超弹性柱晶组织CuAlNi合金丝的弯曲疲劳

用热型连铸法以不同速度连铸了Ф1.5mm的柱晶CuAlNi丝,检验其弯曲疲劳寿命.结果显示：固溶处理后消除了内应力,弯曲疲劳寿命随连铸速度的提高而提高,但超过480mm/min后由于出现等轴晶和加热后的晶粒长大,疲劳寿命下降.弯曲疲劳断裂首先从柱晶晶界开始开裂,然后各晶粒各自断裂.晶界开裂有利于阻隔疲劳裂纹扩展,降低对表面缺陷的敏感性,降低弯曲时在晶界上产生的应变量,从而提高疲劳寿命.     

Effect of Impact-Induced Strain on the Stress-Induced Martensitic Transformation of Superelastic NiTi Shape-Memory Alloy Wires

The stress-induced martensitic (SIM) transformation of NiTi shape-memory alloy wires has been studied as a function of the maximum strain induced during tensile deformation at impact and quasi-static strain rates. The SIM transformation stresses are higher at impact than at quasi-static strain rates. Only the lower plateau strength is sensitive to the maximum strain achieved during transformation when this is higher than necessary to complete the SIM transformation. For the same maximum strain achieved, the deformation energy (E _d) and recoverable strain energy (E _r) are greater at impact than at quasi-static strain rates, and the dissipated energy (W _d) is slightly lower at impact, reaching values close to those obtained at quasi-static strain rates.     

Superelastic Porous NiTi with Adjustable Porosities Synthesized by Powder Metallurgical Method

The effects of cold compaction pressure, TiH₂, and NH₄HCO₃ on the physical and mechanical properties of porous NiTi fabricated by capsule-free-hot isostatic pressing (CF-HIP) were investigated. The porosities vary from 20 to 48% depending on the pressure. Porous NiTi with as great as 42% porosity and irregular pores can be prepared by the addition of TiH₂, whereas 48% porosity and circular pores can be achieved by NH₄HCO₃ addition. All the porous NiTi fabricated by CF-HIP using these three methods exhibit superelastic behavior that closely matches human bones.     

Manipulating Precipitation Through Thermomechanical Treatment to Control Corrosion Behavior of an Al–Cu–Mg Alloy

Controlling the precipitation through thermomechanical treatment is an important method to improve the corrosion resistance of Al–Cu–Mg alloys. In this study, the corrosion behaviors of Al–Cu–Mg alloys in the solution-treated state and retrogressiontreated state under cold rolling deformation and then natural aging were investigated. In the solution-treated series alloys, the cold-rolled deformation improved the resistance to intergranular corrosion by suppressing the precipitation of the S-phas...     

Effects of Loading and Constraining Conditions on the Thermomechanical Fatigue Life of NiTi Shape Memory Wires

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermal activation (thermomechanical fatigue) is central to the rational design of smart actuators based on these materials. Test results on SMAs under thermomechanical fatigue are scarce in the technical literature, and even the few data that are available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by antagonist SMA elements, their stress states are far from being constant in operation. The mismatch between actual working conditions and laboratory settings leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality by completing an experimental campaign involving four fatigue test conditions, which cover most of the typical situations occurring in practice: constant stress, constant-strain, constant stress with limited maximum strain, and linear stress-strain variation with limited maximum strain. The results from the first three test settings, recovered from the previously published works, are critically reviewed and compared with the outcome of the newly performed tests under the fourth arrangement (linear stress-strain variation). General design recommendations emerging from the experimental data are put forward for engineering use.     

Corrosion of NiTi Wires with Cracked Oxide Layer

Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO₂ surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.     

