Characterization of phase transformation in shape memory alloys by atomic force microscope

An atomic force microscope operated at various temperatures is introduced to evaluate phase transformation temperature and to observe microstructure for a shape memory alloy at same time in this paper. A commercial hot-rolled TiNi shape memory alloy bar is ground, polished and etched. The specimen is then observed by atomic force microscopy at the temperature range of 20–100 C in nitrogen gas. The topographies of a TiNi specimen show twinning martensite with rough surface and smooth austenite at various temperatures. The shape memory effect of the TiNi alloy is analyzed based on the shifts of the topographies obtained at various temperatures, which are used to evaluate the phase transformation temperature between martensite and austenite. The phase transformation temperature is also confirmed in a differential scanning calorimeter (DSC) experiment.     

Transformation plasticity and shape memory effects in TiNi alloy after low-temperature treatment

The effect of low-temperature annealing on the structure, kinetics of martensitic transformations, and functional properties of an equiatomic TiNi shape memory alloy is studied. Low-temperature annealing of the TiNi alloy is shown to decrease the temperature of the end of the forward martensite transformation M _f and the temperature of the onset of the reverse transformation A _s , which increases the transformation temperature range. As a result, the shape memory effect is improved due to a decrease in the irreversible strain. These phenomena are assumed to be caused by the hardening of the TiNi alloy induced by low-temperature annealing.     

60keV质子辐照对TiNi记忆合金薄膜马氏体相变的影响

 利用磁控溅射的方法在氧化后的单晶Si基片上制备了TiNi形状记忆合金薄膜，利用示差扫描量热法和原位X射线衍射研究了薄膜的马氏体相变特征。通过60keV质子注入（辐照）薄膜样品研究了H+离子对合金薄膜马氏体相变特征的影响，结果表明氢离子注入后引起了马氏体相变开始Ms和结束点Mf以及逆马氏体相变开始As和结束温度Af的下降，而对R相变开始Rs和结束温度Rf影响不大。掠入射X射线衍射表明H+离子注入后有氢化物形成。H⁺离子注入形成的氢化物是引起相变点的变化的主要因素。     

Temperature evolution in deformed shape memory alloy

Temperature changes during tensile test and simple shear test of TiNi shape memory alloys loaded at various strain rates and at different temperatures have been presented. The temperature changes were measured by recording infrared radiation emitted by the surface of the specimen. It was found that the martensite transformation was accompanied by an increase in temperature while the reversible transformation––temperature decrease.     

Large-deformation analysis in microscopic area using micro-moiré methods with a focused ion beam milling grating

In this study, the focused ion beam (FIB) milling method is applied to fabricate sub-micron grating on TiNi shape memory alloy materials. With self-made FIB milling gratings, scanning electron microscope (SEM) micro-moiré and digital moiré methods are successfully used to measure large deformation of porous TiNi shape memory alloys (SMA) in uni-axial compressive tests. The principles of the SEM micro-moiré method and digital moiré method are introduced, and applied to calculate large strain. The full field deformation around shear bands can be measured precisely. During the investigation, the phenomenon of furcated moiré fringes was found, and a corresponding explanation is given in this paper. The furcated fringes are generated in the locations of combined shear bands where sudden changes of strain occur. Successful results also verify that the FIB milling gratings are suitable for micro-moiré measurement and can generate high quality moiré fringes.     

Compton profiles and nesting of Fermi surfaces for B2-TiNi and B2-TiPd

The band structures of the shape memory alloys B2-TiNi and B2-TiPd are calculated by the full potential linearized augmented plane wave method with the local density approximation. The theoretical Compton profiles for B2-TiNi and B2-TiPd are calculated. In addition, the three-dimensional (3D) occupation number densities obtained by Lock-Crisp-West (LCW) analysis are presented for the first time. These 3D occupation number densities are in good agreement with the Compton scattering experiment for TiNi. Both shape memory alloys are based on martensitic transformation, which is caused by soft phonons. The charge-density wave is created by nesting of Fermi surfaces, which leads to phonon softening. To examine the nesting vectors quantitatively, we calculate the generalized susceptibility χ(q). χ(q) shows peaks at 0.315[110]2π/a and 0.4[111]2π/a for TiNi and at 0.275[110]2π/a and 0.395[111]2π/a for TiPd. Although the nesting vector in the [110] direction agrees with that from the phonon experiment, the nesting vector in the [111] direction differs from that in the experimental results.     

Physicomechanical criteria in devising shape-memory medical materials

Special metals showing shape memory are considered for use in contact with tissue. Basic criteria for them are devised. When a shape-memory material is used over long periods, one needs to combine porosity with good permeability and superelasticity. Only alloys based on TiNi are at present really promising for such porous elastic implanted structures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 97–100, March, 1989.     

