容性特点,并详细讨论了NiTi形状记忆合金的表面改性问题,对NiTi形状记忆合金的医用前景进行了展望。     

NiTi形状记忆合金薄膜的制备及形变特性

研究了适用于微器件的溅射态NiTi形状记忆合金薄膜.讨论了溅射工艺及织构对薄膜结构和相变特征的影响.利用薄膜热相变特性制成了微驱动器,观察并分析了该器件的形变特性.结果表明:原位加热溅射可以获得具有织构的晶化薄膜;用该薄膜制备的驱动器回复率为0.76%.     

Mechanical response of nitrogen ion implanted NiTi shape memory alloy

In the paper a change of material (mechanical) parameters of NiTi shape memory alloy subjected to ion implantation treatment is investigated. The spherical indentation tests in micro- and nano-scale and tension test have been performed to study an evolution of local superelastic effect in different volumes of nonimplanted and nitrogen ion implanted NiTi alloy. The differential scanning calorimetry has been applied to measure the change of characteristic temperatures due to ion implantation treatment. The structure of implanted material has been investigated using electron microscopy technique. It has been found that the ion implantation process changes the properties not only in a thin surface layer but also in bulk material. In the layer the pseudoelastic effect is destroyed, and in the substrate is preserved, however its parameters are changed. The characteristic phase transformation temperatures in substrate are also modified.     

化学修饰对NiTi形状记忆合金氧化膜的影响

用X光电子能谱（XPS）研究了NiTi形状记忆合金经酸、碱处理后表面氧化膜成分和结构的变化。结果表明，未经处理的NiTi合金表面最外层氧化膜主要由TiO2、TiO和少量的Ni组成，酸、碱处理后，最外层氧化膜由TiO2、Ni2O3组成，但经碱处理后，氧化膜的厚度大大增加。     

Sputter deposition of NiTi thin film shape memory alloy using a heated target

In this paper we present a novel method for depositing NiTi thin film by DC sputtering. The film has transformation temperatures very close to that of the target. The new process involves heating the target and does not require compositional modification of the NiTi target. Results from X-ray diffraction, differential scanning calorimeter, four-point probe, Rutherford backscattering, and transmission electron microscopy are presented. These results indicate that compositional modification can be produced by varying the target temperature. Films produced from hot targets have compositions similar to the target while films produced from cold targets were Ti deficient. Films that were produced by gradual heating of the target have compositional gradation through the film thickness. The gradated films exhibit the two-way shape memory effect.     

Densification and microstructural evolution of spark plasma sintered NiTi shape memory alloy

The effect of particle size and sintering temperature on the densification and microstructural characteristics of nickel-titanium shape memory alloy (NiTi-SMA) has been investigated using spark plasma sintering (SPS) process. The Ni and Ti elements in different particle sizes were alloyed in the composition of Ni_50.6Ti_49.4. The milled NiTi powders were consolidated using SPS process in a temperature range of 700–900?°C. The densification was characterized by plotting temperature, current and relative displacement of punch as a function of holding time. The results showed that a maximum relative density of ～98% can be achieved for NiTi-SMA with an average particle size of 10?µm at a sintering temperature of 900?°C. The microstructure of the sintered NiTi-SMA was examined using scanning electron microscope (SEM) and composition of NiTi alloy was analyzed using energy dispersive spectroscopy (EDS) analysis. The effect of sintering temperature on the microstructural evolution and transformation was also studied.     

NiTi形状记忆合金化学镀CoNiWP薄膜及磁性研究

采用化学镀法在NiTi形状记忆合金基体上镀覆CoNiWP磁性薄膜。用振动磁强计测试了样品的磁性能，结果表明NiTi形状记忆合金外层镀覆的金属薄膜是无定形结构并具有较好的磁性，其矫顽力是随化学镀进程逐渐增大到一定值后再逐渐下降。用扫描电镜表征镀层的形貌，结果表明化学镀开始初期，镀层颗粒比较细致，后来逐渐长大成晶胞状，当镀层完全覆盖基材后，再增加厚度会导致矫顽力下降。     

Analysis of the mechanical and shape memory behaviour of nitrogen ion-implanted NiTi alloy

Experimental results of an accumulation and return strain behaviour of the modified surface of NiTi alloy, as well as mechanical and shape memory behaviour, are shown in this paper.Surface of equiatomic NiTi shape memory alloy (in martensitic form) has been modified by high-dose ion-implantation technique using nitrogen ion beam. The low-energy (65 keV) and following high doses have been used: 1×10¹⁷, 5×10¹⁷ and 1×10¹⁸ J/cm². Correlation between subsurface layers elemental composition of NiTi alloy, microstructure and shape memory properties is shown.     