Thermomechanical Properties of Porous NiTi Alloy Produced by SHS

Samples obtained from relatively large powders (<150 µm), with total porosity in the range 30-68%, were characterized mainly from a morphological point of view. Total porosity, as well as pore size, shape and distribution, was analyzed. Sample microstructure was also investigated, indicating that the main phase produced during the self-propagating high-temperature synthesis (SHS) reaction is Ti reach NiTi phase, as confirmed by calorimetric analyses. Moreover, the presence of secondary phases, suggested by the low transformation enthalpy, was confirmed by SEM observations. In fact, EDS microanalyses and EBSD mapping helped in the identification of such secondary phases, such as Ni₃Ti, Ti₂Ni and Ti₄Ni₂O _x . Other samples were successively produced starting from the same powders but introducing a different powder compression methodology and operating conditions. In this way, the obtained samples showed higher porosity featured by more uniform size, shape and distribution while, from a micro-structural point of view, no significant differences were observed. Mechanical compression tests were carried out at room temperature and, on selected samples, also above A _f in order to highlight the influence of pore shape and distribution. Results obtained at room temperature show that the mechanical properties decrease with the porosity augmentation. For higher temperatures, the samples presented a pseudoelastic behavior. Dilatometric tests were also performed and the results well indicated the martensite to austenite transformation at the same temperature showed by the DSC analyses. Thermal analysis was completed by evaluating the thermal diffusivity temperature and porosity dependence using an experimental-numerical approach especially developed.     

Study on Thermomechanical Treatment, Mechanical Properties and Fatigue of Nitinol Superelastic Thin Sheet

A wide range of different thermomechanical treatments was performed on commercially available superelastic Nitinol thin sheet. The ingot composition in the range of standard superelastic material with about 50.8?at.% Ni, balance Ti, was used to manufacture a series of samples with different thermomechanical conditions. Production parameters such as cold work, heat treatment temperature, and heat treatment time were varied. All finished samples were of the same final thickness of 0.3?mm and received the same industrial surface finishing process to obtain a smooth, defect, and oxide-free, shiny surface. Before carrying out the laser cutting, the material was characterized by tensile testing, DSC, and bend-and-free recovery test. Miniature dogbone specimens were cut from the as-manufactured sheets in both directions, longitudinal as well as transverse to the rolling direction. These samples were surface finished using standard deburring and electropolishing processes. For some specific parameter combinations, there were also samples taken at 45?? to the rolling direction. All qualified samples were then exposed to fatigue testing in a bending mode until fracture or run-out. The results showed there is a significant effect on the fatigue performance of the samples from both the applied thermomechanical treatment as well as the sheet anisotropy. It is also obvious that the achieved strain data is on average lower than the data obtained in comparable studies on tube or wire, which can be attributed to the different test setup (bending mode in air at 37???C) as compared to most other studies as well as the larger surface.     

Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing

Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni₅₀Ti₅₀ shape memory alloy were studied. Several specimens were produced by copper boat vacuum induction melting. The homogenized specimens were hot rolled and annealed at 900 °C. Thereafter, annealed specimens were subjected to cold rolling with different thickness reductions up to 70%. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni₅₀Ti₅₀ alloy. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed. Results showed that with increasing thickness reduction during cold rolling, the recoverable strain of Ni₅₀Ti₅₀ alloy was increased during superelastic experiments such that the 70% cold rolled–annealed specimen exhibited about 12% of recoverable strain. Moreover, with increasing thickness reduction, the critical stress for stress-induced martensitic transformation was increased. It is noteworthy that in the 70% cold rolled–annealed specimen, the damping capacity was measured to be 28 J/cm³ that is significantly higher than that of commercial NiTi alloys.     

Mechanical Analysis of Hybrid Textile Composites with NiTi Wires

Hybrid textile structures composed by polyamide (PA), Lycra (or Elastane EL), and NiTi thin wires were manufactured. The fabrics were realized by knitting Lycra (EL) as weft and warp filaments and a coupled PA/NiTi yarn through in-laying technique. Superelastic NiTi wire with diameter of 50 μm in both straight annealed and snake-like form and shape memory with a snake-like memorized form were used. The polyamide filament used coupled with the NiTi wire has a diameter around 140 μm, while the textile structure is composed by various Lycra filaments, characterized by different diameters. The textile structures were realized by a circular machine appropriately equipped and used in order to minimize problems related to the use of the thin NiTi alloy filament. To study the influence of NiTi filaments on mechanical properties, specimens taken from textile fabric were analyzed by using a DMA Q800 TA Instruments, equipped with a tension film clamp, in static and dynamic conditions. Force/strain measurements up to 150% in elongation and Tandelta versus frequency were carried out on fabrics with and without NiTi filaments. Finally, some tests strain recovery under load versus temperature are conducted.     

