A study on the variations of the electrical resistance for NiTi shape memory alloy wires during the thermo-mechanical loading

In order to use NiTi wires as strain or stress sensors, the variations of electrical resistance of Ti-50.8 at%Ni wires during thermo-mechanical loading are studied in this paper. The results show that it is possible to use the NiTi wires as strain sensors, but it is difficult to use NiTi wires as stress sensors.     

Experimental studies on electrical discharge wire cutting of Ni-rich NiTi shape memory alloy

The experimental investigation explores the effect of electrical discharge wire cutting (EDWC) variable parameters such as spark gap voltage, wire tension, pulse off time, wire feed rate, and pulse on time on the surface roughness, average cutting rate, and metallographic changes of Ni_55.95Ti_44.05 shape memory alloy (SMA). The spark gap voltage, pulse off time, and pulse on time have the significant effect on the surface roughness and average cutting rate, whereas wire tension and wire feed rate have the trifling effect. Ni_55.95Ti_44.05 SMA’s surface after EDWC is characterized by many discharge craters, microcracks, voids, and white layer of resolidified molten material. The elemental composition analysis of white layer using energy-dispersive spectroscopy divulges the deposition of the foreign element from the brass wire as well as the dielectric on the surface after EDWC. The machined surface as well as the wire electrode surface consists of various compounds of Ti, Ni, Zn, and Cu which have been identified by X-ray diffraction peak analysis.     

Loading rate dependency of maximum nanoindentation depth in nano-grained NiTi shape memory alloy

We examined the effect of loading rate on maximum nanoindentation depth for nano-grained superelastic NiTi shape memory alloy during stress-induced phase transformation. The depth decreases significantly with increasing loading rate. It is argued that the observed rate dependency is attributed to the release and transfer of latent heat during indentation and the temperature dependence of the material's transition stress. Dimensional analysis further shows that the depth is mainly governed by the normalized average stress in the phase transition zone and the ratio of heat conduction time over loading time. Experimental results support the rationale.     

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.     

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.     

Effect of martensite transformation on fracture behavior of shape memory alloy NiTi in a notched specimen

In this paper, the finite element calculation of the stress–strain distribution in front of a notch tip were carried out for two materials. One is a shape memory alloy NiTi with the stress-induced martensite transformation, and another is a fully transformed martensite NiTi without the transformation. Based on the results obtained, and combining a model of the fracture process zone, effect of martensite transformation on the fracture behavior of the shape memory alloy NiTi in a notched specimen of plane stress state is comparably analyzed. The results show that the martensite transformation increases the load to produce plastic deformation in the transformed martensite at the notch tip and decreases the maximum normal stress and plastic strain near the notch tip, and tends to suspend the crack nucleation and propagation in the fully transformed martensite in front of the notch tip, and thus increases the fracture load and improves the toughness. A quantitative analysis based on the model of the fracture process zone shows that the martensite transformation in the SMA NiTi causes about 47% increase in the apparent fracture toughness.     

Debonding initiation in a NiTi shape memory wire–epoxy matrix composite. Influence of martensitic transformation

The effect of martensitic transformation on the debonding initiation in single shape memory NiTi wire–epoxy matrix composite was studied. Three different heat treatment cycles were performed in order to obtain wires with different transformation characteristics. The composite samples were prepared using NiTi wires and steel wire for comparison. The single-wire specimens were subjected to the pull-out test. The in situ observations of the interfacial debonding and sliding behaviour during the test were carried out.     

Effects of electropulsing treatment on mechanical properties in Ti rich TiNi shape memory alloy

Abstract

The influence of electropulsing treatment on thermomechanical behaviour in terms of transformation temperatures and mechanical properties of a cold worked Ti rich TiNi shape memory alloy was studied. The results showed that the transformation temperatures increase with increasing current intensity. The superelasticity of the alloy is improved after the electropulsing treatment, and the hardness of the alloy decreased with increasing current intensity. The microstructure of the alloy showed that the above changes can be attributed to the decreasing dislocation density due to the current pulses.     

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.     

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%.     

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．     

Ambient effect on damping peak of NiTi shape memory alloy

We report the effects of ambient condition and loading rate on the damping capacity of a superelastic nickel-titanium shape memory alloy during stress-induced martensitic phase transformation with release and absorption of latent heat. The damping capacity was measured via a tensile loading-unloading cycle in the strain-rate range of 10^− 5-10^− 1/s and three ambient conditions: still air and flowing air with velocities of 2 m/s and 17 m/s. It is found that, for each ambient condition, the maximum damping capacity (damping peak) is achieved at the strain rate whose loading time (t_T) is close to the characteristic heat-transfer time (t_h) of the ambient condition.     

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。     

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.     

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.     

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.     

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。     

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.     

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.     

TiNi基形状记忆合金的温度记忆效应研究进展

TiNi形状记忆合金经过一次不完全相变循环后将对随后的相变过程产生很大的影响，因此近年来由不完全相变诱发的特殊的温度记忆效应（TemperatureMemory Effect，TME）现象引起人们的关注：如果从马氏体到母相的逆相变在第一次加热过程中在温度瓦处被中断，而后冷却到马氏体相变终了温度以下，在随后的加热过程会出现被一个动力学中断点Ts分开的两阶段逆相变，Ts可“记住”Ti综述了近年来TiNi基形状记忆合佥的TME研究新进展，阐述了具有不同马氏体相变特征的TiNi基记忆合金中TME的特点及其机理。