Effect of high doses of N<Superscript>+</Superscript>, N<Superscript>+</Superscript> + Ni<Superscript>+</Superscript>, and Mo<Superscript>+</Superscript> + W<Superscript>+</Superscript> ions on the physicomechanical properties of TiNi

The surface layer of an equiatomic TiNi alloy, which exhibits the shape memory effect in the martensitic state, is modified with high-dose implantation of 65-keV N⁺ ions (the implantation dose is varied from 10¹⁷ to 10¹⁸ ions/cm²). TiNi samples are implanted by N⁺, Ni⁺-N⁺, and Mo⁺-W⁺ ions at a dose of 10¹⁷–10¹⁸ cm⁻² and studied by Rutherford backscattering, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction (glancing geometry), and by measuring the nanohardness and the elastic modulus. A Ni⁺ concentration peak is detected between two maxima in the depth profile of the N⁺ ion concentration. X-ray diffraction (glancing geometry) of TiNi samples implanted by Ni⁺ and N⁺ ions shows the formation of the TiNi (B2), TiN, and Ni₃N phases. In the initial state, the elastic modulus of the samples is E = 56 GPa at a hardness of H = 2.13 ± 0.30 GPa (at a depth of 150 nm). After double implantation by Ni⁺-N⁺ and W⁺-Mo⁺ ions, the hardness of the TiNi samples is ∼2.78 ± 0.95 GPa at a depth of 150 nm and 4.95 ± 2.25 GPa at a depth of 50 nm; the elastic modulus is 59 GPa. Annealing of the samples at 550°C leads to an increase in the hardness to 4.44 ± 1.45 GPa and a sharp increase in the elastic modulus to 236 ± 39 GPa. A correlation between the elemental composition, microstructure, shape memory effect, and mechanical properties of the near-surface layer in TiNi is found.     

Two-Way Shape Memory Effect in Rapidly Quenched Highly Doped Alloys of TiNi–TiCu System upon Laser Treatment

Using an original approach, a pronounced two-way shape memory effect is created for the first time in rapidly quenched alloys of the quasibinary TiNi–TiCu system with copper contents of up to 38 at %. The technique includes the combined effect of the dynamic crystallization of the amorphous state by application of a single pulse of electric current of 10 ms duration and pulsed laser radiation (λ = 248 nm, τ = 20 ns). The obtained composite structure materials can be used to create different micromechanical devices, especially microswitches and microtweezers for gripping micro-objects.     

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.     

Orientational dependence of shape memory effects and superelasticity in CoNiGa,NiMnGa, CoNiAl,FeNiCoTi, and TiNi single crystals

The dependence of deforming stresses, shape memory effect (SME), and superelasticity (SE) on the orientation of the single crystal axis, test temperature, and disperse particle size is examined for CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals. The orientational dependence of SME, SE, and temperature interval of the development of martensitic transformations (MT) under loading and SE is established. The influence of disperse particles on magnitudes of SME, SE, and mechanical hysteresis is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 4–20, September, 2004.     

Effect of the temperature and way of preliminary torsion in the austenitic state on the shape memory effect in TiNi alloy

The effect of temperature and direction of preliminary torsion in the austenitic state on the degree of strain recovery upon heating of a TiNi alloy has been investigated. It is shown that an increase in the preliminary deformation temperature from 500 to 700 K leads to an increase in the degree of shape recovery upon heating of the material studied. In particular, a 20% strain at a temperature of 500 K decreases the recovery coefficient by 20%, whereas the same preliminary strain at 700 K deteriorates the shape recovery by only 4%. It is established that, applying preliminary torsion in the austenitic and martensitic states in opposite directions, one can obtain an increase in the shape memory strain with an increase in the preliminary plastic strain. Thus, at some plastic strains (λ _pl > 10%), the strain recovered upon heating may even exceed the strain set in the martensitic state.     

周期性多层膜合金化制取的TiNi形状记忆薄膜的室温微结构特征

用掠入射X射线衍射及X射线反射对磁控溅射制取的等原子比Ni／Ti周期性多层膜晶化热处理 后的TiNi形状记忆薄膜室温微结构进行了研究.TiNi形状记忆薄膜在深度方向的相分布和元 素分布是不均匀的，都是一种多层结构.室温下其微结构特征为最外层是Ti氧化膜，再下层 是Ti₃Ni₄,B19’马氏体相和少量的B2奥氏体相的三相混合物，靠近 基体为主要相成分马氏体，最后是Ni和Si界面反应层.X射线反射率的拟和结果显示薄膜微结 构的分析是合理的.薄膜中相深度分布的不均匀性主要是动力学因素决定的. 关键词： 相深度分布 形状记忆 TiNi 多层膜     

Neutron-irradiation-induced shape memory effect in a TiNi alloy

The recovery of inelastic strains in Ti-Ni alloy samples irradiated in a nuclear reactor under isothermal conditions was studied. Before irradiation, the cylindrical samples were compressed to a residual strain of 3–6% in the martenstici state at room temperature. The samples were irradiated at a temperature of 45°C, which does not exceed the temperature of the onset of the reverse martensitic transformation A _S . Irradiation with a fastneutron fluence of 5 × 10²⁰ cm^?2 is established to result in the recovery of the residual strain. The value of the recoverable strain is comparable to that observed under the conditions of the shape memory effect on heating of the deformed alloy and even somewhat exceeds it. The obtained data show that neutron irradiation can induce the shape-memory effect in the TiNi alloy. This is due to a decrease in the temperatures of the martensitic transformations under irradiation.     

