Transformation hysteresis and shape memory effect of an ultrafine-grained TiNiNb shape memory alloy

In present work, transformation hysteresis and shape memory effect of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ alloy processed by ECAP were studied. After deformation, the ECAPed sample showed a much wider transformation hysteresis than the initial sample due to the enlarged strength mismatch between matrix and β-Nb phase. The shape memory effect and its cycling stability of the ECAPed sample were obviously improved.     

NiTi形状记忆合金的热电行为

研究了形状记忆合金线的热电行为。当合金线被加热到温度高于其相变温度时,由于相的转变而产生一个大的机械力。将 SMA 线用作驱动器,并研究了不同的参数及它们之间的关系。在不同的压力水平下,这些变化的参数分别是弹性应变(位移)、温度磁滞和电阻。通过传递热模型得到安全的加热电流并预测了相变温度。在自然空气对流的条件下,采用680 mA的电流加热、冷却合金线796 s。在43 MPa的应力水平下,应变恢复率为4.33%,相应的电阻变化为11.2%。在加热、冷却循环过程中,电阻变化与位移和电流分别呈线性关系。该研究有助于精确控制有、无外部传感器反馈的SMA线驱动器。     

Resistance welding of NiTi shape memory alloy tubes

Resistance welding is studied as a technique to join NiTi shape memory alloy thin-walled tubes. Properties of the welded metal are compared to those of the base metal by means of differential scanning calorimetry measurements, microstructure observations and nanoindentation tests. Transformation behaviour, microstructure and mechanical properties of the joint are significantly affected by the welding process. The effect of post-welding thermomechanical treatment is studied. After solution treatment followed by ageing, the transformation behaviour of the joint and of the initial material is almost identical. Strength of the joint is analysed by means of tensile and compression tests performed on as welded and heat treated tubes. This study proves that resistance welding is a feasible technique to create complex structures with low density and high reversible elasticity.     

Crystallization of NiTi shape memory alloy sputtering-deposition film

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Compression behavior of porous NiTi shape memory alloy

Porous NiTi alloy with several different porosities was processed by spark plasma sintering. The compression behavior of the porous NiTi was examined with the aim of using it possibly as a high energy absorbing material. A model for the macroscopic compression behavior of porous shape memory alloy (SMA) is presented in this work, where Eshelby’s inhomogeneous inclusion method is used to predict the effective elastic and superelastic behavior of a porous SMA based on the assumption of stress–strain curve. The analytical results are compared with experimental data for porous NiTi with 13% porosity, resulting in a reasonably good agreement.     

峰值应力对NiTi形状记忆合金循环变形塑性行为的影响

对单轴循环加载下,峰值应力对NiTi形状记忆合金(SMA)塑性行为的影响进行了研究。考虑到实际工况,对NiTi形状记忆合金在3种应力条件下(恒定峰值应力、阶梯峰值应力以及震荡峰值应力)的峰值应变、残余应变以及耗散能的变化进行实验分析。对于恒定峰值应力,发现峰值应力越高,NiTi形状记忆合金的超弹性劣化现象就越明显;对于阶梯峰值应力和震荡峰值应力,发现加载历史对于NiTi形状记忆合金塑性行为有显著影响。相较于恒定峰值应力,非恒定峰值应力可以引起更明显的NiTi形状记忆合金的超弹性劣化。     

DSC study on temperature memory effect of NiTi shape memory alloy

A systematic study on the temperature memory effect （TME） in a polycrystalline NiTi shape memory alloy was presented. The investigation was carded out through a series of differential scanning calorimeter （DSC） tests. Two types of tests were conducted, namely single-step test and multi-step test. The influence of the step temperature on the peak/trough temperatures in the subsequent heating process and the associated energy absorption/release in the phase transformations was investigated. Using a simple theoretical model, the exact mechanism behind TME was studied.     

Electrochemical behavior of YAG laser-welded NiTi shape memory alloy

Electrochemical behaviors of laser-welded Ti-50.6%Ni（mole fraction） shape memory alloy and the base metal in 0.9% NaCl solution were investigated by electrochemical techniques as corrosion potential measurement, linear and potentiodynamic polarization. The results indicate that the laser-welded NiTi alloy is less susceptible to pitting and crevice corrosion than the base metal, which is demonstrated by the increase in polarization resistance（Rp） and pitting potential（φpit） and decrease in corrosion current density（Jcorr） and mean difference between φpit and φprot values. It is confirmed by scanning electron microscope micrographs that pits could be observed on the surface of base metal but not on the surface of laser-welded alloy after potentiodynamic tests. An improvement of corrosion resistance of laser-welded NiTi alloy could be attributed to almost complete dissolution of inclusions upon laser welding.     

