浅析NiTi形状记忆合金的疲劳机理

由于NiTi形状记忆合金具有可逆的热弹性马氏体相变效应等不同与一般金属的特性，使得其疲劳机理也不同于一般的工程材料。为此，本文侧重分析了温度变化对NiTi形状记忆合金疲劳机理的影响和应力诱发马氏体相变等特征相变过程，以及相关的研究方法与主要结果，将为进一步研究其疲劳过程及增韧工艺提供理论参考依据．     

重熔镍钛合金的形状记忆效应分析

采用不同的工艺及设备对近等原子比的形状记忆合金进行重熔,获得铸态组织形状记忆合金,对采用不同坩埚所制备出的NiTi合金进行对比分析,采用金属坩埚所得到的形状记忆合金的成分满足医疗器械及外科植入物用NiTi形状记忆合金的成分要求,且具有良好的形状记忆效应。     

Oxide Scales Formed on NiTi and NiPtTi Shape Memory Alloys

Ni-49Ti and Ni-30Pt-50Ti (nominal at. pct) shape memory alloys (SMAs) were isothermally oxidized in air over the temperature range of 773?K to 1173?K (500?°C to 900?°C) for 100?hours. The oxidation kinetics, presented in detail in a companion study, show ~4 times reduction in oxidation rate due to Pt.[1] The microstructure, composition, and phase content of the scales and depletion zones were determined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). A relatively pure TiO₂ rutile structure was identified as the predominant scale surface feature, typified by a distinct highly striated and faceted crystal morphology, with crystal size proportional to oxidation temperature. The complex layered structure beneath these crystals was characterized by semiquantitative XRD of serial/taper polished sections and SEM/EDS of cross sections for samples oxidized at 973?K (700?°C). In general, graded mixtures of TiO₂, NiTiO₃, NiO, Ni(Ti), or Pt(Ni) metallic dispersoids, and continuous Ni₃Ti or Pt-rich metal depletion zones, were observed from the gas surface to the substrate interior. Overall, substantial depletion of Ti occurred due to the formation of predominantly TiO₂ scales. It is proposed that the Ni-30Pt-50Ti alloy oxidized more slowly than the binary Ni-49Ti alloy by decreasing oxygen and titanium diffusion through the thin Pt-rich layer.     

On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys

Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19′ when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19′ transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19′) and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes.     

Influence of Composition and Thermomechanical Training Process on the Transformation Behavior and Shape Memory Properties of NiTi Based Alloys

The effects of composition, specially the Cu element and thermomechanical training process on the transformation and one way shape memory behavior (OWSM) of NiTi based alloys were investigated using differential scanning calorimetry and microstructural analysis. The hot rolled strips with different compositions were trained under various applied pre-strains by bending test at martensitic state. It was found that the presence of Cu in the NiTi alloy results in an improvement of the OWSM effect by reducing the transformation hysteresis and forming the longer martensitic variants with lower interfacial energy than the NiTi binary alloys. Increasing the applied pre-strain and the number of thermomechanical training cycles caused OWSM effect to decrease. Although the transformation temperature increased at the higher applied pre-strain, it was reduced during the thermomechanical training process.     

NiTi合金腐蚀性能研究

研究了稀氯化物溶液(模拟人体生理环境)中NiTi合金的腐蚀行为。NiTi合金的镍离子析出量小于316L不锈钢。NiTi—B合金的耐腐蚀性能优于NiTi—A合金。砂纸抛光 氧化处理的NiTi合金的耐腐蚀性能优于砂纸抛光处理的NiTi合金。NiTi合金的耐腐蚀性能随热处理温度的升高及热处理时间的延长而提高；在500℃／90min时，NiTi合金耐腐蚀性能最佳。NiTi合金的耐腐蚀性能与其表面形成的氧化膜有关。     

镍钛形状记忆合金的开发应用

综述了镍钛形状记忆合金在国内外的开发应用情况。对今后的发展提出了建议。     

Direct Numerical Determination of the Asymptotic Cyclic Behavior of Pseudoelastic Shape Memory Structures

The design of shape memory alloys (SMAs) structures against fatigue requires the computation of the stabilized mechanical state. The classical computation method, based on a plasticity-like algorithm, requires a step-by-step calculation, leading to prohibitive computation time to reach this stabilized state. To overcome this issue, we propose to extend the direct cyclic method (DCM), for elastoplastic structures, for use with the Zaki-Moumni (ZM) model for SMAs. DCM is a large time increment method in which a periodicity condition is enforced on the state variables. Comparison with the classical incremental approach shows considerable reduction in computation time.     

