镍钛形状记忆合金的开发应用

综述了镍钛形状记忆合金在国内外的开发应用情况。对今后的发展提出了建议。     

NiTi形状记忆合金超弹性的研究现状

综述了NiTi形状记忆合金在有关超弹性性能方面的基础研究、应用研究、力学行为、磨损性能以及数学建模等问题。     

Nanostructured NiTi Shape Memory Alloy Via Ni/Ti Nanolamination

Ultrafine grained NiTi shape memory alloy was consolidated from Ni/Ti laminates via accumulative roll bonding (ARB) followed by hot isostatic pressing (HIP). Due to the extensive plastic deformation arising from ARB and the attainment of ultra-thin layers of both Ti and Ni laminates, the subsequent HIP processing time was significantly reduced. Differential scanning calorimetry (DSC) analysis confirms that the shape memory effect was obtained in the consolidated NiTi alloy.     

Laser and Surface Processes of NiTi Shape Memory Elements for Micro-actuation

In the current microtechnology for actuation field, shape memory alloys (SMA) are considered one of the best candidates for the production of mini/micro devices thanks to their high power-to-weight ratio as function of the actuator weight and hence for their capability of generating high mechanical performance in very limited spaces. In the microscale the most suitable conformation of a SMA actuator is given by a planar wavy formed arrangement, i.e., the snake-like shape, which allows high strokes, considerable forces, and devices with very low sizes. This uncommon and complex geometry becomes more difficult to be realized when the actuator dimensions are scaled down to micrometric values. In this work, micro-snake-like actuators are laser machined using a nanosecond pulsed fiber laser, starting from a 120-μm-thick NiTi sheet. Chemical and electrochemical surface polishes are also investigated for the removal of the thermal damages of the laser process. Calorimetric and thermo-mechanical tests are accomplished to assess the NiTi microdevice performance after each step of the working process. It is shown that laser machining has to be followed by some post-processes in order to obtain a micro-actuator with good thermo-mechanical properties.     

Mechanical Properties of TiTaHfNbZr High-Entropy Alloy Coatings Deposited on NiTi Shape Memory Alloy Substrates

TiTaHfNbZr high-entropy alloy (HEA) thin films with thicknesses of about 750 and 1500 nm were deposited on NiTi substrates by RF magnetron sputtering using TiTaHfNbZr equimolar targets. The thorough experimental analysis on microstructure and mechanical properties of deposited films revealed that the TiTaHfNbZr films exhibited amorphous and cauliflower-like structure, where grain size and surface roughness increased concomitant with film thickness. More importantly, the current findings demonstrate that the TiTaHfNbZr HEA films with mechanical properties of the same order as those of the NiTi substrate constitute promising biomedical coatings effective in preventing Ni release.     

镍钛形状记忆合金薄膜的相变温度和力学性能

利用直流磁控溅射法溅射沉积了NiTi形状记忆合金薄膜，对NiTi形状记忆合金薄膜进行晶化热处理以使其获得形状记忆效应。研究了550℃晶化热处理1h后Ti-48.2%Ni薄膜的相变温度和力学性能，研究结果表明，NiTi薄膜550℃晶化热处理1h后，升温过程中发生M→A的相变，而降温过程中则先发生A→R相变，再发生R→M的相变，薄膜的断裂强度随测试温度的升高而增大，残余应变随温度的升高而减小，当温度大于Af点时，表现为超弹性。     

High Strain Rate Compression of Martensitic NiTi Shape Memory Alloy at Different Temperatures

The compressive response of martensitic NiTi shape memory alloy (SMA) rods has been investigated using a modified Kolsky compression bar at various strain rates (400, 800, and 1200 s^?1) and temperatures [room temperature and 373 K (100 °C)], i.e., in the martensitic state and in the austenitic state. SEM, DSC, and XRD were performed on NiTi SMA rod samples after high strain rate compression in order to reveal the influence of strain rate and temperature on the microstructural evolution, phase transformation, and crystal structure. It is found that at room temperature, the critical stress increases slightly as strain rate increases, whereas the strain-hardening rate decreases. However, the critical stress under high strain rate compression at 373 K (100 °C) increase first and then decrease due to competing strain hardening and thermal softening effects. After high rate compression, the microstructure of both martensitic and austenitic NiTi SMAs changes as a function of increasing strain rate, while the phase transformation after deformation is independent of the strain rate at room temperature and 373 K (100 °C). The preferred crystal plane of the martensitic NiTi SMA changes from (\( 1\bar{1}1 \))_M before compression to (111)_M after compression, while the preferred plane remains the same for austenitic NiTi SMA before and after compression. Additionally, dynamic recovery and recrystallization are also observed to occur after deformation of the austenitic NiTi SMA at 373 K (100 °C). The findings presented here extend the basic understanding of the deformation behavior of NiTi SMAs and its relation to microstructure, phase transformation, and crystal structure, especially at high strain rates.     