热型连铸超弹性CuAlNi合金丝的拉伸疲劳性能

用热型连铸法以不同速度连铸了φ1.5的柱晶CuAlNi合金丝,研究其拉伸疲劳性能.结果显示：铸态下,连铸速度由240mm/min提高到360mm/min时合金丝由于晶粒细化提高了疲劳寿命;铸态下少量γ2对拉伸疲劳没有显著影响;疲劳拉伸中柱晶晶界开裂,阻隔了疲劳裂纹扩展,降低了疲劳裂纹对表面缺陷的敏感性,有利于提高疲劳寿命.     

热型连铸超弹性CuAlNi合金丝的拉伸疲劳性能

用热型连铸法以不同速度连铸了φ1．5的柱晶CuAlNi合金丝,研究其拉伸疲劳性能。结果显示:铸态下,连铸速度由240mm／min提高到360mm／min时合金丝由于晶粒细化提高了疲劳寿命;铸态下少量γ2对拉伸疲劳没有显著影响;疲劳拉伸中柱晶晶界开裂,阻隔了疲劳裂纹扩展,降低了疲劳裂纹对表面缺陷的敏感性,有利于提高疲劳寿命。     

Influence of Post-Weld Annealing on Transformation Behavior and Mechanical Properties of Laser-Welded NiTi Alloy Wires

The influence of annealing on the transformation behavior, mechanical, and functional properties of laser-welded NiTi wires was investigated. The results show that Ti₃Ni₄ precipitates occur after post-weld annealing and coarsen with increasing annealing temperature. The as-welded specimen exhibits one-step B2 → B19′ transformation, while the annealed ones show two-step B2 → R → B19′ transformation. Annealing at 400 °C for 1 h can improve the tensile strength and superelasticity of the welded joints. However, these properties decrease when annealing at 500 °C for 1 h. The change of mechanical and functional properties after annealing is attributed to the different size of Ti₃Ni₄ precipitates. Annealing to produce smaller coherent precipitates (10 nm) improves the mechanical and functional properties of the welded joints. As the Ti₃Ni₄ precipitates coarsen, the mechanical and functional properties decrease.     

热型连铸CuAlBe超弹性合金丝的力学性能

研究热型连铸柱状晶CuAlBe超弹性合金的性能并与单晶相比较。用热型连铸法制备了柱状晶CuAlBe超弹性合金丝,用循环拉伸试验检测其力学性能,用纯弯曲疲劳试验检测其弯曲疲劳寿命。结果表明,柱状晶CuAlBe合金丝的可恢复应变可达到15％,与单晶接近。超过这一应变量将导致马氏体稳定化,产生残余变形,导致卸载时拉伸曲线上出现锯齿状峰。柱状晶的弯曲疲劳寿命与单晶的为同一数量级,比机械加工表面的单晶高,而比电解抛光表面的单晶低。由此可见,柱状晶CuAlBe的性能与单晶的相近。     

Investigation on the Hysteretic Behavior of NiTi Shape Memory Wires Actuated Under Quasi-Equilibrium and Dynamic Conditions

The functional characterization of SMAs for actuation is typically performed by measuring the specimen deformation under constant load during a controlled thermal cycling across transformation temperatures. Under dynamic actuation, transformation temperatures different from those measured in quasi-equilibrium conditions have been observed. The aim of this work is to better investigate and understand these phenomena. Direct and indirect heating of shape memory wires under several loading conditions are examined in detail. According to the experimental results, the hypothesis is to consider the observed differences as an effect of the thermal cycling rate on the internal friction. However, the presented data seem do not fully confirm this idea. Further experiments will be carried out in order to directly measure the internal friction of the material under the same working conditions.     

