NiTi shape memory alloys coated with calcium phosphate by plasma‐spraying. Chemical and biological properties

Plates of superelastic nickel‐titanium shape memory alloy (NiTi) were coated with calcium phosphate (hydroxyapatite) by high‐temperature plasma‐spraying. The porous layer of about 100 μm thickness showed a good adhesion to the metallic substrate that withstood bending of the plate but detached upon cutting the plate. The biocompatibility was tested by cultivation of blood cells (whole blood and isolated granulocytes [a subpopulation of blood leukocytes]). As substrates, pure NiTi, plasma‐spray‐coated NiTi and calcium phosphate‐coated NiTi prepared by a dip‐coating process were used. The adhesion of whole blood cells to all materials was not significantly different. In contrast, isolated granulocytes showed an increased adhesion to both calcium phosphate‐coated NiTi samples. However, compared to non‐coated NiTi or dip‐coated NiTi, the number of dead granulocytes adherent to plasma‐sprayed surfaces was significantly increased for isolated granulocytes (p<0.01).     

Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one‐way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni₅₀Ti₅₀ and ternary Ni₄₀Ti₅₀Cu₁₀ SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape‐constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring‐making are documented. The mechanical and functional characteristics of the wires are investigated in the stress‐strain‐temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.     

Influence of compression aging on phase transition temperatures in single crystalline Ni‐rich NiTi shape memory alloys

The present paper considers the phase transition behavior of a single crystal Ni‐rich NiTi alloy which was compression aged to produce one single family of Ni₄Ti₃ precipitates. The single crystal material was produced in a two stage process. Polycrystalline material was first melted under an inert atmosphere and remelted when single crystals were produced. Compression aging treatments in <111>‐orientation were carried out in order to suppress all but one family of Ni₄Ti₃‐precipitates which nucleate and grow on {111}‐planes of the B2 matrix. The objective of this study is to investigate the influence of Ni₄Ti₃‐precipitates on the martensitic transformation behavior. It was previously shown that grain boundaries provide heterogeneous nucleation sites for the formation of Ni₄Ti₃; this results in heterogeneous microstructures which undergo multiple step martensitic transformations. Single crystals avoid grain boundaries and the present study aims at clarifying how homogeneously precipitated particles affect martensitic transformations.     

Bending rotation HCF testing of pseudoelastic Ni‐Ti shape memory alloys

In the present work, a computer‐controlled test rig for simultaneous fatigue testing of several pseudoelastic NiTi wires through bending rotation is described. Bending rotation fatigue (BRF) testing represents a displacement‐controlled experiment where a straight wire is bent into a semi‐circle und forced to rotate around its axis. Thus, each point on the wire surface is subjected to alternating tension and compression. A test rig, which allows to control loading amplitudes, rotation frequencies and temperatures is described. We report preliminary results of an experimental program, which aims for a better understanding of fatigue lives, crack initiation, and crack growth in pseudoelastic NiTi wires. It was found that a good surface quality is of utmost importance to avoid early crack initiation. Wöhler curves of pseudoelastic NiTi wires typically show two different regimes depending on the maximum imposed surface strain during bending rotation fatigue testing. Larger strain amplitudes, which are associated with macroscopic formation of stress‐induced martensite, result in relatively low fatigue lives (LCF regime). In contrast, cycle numbers exceeding 10⁷ were obtained for strain amplitudes where no large scale stress‐induced formation of martensite occurred (HCF regime).     

Experimental study of one‐dimensional superelastic capability of a nearly equi‐atomic NiTi shape memory alloy

A series of experimental studies have been carried out on nearly equi‐atomic NiTi shape memory alloy wires. The effects of fatigue cycles, displacement rates as well as testing temperatures on the superelastic capabilities have been studied. Under cycling loading, the threshold stresses for martensitic transformation decrease and the residule strains increase. Saturation is reached after 100 cycles. With increasing displacement rates, the critical stresses required for the martensitic transformation increase and the slopes of the upper plateau in the stress‐strain curves rise. The dissipated energy increases rapidly with increasing displacement rate, reaches a maximum value at around 6.0mm/min and then decreases as the displacement rate continues to increase. Tests also show that the threshold stresses characterizing the forward/reverse transformations increase linearly with increasing test temperature.     