Effect of Intermittent Overload Cycles on Thermomechanical Fatigue Life of NiTi Shape Memory Alloy Wire

Effect of intermittent overload cycles on fatigue behavior of NiTi shape memory alloy wire during thermomechanical cycling (TMC) has been evaluated. Results showed that fatigue life of NiTi is enhanced when the intermittent overload is above certain minimum level. An enhancement in fatigue life by ~50 pct is observed when the overload ratio is 2.0. Accumulation of plastic strain in the material under such TMC condition is found to be relatively high compared to that of TMC with no overload cycles.     

Ti-Ni-Ce合金的相变行为及其形状记忆效应

通过示差扫描量热仪(DSC)等试验手段,系统分析了添加稀土元素Ce后所形成的Ti-Ni-Cex(x＝2 at%、3 at%、4 at%)系合金的相变行为和形状记忆效应,以及相变温度和相变潜热与稀土含量之间的关系.研究结果表明,相变温度(Ms,Mf,As,Af)受稀土含量的影响较大,总的趋势是先升高后下降,然后有稍微升高;而对相变潜热(马氏体相变潜热ΔHB2M、逆马氏体相变潜热ΔHMB2)的影响,则是随着稀土元素Ce含量的升高呈现上升的趋势;稀土的添加使Ti-Ni二元合金的相变滞后明显变窄,但与稀土的含量没有明显关系.研究发现,添加稀土后Ti-Ni形状记忆合金优良的形状记忆效应没有改变,而相变温度却有明显的提高.     

重熔镍钛合金的形状记忆效应分析

采用不同的工艺及设备对近等原子比的形状记忆合金进行重熔,获得铸态组织形状记忆合金,对采用不同坩埚所制备出的NiTi合金进行对比分析,采用金属坩埚所得到的形状记忆合金的成分满足医疗器械及外科植入物用NiTi形状记忆合金的成分要求,且具有良好的形状记忆效应。     

镍钛形状记忆合金线材无模拉拔成形过程的电磁场和温度场模拟

无模拉拔成形过程的电磁场和温度场数值模拟对于正确制定加工工艺,精确控制线材质量十分重要。利用ANSYS有限元软件,对镍钛形状记忆合金线材无模拉拔成形过程的电磁场和温度场进行了数值模拟,获得了镍钛形状记忆合金线材无模拉拔成形时的电磁场和温度场信息,并进行了讨论。提出了较合理的镍钛形状记忆合金线材无模拉拔成形工艺参数范围:电流频率59~61 kHz、电流密度(2.95~3.05)×107A.m-2、冷却水与感应加热线圈之间的距离19~22 mm、冷却水流量22~26 ml.s-1。     

Cu-Zn合金的形状记忆效应

通过扭转法和电阻测量法对Cu-Zn合金的形状记忆效应进行了实验研究。发现在M_s点以上,不存在热弹性马氏体的情况下,由应力诱发马氏体可以引起形状记忆效应。另外,在M_s点以下有热弹性马氏体的情况下,根据实验结果可以证明,应力诱发马氏体在形状记忆效应中仍然起主要作用。     