Simulations of Mechanical Response of Superelastic NiTi Helical Spring and its Relation to Fatigue Resistance

Behavior of NiTi shape memory alloys under complex loading is still a subject of both experimental and theoretical investigations. One of the simplest geometries, in which the material is loaded in combined mode and which has also several practical applications, is a simple helical spring. In this contribution, mechanical response of NiTi superelastic spring is analyzed in detail by numerical simulation and the results are compared to experiments. The simulations show complex stress state, which develops during spring stretching. Analyzing fatigue tests with respect to simulated behavior allowed us to find relation between fatigue resistance and periodic changes in volume fraction of martensite induced by cyclic mechanical loading. The work also underlines an extension of the range of stroke amplitudes guaranteeing enhanced life performance of the spring when material transforms through the R-phase.     

Failure Behavior of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings Under Three-Point Bending Test via Acoustic Emission Technique

In this paper, the failure behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings fabricated by atmospheric plasma spraying (APS-TBCs) under three-point bending (3PB) test has been characterized via acoustic emission (AE) technique. Linear positioning method has been adopted to monitor dynamic failure process of the APS-TBCs under 3PB test. The investigation results indicate that the variation of AE parameters (AE event counts, amplitudes and AE energy) corresponds well with the change of stress–strain curve of the loading processes. The failure mechanism was analyzed based on the characteristics of AE parameters. The distribution of frequency of crack propagation has been obtained. The AE signals came from two aspects: i.e., plastic deformation of substrates, initiation and propagation of the cracks in the coatings. The AE analysis combined with cross-sectional observation has indicated that many critical cracks initiate at the surface of the top-coat. And some main cracks tend to propagate toward the substrate/bond-coat interface. The actual failure mechanism of the APS-TBCs under 3PB test is attributed to the debonding of metallic coating from the substrates and the propagation of the horizontal crack along the substrate/bond-coat interface under the action of flexural moment.     

负压铸渗法制备铸铁表面Ni／WC复合渗层三点弯曲性能测试

采用负压铸渗法制备了铸铁表面不同WC含量的Ni基复合渗层, 观察了渗层与基体结合形貌并对试样进行三点弯曲测试.结果表明:渗层与基体熔合良好;随着渗层中WC含量的增加,渗层的韧性变差,变形的能力变差;弯曲破坏形貌和断口分析表明,不同WC含量的渗层破坏形貌基本相同,断裂机理也相同,基体沿石墨断裂,渗层沿WC断裂.     

Formation of the Nanocrystalline Structure in an Equiatomic NiTi Shape-Memory Alloy by Thermomechanical Processing

The microstructural evolution during cold rolling followed by annealing of an equiatomic NiTi shape-memory alloy was investigated. The high purity Ni₅₀Ti₅₀ alloy was cast by a copper boat vacuum induction-melting technique. The as-cast ingots were then homogenized, hot rolled, and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70 % thickness reduction at room temperature. Post-deformation annealing was conducted at 400 °C for 1 h. The microstructure was characterized using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry techniques. The initial microstructure was free from segregation and Ti- or Ni-rich precipitates and was composed of coarse grains with an average size of 50 μm. The cold rolling of NiTi alloy resulted in a partial amorphization and the deformation-induced grain refinement. A nanocrystalline structure with the grain size of about 20-70 nm was formed during the post-deformation annealing.     

热处理工艺对NiTi合金负热膨胀行为的影响

通过测定热膨胀系数研究了热处理工艺对NiTi合金负热膨胀（NTE）行为的影响，并采用XRD和TEM分析对其机理进行了探讨。研究表明：360℃，130h约束时效时，随着外加应力的增加，NiTi合金的负热膨胀系数先增后减，在250MPa时可获得峰值-87．0×10^-6K^-1；时效温度升高或高温下时效时间延长，其NTE行为变差。母相中Ti3Ni4相定向析出所形成的内应力场有效控制了相变的发生并促使产生体积变化和双程形状记忆效应，二者的联合作用导致了NTE行为的产生。