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。     

High‐Temperature Corrosion of NiTi‐Shape‐Memory Alloys

Semi‐finished products and components made of NiTi‐shape‐memory alloys (NiTi‐SMA) are often subjected to heat treatment after their fabrication. During this heat treatment, oxide layers begin to form which contain a high amount of titanium. In this investigation special attention was drawn to the selective oxidation of Ti because a TiO_X‐layer can represent a Ni‐barrier and may therefore be of special use for medical applications. A comparison of the following three samples was carried out: A sample oxidised at room temperature, another that was heat‐treated in ambient air (600 °C/1min) and a third sample that was subjected to a heat treatment (600 °C/1min) in an atmosphere that oxidises titanium but reduces NiO in order to achieve a selective oxidation of the titanium. The analysis of the oxide layers was carried out by means of x‐ray photoelectron spectroscopy (XPS). It was shown that the ratio of titanium to nickel in the oxide layer can be substantially increased when performing the annealing treatments in a partial reducing atmosphere. Furthermore, a thermo‐gravimetric investigation of the material was carried out at 600 °C in dry air in order to estimate the growth of the oxide layers.     

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.     

A macro-constitutive model of polycrystalline NiTi SMAs including tensile–compressive asymmetry and torsion pseudoelastic behaviors

Experimental results show that material component and loading modes may affect the properties of shape-memory alloys (SMAs) markedly. In order to investigate the influence of loading modes on pseudoelasticity behaviors fully, some experiments of NiTi specimens under pure tension, compression and torsion with the same material component are investigated. In terms of the phenomena observed in experiments, a macro-constitutive model is presented for considering the tension–compression asymmetry of polycrystalline NiTi SMAs. In this study, the macroscopic strain is taken account of elastic strain, macro-transformation strain and macro-temperature strain. The volume fraction of martensite is governed by the reduction in Gibbs’ free energy of the system. In the present model, the fewer experimental data which are used to determine model parameters are needed. The model is not only simple but also fit for using in engineering. The theoretical results are found to be in good agreement with experimental data.     

Micromechanics of Tension‐Compression Asymmetry of Polycrystalline Shape‐Memory‐Alloys

The asymmetry associated with martensitic transformations observed in tension/compression experiments of shape‐memory‐alloys (SMAs) is investigated on the basis of a recently suggested micromechanical model. The approach is based on crystallographic theory and utilizes a framework of energy minimization in a finite deformation context. Polycrystalline NiTi under tension demonstrates smaller phase‐transformation start‐strain, differe phase‐transformation stress‐levels and flatter phase‐transformation stress‐strain slopes than that under compression in our numerical simulation. The phase‐transformation start‐stress is followed to have a linear relationship with respect to the temperature within a certain range. These results agree well with experimental results reported in the literature.     

Damping Factors and Fatigue Notch Factors of Pseudoelastic NiTi‐Shape Memory Alloys

Pseudoelastic NiTi‐ shape memory alloys (SMAs) provide a high damping capacity and can be used in order to achieve a reduction of peak loads being caused by unexpected shock loading. These “pseudoelastic” properties are related to the formation of martensite M from austenite A, which has been induced by stress; they allow to refer to SMAs as functional materials. Furthermore, these functional materials can operate at high stresses and thus, have to withstand severe mechanical loadings like classical structural materials. In combination, these characteristics provide opportunities for technical applications, e.g., to reduce vibrations or to reduce peak loads caused by shock loading. An extensive knowledge of the functional and structural fatigue behaviour of the material is required to design SMA components. NiTi hollow shaft samples and solid shaft samples have been tested under cyclic torsional loading conditions in a load‐controlled mode. By using these two geometries the influence of the sample geometry on the fatigue behaviour can be investigated. In addition, a test programme has been elaborated in order to investigate the behaviour of the material when subjected to bending. The experimental data have been evaluated describing the transformation behaviour induced by stress concerning transformation stress, apparent shear modulus of the austenite G_A and apparent stiffness τ_Ms (describing the slope of the shear stress‐strain‐curve in the transformation range G_A‐M). These parameters naturally depend on the cycle number, the load amplitude as well as the temperature. Engineering failures are often associated with the presence of notches. In this context, torsion tests on notched samples are planned to be carried out in order to assess the resulting data based on the results obtained from the notch free samples. This will allow to derive simple design rules based on fatigue notch factors, which are needed for engineering design.     