Effect of graphite addition on martensitic transformation and damping behavior of NiTi shape memory alloy

In this investigation, the effect of graphite addition on martensitic transformation and damping behavior of Ni₅₀Ti₅₀ (at.%) shape memory alloy has been studied. It is found that martensitic transformation temperature decreases obviously with the addition of graphite. Microstructural observation shows that TiC precipitates and forms whiskers when the carbon content is increased beyond ~ 0.6%. With the increase of graphite content, the damping capacity during reverse transformation increases initially and then decreases while the damping capacity of full martensite is remarkably improved by the addition of graphite particles. It is proposed that the enhancement of damping capacity can be ascribed to the high damping capacity of graphite itself, as well as, the increase of the amount of interface between martensite and austenite can be beneficial to the damping capacity.     

Fabrication and characteristics of porous NiTi shape memory alloy synthesized by microwave sintering

Porous nickel titanium (NiTi) shape memory alloy (SMA) was successfully fabricated by microwave sintering method. This method allows formation of porous structures without using any pore-forming agents. Moreover, microwave sintering of NiTi SMA can be successfully performed at a relatively low sintering temperature of 850 °C and a short sintering time of 15 min. The pore characteristics, microstructure, phase transformation and stress-strain behavior of the porous NiTi SMA were investigated. The porous NiTi SMA exhibited porosity ratios from 27% to 48% and pore sizes range from 50 to 200 μm when using different sintering temperatures and holding times. The predominant B2 (NiTi) and B19′ (NiTi) phases were identified in the porous NiTi SMA. A multi-step phase transformation took place on heating and a two-step phase transformation took place on cooling of the porous NiTi SMA. The irrecoverable strains decreased with increasing sintering temperature, but the holding time had little effect on the stress-strain behavior at 60 °C.     

Carbon nanotube forest growth on NiTi shape memory alloy thin films for thermal actuation

Actuation frequencies in thermally triggered Shape Memory Alloy (SMA) thin films are limited by the slow heat transport into/out of the films. Carbon Nanotubes (CNTs) are known to exhibit an exceptionally high thermal conductivity. Thus, we propose to thermally contact SMA films with CNTs to increase SMA actuation frequencies by enhanced heat transport through the CNTs. The basic requirement for this envisaged nanotube application is to obtain CNT forest growth on a SMA material while retaining a reversible martensitic transformation, as required for Shape Memory Effect exploitation. We show how such growth can be achieved on thin films of the SMA material NiTi. Future work is needed to measure thermal properties and obtainable cycling frequencies of CNT-SMA structures.     

表面氧化及离子注氮对NiTi形状记忆合金耐腐蚀性能的影响

采用电化学测试的方法，研究了表面氧化以及表面氧化-离子注氮两种表面改性方式对NiTi形状记忆合金在人体生理模拟液(Hank′s溶液)中腐蚀行为的影响。腐蚀电位和极化曲线的测量结果表明表面氧化-离子注氮的方法使NiTi合金材料的腐蚀电位正移。雏钝电流密度下降，钝化电位区间扩大，合金表面耐蚀性明显提高。尤其是NiTi合金在进行氮离子注入后，测得击穿电位显著上升．增强了表面膜的抗局部腐蚀能力。因而表面氧化-离子注氮的改性方法可使材料的耐蚀性达到最佳。通过XPS的分析发现，离子注氮后合金表面形成氮化钛相以及富含羟基的化学效应，使NiTi基体的电化学性能得到提高。     

Non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: Experimental observations

Experimental observations are reported for the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy performed under the stress-controlled cyclic tension–torsion loading conditions and at room temperature. The effect of axial mean stress on the evolutions of transformation ratchetting strain and dissipation energy per cycle during the cyclic tests is discussed firstly; and then the dependence of multiaxial transformation ratchetting on the different non-proportionally loading paths (e.g., linear, square, hourglass-typed, butterfly-typed, rhombic and octagonal paths) is investigated. The results show that the multiaxial transformation ratchetting occurs mainly in the axial direction because only the non-zero axial mean stress is used and the mean shear stress is set to be zero in the all prescribed multiaxial loading paths; and the axial peak and valley strains increase with the increasing axial mean stress and also depend significantly on the shapes of loading paths. Comparison with the corresponding uniaxial ones illustrates that the multiaxial stress states are more helpful to promote the development of transformation ratchetting, especially for the non-proportional ones.     