Experimental Investigation of Cyclic Behavior of Concrete-Filled Fiber Reinforced Polymer Tubes

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) have in the last decade been used as girders, beam columns, and piles. The focus of research, however, has been exclusively on their monotonic behavior, with little or no attention to the implications of using CFFT in seismic regions. A total of six CFFT specimens were tested as simple span beam columns under constant axial loading and quasi-static reverse lateral loading in four point flexure. Three of the tubes were made using centrifuge (spin) casting with 12.7?mm thickness with the majority of the fibers in the longitudinal direction, whereas the other three were filament wound with 5?mm thickness and ±55° fiber orientation. One specimen for each type of tube had no internal reinforcement, whereas the other two incorporated approximately 1.7 and 2.5% steel reinforcement ratios, respectively. The two types of tubes represented two different failure modes; a brittle compression failure for the thick tubes with the majority of the fibers in the longitudinal direction, and a ductile tension failure for the thin tubes with off-axis fibers. The study showed that CFFT can be designed with ductility behavior comparable to reinforced concrete members. Significant ductility can stem from the fiber architecture and interlaminar shear in the FRP tube. Moderate amounts of internal steel reinforcement in the range of 1–2% may further improve the cyclic behavior of CFFT.     

Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actuators

The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 7.5, and 10 at. pct Cu) shape memory (SM) spring actuators. We study how the alloy composition and processing affect the actuator stability during thermomechanical cycling. Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures. We present phenomenological equations that quantify both phenomena. We show that cyclic actuator stability can be improved by using precycling, subjecting the material to cold work, and adding copper. Adding copper is more attractive than cold work, because it improves cyclic stability without sacrificing the exploitable actuator stroke. Copper reduces the width of the thermal hysteresis and improves geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent and the product phase. There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable deformation associated with thermomechanical cycling. We interpret this finding on the basis of a scenario in which dislocations are created during the phase transformations that remain in the microstructure during subsequent cycling. These dislocations facilitate the formation of martensite (increasing martensite start (M _S ) temperatures) and account for the accumulation of irreversible strain in martensite and austenite.     

形状记忆合金的发展

形状记忆合金是一种新型功能材料,因为具有温度感知和驱动性能而得到广泛应用.该文阐述了形状记忆合金的发展、分类以及与形状记忆效应有紧密联系的热弹性马氏体相变的原理,最后介绍了形状记忆合金的应用.     

TiNi Shape Memory Alloys with High Sintered Densities and Well-Defined Martensitic Transformation Behavior

This study demonstrates that a high density and a high transformation heat can be attained for PM TiNi. With the use of fine elemental powders, a composition of Ti₅₁Ni₄₉, two-step heating, and persistent liquid-phase sintering at 1553 K (1280 °C), a 95.3 pct sintered density was attained for compacts with a green density of 66 pct. A transformation heat, ΔH, of 31.9 J/g was also achieved, which is much higher than reported previously for sintered TiNi and is approaching the highest ΔH reported to date, 35 J/g, for wrought TiNi with low C, O, and N contents. The main reason for having these properties in powder metal TiNi with higher amounts of C, O, and N is that the extra Ti, that over the equiatomic portion in the Ti-rich Ti₅₁Ni₄₉, forms Ti₂Ni compound, which traps most of the C, O, and N. This results in low interstitial contents and a high Ti/Ni ratio of 50.5/49.5 in the TiNi matrix. The tensile strength, elongation, and shape recovery rate after five training cycles were 638 MPa, 14.6, and 99.1 pct, respectively, despite the presence of Ti₂Ni compounds at grain boundaries.     

磁性形状记忆合金的研究现状及发展

磁性形状记忆合金是一种新发展起来的形状记忆材料，合金具有大恢复应变、大输出应力、高响应频率和可精确控制的综合特性，有望成为压电陶瓷和磁致伸缩材料之后的新一代驱动与传感材料。系统阐述了近年来磁性形状记忆合金的研究进展，并着重介绍了目前研究最多的Ni-Mn-Ga合金的结构、相变、形状记忆效应、制备方法及其在应用方面的一些新的研究成果，并提出了磁性形状记忆合金需要深入研究的问题及其发展方向。     

Aging Effects in Copper-Based Shape Memory Alloys

Aging of three copper-based shape memory alloys was studied by measuring the time dependence of hardness, martensitic transformation temperatures, lattice parameters, and shape memory capability at temperatures in the range 200 to 450°C. The ultimate loss of the shape memory effect in each alloy was preceded by changes in the other properties which resulted from thermally activated processes having activation energies in the range 60 to 80 kJ mol^-1. At temperatures above about 300°C the aging process involved the eventual formation of α and γ₂ phases. Although the activation energy appears to be insensitive to temperature and alloy composition, at lower temperatures other thermally activated processes, such as change in the type or degree of order, may, at least in the initial stages, be significant aging phenomena.     

Ni-Ti形状记忆合金中的相变

根据作者的工作、并结合新近文献,对近等原子Ni-Ti合金中的相变作了系统的综述,包括:相变的顺序,无公度相变,R相变,马氏体相变,合金成分及热循环对相变临界温度的影响,逆相变及沉淀。     

Microstructural Model for the Deformation of Shape Memory Alloys

A model is proposed to describe the phase-structural deformation of shape memory alloys with allowance for the nonuniform strain hardening of the martensite part of representative volume. A scheme is developed to determine the volume fraction of martensite undergoing structural transformation during proportional nonreversible loading. The problem of reactive-stress generation in experiments on orientational transformation with constrained deformation after unloading is resolved.