Drilling of NiTi Shape Memory Alloys

The machinability of NiTi based shape memory alloys has been examined by conducting drilling experiments. For this reason the cutting parameters cutting speed and feed were varied within a wide range. The machining process was evaluated in terms of tool wear, cutting forces and machining quality. The tool wear was analysed with a scanning electron microscope and the influence of machining on the subsurface zone was evaluated by micro hardness measurements.     

On the electropolishing of NiTi braided stents – challenges and solutions

Nickel Titanium (NiTi) alloys possess special mechanical properties and good biocompatibility hence used as base material for the production of vascular stents. Normally, vascular stents are machined from NiTi tubes, using laser cutting processes. Braiding is a promising alternative for the machining of certain NiTi stents. However, a surface finish treatment, such as electropolishing of the braided stents, is still required in order to achieve a medical‐grade surface finish. The thermally‐grown oxide resulting from the shape‐setting heat treatment, following the braiding must be removed. Moreover, electropolishing is required to achieve optimum corrosion resistance. Therefore, the aim of this study is to find suitable parameters for the effective electropolishing of NiTi textile stents. Electropolishing of a device with such a complex geometry is challenging, hence a custom‐designed electrolytic cell was constructed and used in this study. We examined the stent surfaces before and after electropolishing, using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Potentiodynamic tests were performed in NaCl 0.9% solution for as‐received and electropolished samples. The results from the present study indicate an improvement in surface quality of the braided stents after electropolishing. Potentiodynamic tests revealed that electropolishing improves the corrosion resistance of the NiTi stents.     

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。     

Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni_50.8Ti_49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2_2 0 0 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈1 0 0〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M_s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state.     

TiNiCu形状记忆合金双向记忆效应研究

研究了热机械训练温度及定型处理温度对TiNiCu形状记忆合金弹簧双向记忆效应的影响。研究结果表明:在纯马氏体状态进行训练时,双向记忆恢复率随训练次数的增加而增加,并在一定的训练次数后达到饱和;在纯奥氏体状态进行训练,双向记忆恢复率随训练次数迅速增加到某一最大值后随训练次数的增加而减小;在马氏体和奥氏体混合相进行训练时,双向记忆恢复率随训练次数先增加而后减小。经过400～550℃×1h/AC定型处理及热机械训练后最大形状记忆恢复率随定型处理温度升高先增大然后减小。由于马氏体再取向时引入的位错有利于双向记忆效应,热诱发和应力诱发的马氏体变体数量不同,引起了在不同状态训练诱发的双向记忆效应随训练次数变化的差异。     

Investigation of spatial inhomogeneities of the thermal properties of a NiTi shape memory alloy device

Local changes of the phase transition on a SM one‐way actuator have been investigated by time‐dependent IR thermography and compared with the local behavior of thermal properties studied by means of scanning thermal microscopy and modulated IR radiometry at room temperature and at 120 °C. The results point towards large spatial inhomogeneities of the shape memory properties developing with increasing temperature and which are the reason that only parts of the material transform into the austenite phase.     

多元氮化物薄膜及其沉积工艺对NiTi记忆合金性能和组织的影响

采用真空阴极电弧沉积技术,在NiTi记忆合金表面沉积了TiAlBN和TiAlCrFeSiBN多元膜和TiN薄膜,研究了薄膜成份及沉积工艺对NiTi合金性能和组织的影响.结果表明,在NiTi合金表面沉积TiAlBN和TiAlCrFeSiBN多元膜和TiN薄膜均可降低合金在Hank溶液中的Ni溶出速率,其中多元膜的Ni溶出速率最小;提高偏压对沉积了TiAlBN多元膜的NiTi合金的Ni溶出速率无明显影响,但使沉积了TiAlCrFeSiBN膜的NiTi合金的Ni溶出速率降低.在TiAlBN和TiAlCrFeSiBN多元膜表面存在较多细小的钛滴和孔隙,钛滴与薄膜基体之间的融合良好;在TiN薄膜表面存在一些大钛滴和孔隙,钛滴与薄膜基体之间的融合不好.镀膜后,NiTi基体的加热相变点移向低温区,其幅度与薄膜成份及沉积工艺有关,提高偏压使沉积了两种多元膜的NiTi基体的相变点移动幅度增大,但却使沉积了TiN膜的NiTi基体的相变点的移动幅度减小.镀膜过程均使NiTi中的M体尺寸增大.