预应变对TiNi丝/Ni基复合材料的内耗行为的影响

本文对预应变TiNi形状记忆合金N／Ni基复合材料的内耗行为进行了研究。TiNi丝／Ni基复合材料是将TiNi丝作为阴极，Ni基作为阳极通过化学电镀法制成的。结果表明：随着TiNi丝预应变的增加，复合材料的相变内耗峰逐渐增宽。由于TiNi丝与Ni基之间的热膨胀系数不匹配以及回复力的产生，使复合材料在高温段的内耗有一个急剧增加过程。与TiNi合金相比，TiNi／Ni基复合材料的整体内耗整体随着温度的升高而增加的。     

Reversible change of shape in mabtensitic transformations

The thermal stability of the reversible changes in shape of opposite sign, obtained after preliminary plastic deformation of TiNi above and below M_d, is investigated. It is shown that the stability of the dislocational structure responsible for reversible changes in shape in martensitic transformations is determined by the phase state of the material in the course of its formation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 95–101, May, 1977.     

Effect of electropulsing treatment on the microstructure and superelasticity of TiNi alloy

Cold-rolled TiNi alloy was treated by electropulsing with different process parameters. The superelasticity, electrical resistivity and microstructure evolution of the TiNi alloy were investigated. The recrystallization is found to happen in the cold-rolled TiNi alloys during electropulsing treatment (EPT) processing and it took 9 s to complete the crystallization in the EPT process. In addition, the grain size increases with the charge voltage and the discharge frequency of the electropulse. The maximum superelasticity of 6.94 % is gained when the TiNi alloy is treated by the electropulsing with the frequency of 300 Hz. The short recrystallization time of the alloy is attributed to the acceleration of atomic diffusion and the movement velocity of the grain boundary by electropulsing. This research provides an in-depth understanding of how the electropulsing affects the microstructure and superelasticity of TiNi alloy.     

Synthesis and characterization of laser ablated TiNi films

TiNi thin films with BaTiO₃ and PbZr_0.52Ti_0.48O₃ (PZT) as buffer layers were deposited on Si(100) substrates by the pulsed laser deposition (PLD) method. Buffer layers (BaTiO₃ and PZT) were deposited at 600 °C in oxygen (O₂) environment and TiNi films were deposited on the top of the buffer layer in presence of 15 mTorr nitrogen (N₂) at various deposition temperatures (50, 300, and 500 °C). Synthesis and characterization of TiNi films were investigated from the crystallographic point of view by using X-ray diffractometer (XRD) and atomic force microscope (AFM) techniques. It is found that buffer layer of BaTiO₃ and PZT have improved the crystallinity of TiNi films deposited at higher temperatures. The TiNi/PZT film was uniform compared to TiNi/BaTiO3 film with the exception of agglomerates that appeared throughout the layer.     

Interfacial reactions of rf-sputtered TiNi thin films on (100) silicon with a SiN diffusion barrier

Silicon nitride (SiN) with a 50?nm thickness on Si(100) as a thermal barrier was obtained by plasma-enhanced chemical vapour deposition (PECVD). TiNi thin films were rf sputtered on a SiN/Si substrate and then annealed at 400–700°C for 30?min. Their interfacial reactions were studied using transmission electron microscopy, X-ray diffraction and Auger electron spectroscopy analyses. Experimental results show that the thickness of reaction layer in TiNi/SiN/Si specimens is clearly reduced, compared with that in TiNi/Si specimens under the same annealing conditions. The significant effect of the SiN layer as a diffusion barrier in TiNi/SiN/Si can be recognized. N and Si atoms diffuse from the SiN layer to react with TiNi films at 500°C and 600°C respectively. The TiN_{1 ? x} phase is formed in specimens annealed at 500°C, and mixed Ti₂Ni₃Si and Ti₄Ni₂O compounds are found at 600°C. In the specimen annealed at 700°C, the reaction layer has sublayers in the sequence TiNi/Ti₄Ni₂O/Ti₂Ni₃Si/TiN_{1 ? x} /SiN/Si. The SiN thermal barrier obtained by PECVD caused quite different diffusion species to cross the interfaces between TiNi/SiN/Si and TiNi/Si specimens during the annealing.     

Effect of thermo-mechanical process on structure and high temperature shape memory properties of Ti–15Ta–15Zr alloy

The effect of thermo-mechanical treatment on microstructure evolution, martensite transformation, and shape memory behavior of Ti–15Ta–15Zr high temperature shape memory alloy were investigated. Different martensite morphologies were found with different annealing temperatures. The Ti–15Ta–15Zr alloy exhibits almost perfect shape memory recovery strain of 6% after annealing at 973 K for 0.5 h.