医用NiTi形状记忆合金的腐蚀特性

研究了0.9%NaCl生理液、Hank's模拟体液和Tyrode's模拟血液的pH值和Cl-离子浓度对医用NiTi形状记忆合金Ni离子腐蚀溶出的影响.结果表明,三种模拟体液的氧化还原电位不同,使Ni离子释放呈现出差异,Tyrode's模拟血液中Ni离子释放量较高;随着pH值的增大Ni的Flade电位降低,从而使Ni离子释放量降低;随着Cl-浓度增加,Ni离子释放量增大,呈现出较强的Ni选择性腐蚀行为,而Ti的腐蚀微弱,点蚀是其主要的腐蚀形式.     

A physical mechanism based constitutive model for temperature-dependent transformation ratchetting of NiTi shape memory alloy: One-dimensional model

In this paper, the effect of test temperature on the transformation ratchetting of super-elastic NiTi shape memory alloy was first investigated in the cyclic tension-unloading tests. It is shown that all the residual strain, dissipation energy, the start stress of martensite transformation and their evolutions during the cyclic loading depend greatly upon the test temperature. Based on the experimental observations, a new one-dimensional constitutive model is constructed by considering two different inelastic deformation mechanisms (i.e., martensite transformation and transformation-induced plasticity). The proposed model employs a new evolution rule of transformation-induced plasticity which considers the physical mechanism of the plastic deformation, i.e., the dislocation slipping in the austenite phase near the austenite–martensite interfaces. Furthermore, the interaction between dislocation and martensite transformation is also taken into account in the proposed model. The capability of the proposed model to predict the uniaxial temperature-dependent transformation ratchetting of NiTi shape memory alloy is verified by comparing the predictions with the experimental data.     

Selective laser melting of NiTi alloy with superior tensile property and shape memory effect

It is a challenge to develop complex-shaped NiTi shape memory alloy parts by traditional processing methods, due to the poor machinability of NiTi alloy. It is reported that selective laser melting (SLM) of additive manufacturing could overcome this problem. However, the reported SLM-produced NiTi exhibits poor tensile ductility due to the inner defects and adverse unidirectional columnar grains from SLM process. In this work, the defect-less SLM-NiTi with nondirective columnar grains was fabricated by optimizing the intraformational laser scanning length and interformational laser scanning direction. The obtained lath-shaped SLM-NiTi sample exhibits tensile strain of 15.6%, more than twice of the reported maximum result (˜7%). Besides, the SLM-NiTi part with complex geometry displays a shape memory recovery of 99% under compressive deformation of 50%.     

激光表面改性对NiTi形状记忆合金腐蚀行为的影响

采用连续波Nd：YAG固体激光,在NiTi形状记忆合金表面制备出表面致密、无7L洞和裂纹的氮化层,测试了这种激光改性层在37℃模拟人体体液Hank’s溶液中的电化学阳极极化曲线和交流阻抗谱,研究了改性层的腐蚀行为．结果表明,NiTi合金氮化层的腐蚀电位和击穿电位正移,反应转移电阻明显提高,而腐蚀电流及界面电容下降．这说明激光气体氮化有效地改善了NiTi形状记忆合金在模拟人体体液中的电化学抗腐蚀性能．     

The effect of grain orientation on the tensile‐compressive asymmetry of polycrystalline NiTi shape memory alloy

The purpose of the present study is to thoroughly understand the influence of crystallographic texture on the stress‐strain asymmetric behavior of polycrystalline NiTi shape memory alloy under tension and compression. To do this, a 3D thermo‐mechanical model has been implemented in a finite element program and textured and untextured polycrystalline NiTi have been considered. In our polycrystalline finite element model, each element represents one grain and a set of crystal orientations which approximate the initial crystallographic texture of the NiTi are assigned to the elements. From the calculated results, it is found that the crystallographic texture is the important reason for the tension‐compression asymmetry. For the textured polycrystal, the tension‐compression asymmetry can be observed clearly, but for the polycrystal containing randomly oriented grains, the stress‐strain curves show low levers of asymmetry between tensile and compressive loading, and the evolutions of martensite volume fractions are similar under two stress states.     

Pulsed laser deposition of NiTi shape memory alloy thin films with optimum parameters

A series of experiments was carried out to optimize the pulsed laser deposition parameters for the fabrication of high quality NiTi shape memory alloy thin films. Smooth NiTi shape memory alloy thin films were deposited at high growth rate with optimum deposition parameters based on the analysis of the relationships among the morphology of the target surface and the deposited thin film, the laser energy, the target–substrate distance, the thin film composition and its growth rate. Crystal structures and phase transformation temperatures of the annealed Ni_49.7Ti_50.3 thin film were characterized by using X-ray diffraction and differential scanning calorimetry, respectively. The martensitic transformation temperature of the crystallized Ni_49.7Ti_50.3 thin film is found to be lower than room temperature and 27°C lower than that of the NiTi target material. These results are attributed to the refined grain size of the thin film and its composition, which deviates slightly from Ni₅₀Ti₅₀.