Thermomechanical behavior of an Fe-Based shape memory alloy :Transformation conditions and hystereses

The transformation conditions of an Fe-based polycrystalline shape memory alloy are determined experimentally under the multiaxial stress state. The martensite start condition is represented as an oval conical surface in the axial stress-shear stress-temperature space, meaning the transformation condition is not the von Mises type condition in plasticity and clearly depends on the third invariant of the stress tensor. The austenite start and finish conditions are also determined as the planes in the multiaxial stress-temperature space. The uniaxial stress-strain-temperature hysteretic behavior determined under the uniaxial thermomechanical loads is simulated by introducing the internal state variables which represent the status of the microscopic internal structures.     

NiTi形状记忆合金的应变补偿物理本构模型

采用Gleeble-3500热模拟试验机对NiTi形状记忆合金进行了等温恒应变速率压缩试验,研究其在变形温度为700～900℃、应变速率为0.001～10 s-1和最大高度下压量为70％下的高温热变形行为;建立了引入物理参量的应变补偿本构模型.结果 表明:合金的流变应力具有负温度相关性和正应变速率敏感性,在应变速率为0...     

Microstructure of stress-induced martensite in nanocrystalline NiTi shape memory alloy

The microstructure of stress-induced martensite(SIM) in the nanocrystalline NiTi alloy was investigated by means of transmission electron microscopy(TEM). The result shows that the multi-variant structure of the martensite is suppressed and only single-variant martensitic twins form after tensile deformation when the grain size is smaller than80 nm. The normal directions of the(001)B190twin planes are all within the range of 45° from the axial direction of the wire. The angle between twin crystals(1"11)Mand(111)Tof the SIM is also found to be smaller than that of thermally induced martensite in nanocrystalline NiTi.     

NiTi形状记忆合金沙蚀磨损的机敏行为

研究了Ti 50 .94%Ni(摩尔分数 )形状记忆合金的耐磨性 ,试验温度为 70℃ ,砂水比为 0 .2 ,累计磨损时间 30h。结果表明 ,NiTi合金表现出良好的耐磨性 ,扫描电镜仅观察到轻微的磨损。采用四棱锥金刚石压头模拟砂粒与试样的接触过程 ,并用原子力显微镜对压痕进行定量研究 ,观察发现 ,NiTi合金压痕呈星形 ,反映出明显的弹性变形特征 ,说明材料应力应变非线性关系和伪弹性变形行为对砂蚀磨损耐磨性具有重要贡献 ,进一步证明形状记忆合金的机敏摩擦学特性。     

Equal channel angular extrusion of NiTi shape memory alloy tube

As a new attempt, equal channel angular extrusion (ECAE) of nickel–titanium shape memory alloy (NiTi SMA) tube was investigated by means of process experiment, finite element method (FEM) and microscopy. NiTi SMA tube with the steel core in it was inserted into the steel can during ECAE of NiTi SMA tube. Based on rigid-viscoplastic FEM, multiple coupled boundary conditions and multiple constitutive models were used for finite element simulation of ECAE of NiTi SMA tube, where the effective stress field, the effective strain field and the velocity field were obtained. Finite element simulation results are in good accordance with the experimental ones. Finite element simulation results reveal that the velocity field shows the minimum value in the corner of NiTi SMA tube, where severe shear deformation occurs. Microstructural observation results reveal that severe plastic deformation leads to a certain grain orientation as well as occurrence of substructures in the grain interior and dynamic recovery occurs during ECAE of NiTi SMA tube. ECAE of NiTi SMA tube provides a new approach to manufacturing ultrafine-grained NiTi SMA tube.     

Corrosion properties of laser surface melted NiTi shape memory alloy

A Nd-YAG laser was used to melt the surface of NiTi shape memory alloy in air and argon environment respectively. The laser surface melted (LSM) layer is free of porosity and crack. The corrosion resistance of the LSM specimen in 3%NaCl solution has been improved significantly and is attributed to the increased amount of TiO₂ and Ti/Ni ratio on the surface.     

添加铜箔中间层的NiTi形状记忆合金超声波焊接

采用超声波焊接方法对添加铜箔中间层的NiTi形状记忆合金进行搭接焊,研究接头的表面形貌、界面形貌、相变行为、抗拉性能和断裂特点. 结果表明,超声波焊接能实现添加铜箔中间层的薄片状NiTi形状记忆合金的有效连接. NiTi焊缝表面存在明显压痕,界面处结合良好,无金属间化合物生成. NiTi形状记忆合金超声波焊缝呈可逆单相转变过程,接头强度达到母材的65%,断裂位置在母材区,断口形貌呈现韧性断裂特征. 超声波焊接方法将有效解决NiTi形状记忆合金采用传统熔焊方法时产生脆性金属间化合物的这一问题,为NiTi记忆合金的连接提供了新的解决途径.     