Oxide Scales Formed on NiTi and NiPtTi Shape Memory Alloys

Ni-49Ti and Ni-30Pt-50Ti (nominal at. pct) shape memory alloys (SMAs) were isothermally oxidized in air over the temperature range of 773?K to 1173?K (500?°C to 900?°C) for 100?hours. The oxidation kinetics, presented in detail in a companion study, show ~4 times reduction in oxidation rate due to Pt.[1] The microstructure, composition, and phase content of the scales and depletion zones were determined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). A relatively pure TiO₂ rutile structure was identified as the predominant scale surface feature, typified by a distinct highly striated and faceted crystal morphology, with crystal size proportional to oxidation temperature. The complex layered structure beneath these crystals was characterized by semiquantitative XRD of serial/taper polished sections and SEM/EDS of cross sections for samples oxidized at 973?K (700?°C). In general, graded mixtures of TiO₂, NiTiO₃, NiO, Ni(Ti), or Pt(Ni) metallic dispersoids, and continuous Ni₃Ti or Pt-rich metal depletion zones, were observed from the gas surface to the substrate interior. Overall, substantial depletion of Ti occurred due to the formation of predominantly TiO₂ scales. It is proposed that the Ni-30Pt-50Ti alloy oxidized more slowly than the binary Ni-49Ti alloy by decreasing oxygen and titanium diffusion through the thin Pt-rich layer.     

浅析NiTi形状记忆合金的疲劳机理

由于NiTi形状记忆合金具有可逆的热弹性马氏体相变效应等不同与一般金属的特性，使得其疲劳机理也不同于一般的工程材料。为此，本文侧重分析了温度变化对NiTi形状记忆合金疲劳机理的影响和应力诱发马氏体相变等特征相变过程，以及相关的研究方法与主要结果，将为进一步研究其疲劳过程及增韧工艺提供理论参考依据．     

Stress-Induced Shape Changes and Shape Memory in the R and Martensite Transformations in Equiatomic NiTi

Equiatomic NiTi wire was cooled below T_R, the critical temperature for theB R transition, and then stressed in situ in a custom-built X-ray diffraction stage to study the shape memory phenomenon. Tensile stressing the specimen causes a shifting of X-ray intensity from l_R to -l_R. This is rationalized in terms of domain growth under external stress, resulting in a preferred arrangement of domains in theR -phase, which dimensionally accommodates the applied force. A recoverable strain of ~1.37 pet is accommodated by theR-phase, 0.56 pct of which is recovered immediately after the stress is removed and the remaining 0.81 pct on heating to above T_R. This amount of strain is rationalized in terms of the difference in d-spacings and multiplicity factors between {111}_R and {-111}_R. The springback is primarily associated with the reversal of domain alignment while the shape memory on heating is primarily due to the return of the phase to cubic symmetry. Straining beyond 1.37 pct induces stress-assisted marteniste formation up to 5.47 pct Δl/l, the maximum strain achieved in this series of experiments. This results in a second stage of shape recovery on heating through theA, — A_f temperature range. Only 15 vol pct of stress-assisted martensite accounts for nearly all of the additional ~4 pct change in Δl/l. This emphasizes the important role of the martensitic transformation in achieving large changes in macroscopic length in the shape memory phenomenon.     

Crystallography of Martensite in TiAu Shape Memory Alloy

The twin structure, habit plane orientation, and morphology of B19 martensite in TiAu, which is a candidate shape memory alloy (SMA) for high-temperature and biomedical applications, were investigated by conventional transmission electron microscopy. Almost all internal twins were {111} type I twins as lattice-invariant deformation (LID). The <211> type II twin was scarcely observed in TiAu, unlike in TiPd and TiPt SMAs. The habit plane roughly corresponded to the twinning plane (K ₁ plane) of the <211> type II twin because of the superb lattice parameter ratio of TiAu. As a result, an energy-minimizing microstructure referred to as “twins within twins” appears as the major microstructure. The selection rules for the twinning of LID are also discussed considering the results of extensive studies on LID in SMAs.     

In Situ Synthesis and Characterization of Shape Memory Alloy Nitinol by Laser Direct Deposition

Nitinol is well known for its unique shape-memory and super-elastic properties along with its excellent biomechanical compatibility and corrosion resistance. In this study, a laser direct deposition technique was explored to synthesize high-quality, near-net-shape nitinol components directly from elemental nickel and titanium powders as opposed to using expensive prealloyed nitinol powder. The systematic characterization of samples was done using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Transformation temperatures were obtained using differential scanning calorimetry (DSC). With an optimum ratio of nickel and titanium powder mixture, optimal laser parameters, and post-heat treatment, samples with homogeneous and nearly fully dense NiTi phase were synthesized with less unwanted secondary phases occupying less than 3.2 pct volume fraction. Furthermore, these results were compared with those obtained for samples deposited using prealloyed nitinol powder. This technique offers maximum flexibility and cost benefit in the manufacturability of near-net-shape nitinol components.