铸态镍钛形状记忆合金的断裂行为及显微组织(英文)

通过真空自耗电极熔炼法制备等原子比镍钛形状记忆合金。为了研究其断裂力学性能,进行铸态镍钛形状记忆合金的拉伸和压缩实验。为了更好地理解镍钛形状记忆合金的组织演变及断裂行为,分析铸态镍钛形状记忆合金及其断裂样品的显微组织。在拉伸加载下,镍钛形状记忆合金在750°C时具有较高的塑性,表现为韧性断裂,但在室温和-100°C时表现出较差的塑性,具有解理断裂和穿晶断裂的特征。在-100°C的压缩加载下,铸态镍钛形状记忆合金发生剪切断裂,剪切断裂面法线与压缩轴呈45°,具有解理断裂的特征,裂纹经由穿晶断裂而扩展。     

NiTi形状记忆合金表面TiO2薄膜的制备及其血液相容性

采用溶胶-凝胶法在NiTi合金表面制备了TiO2薄膜,用DSC分析了凝胶在加热过程中发生的转变.结果表明:在NiTi表面形成金红石型TiO2薄膜;覆盖有TiO2薄膜的NiTi合金与NiTi基体及316L不锈钢、Ti6Al4V相比,凝血时间延长,溶血率下降,说明TiO2膜可提高NiTi合金血液相容性.     

镍钛形状记忆合金在局部包套压缩下的塑性屈服

作为一种崭新的尝试,局部包套压缩被应用于实现镍钛形状记忆合金在室温下的大塑性变形。基于主应力法和塑性屈服准则,分析了镍钛形状记忆合金局部包套的压缩塑性力学。采用透射电镜、高分辨透射电镜和扫描电镜研究镍钛形状记忆合金在局部包套压缩下的显微组织演变和变形行为。静水压力随着包套外径的增加而增加,有效地抑制了显微裂纹的萌生和扩展,有助于提高镍钛形状记忆合金的塑性,避免了脆性断裂的发生。在0．15～0．50的真实应变范围内,镍钛形状记忆合金在三向压应力状态下的塑性变形满足密席斯塑性屈服准则。在更大的塑性应变下,由于非晶相的出现,镍钛合金不能满足密席斯塑性屈服准则。     

Nanomechanical and electrochemical properties of ZrN coated NiTi shape memory alloy

Zirconium nitride (ZrN) thin films were deposited on NiTi and Si substrates in the 23–570 °C temperature range by direct current reactive magnetron sputtering using N₂/Ar gas mixture. The film hardness, corrosion behavior, phase composition, and texture were determined. The deposited films were composed mainly by the cubic ZrN phase, whose texture varies with substrate temperature, changing progressively from (111) to (200) texture as the temperature increases. The hardness of the films is influenced by the texture and has a linear relationship with the ratio of the texture coefficients P(111)/P(200). The higher hardness is obtained for ZrN thin films with (200) texture. Electrochemical tests show that NiTi coated with (200)-oriented ZrN films has higher tendency to passivation and greater stability of the passive film as compared to (111)-oriented ZrN films, despite no abrupt changes was observed when the texture changes from (111) to (200).     

Functionally-graded shape memory alloy by diffusion annealing of palladium-coated NiTi plates

Diffusion annealing of palladium-coated Ti-Ni plates was performed at temperatures ranging from 900 °C to 1,000 °C, to accomplish a compositional gradient in Ti-rich, Ti-Ni shape memory alloys. The aim of this study was to increase the transformation temperatures and transformation temperature intervals. Palladium diffusion profiles were measured by energy dispersive spectroscopy, and the corresponding approximate diffusion coefficients of the annealed specimens were calculated. The Gaussian solution of Fick’s second law for the one-dimensional lattice diffusion of a tracer was used. The transformation behavior studies were performed by differential scanning calorimetry. It was depicted that annealed specimens show longer transformation intervals compared to the bare alloy. In addition, annealed specimens showed improved shape memory properties that were attributed to the lower amount of Ti₂Ni precipitates in the diffusion layer. The shape memory behaviour of the samples was detected using micro-indentation at room temperature, followed by heating them above the austenite formation temperature to calculate the shape recovery ratio.