Ni-Mn-Ga-V高温形状记忆合金研究

本文采用光学显微镜、X射线衍射仪、扫描电子显微镜、透射电子显微镜以及压缩力学试验机系统的研究了钒元素对(Ni56Mn25Ga19)100-xVx (x = 0, 1, 3) (at.%)高温形状记忆合金性能的影响。结果表明,由于钒取代方法的不同,我们所研究合金的组织结构和某些性能与已报到的Ni56Mn25Ga19-xVx合金差异较大。未掺杂钒元素时,试验合金由单相非调制四方结构马氏体构成,当钒含量为1at.%和3at.%时,合金由四方结构马氏体和竹叶状γ相构成。随着钒含量的增加,γ相的数量增多,但其尺寸和形状保持不变。钒掺杂改善合金的力学性能和形状记忆效应。 (Ni56Mn25Ga19)99V1合金变形10%后加热,可以得到6.7%的可逆应变。     

Kinetic arrest behavior in martensitic transformation of NiCoMnSn metamagnetic shape memory alloy

High-field magnetic measurements were carried out in order to investigate behaviors of field-induced reverse martensitic transformation and kinetic arrest of NiCoMnSn metamagnetic shape memory alloy. In the thermomagnetization curves, it was confirmed that the reverse martensitic transformation temperature decreases 67 K by applying magnetic field of 5 T, while in the magnetic field cooling process under 5 T, martensitic transformation does not occur down to low temperatures. Equilibrium magnetic field, defined from the critical magnetic fields of the metamagnetic evidence in the magnetization curves, exhibits almost constant below about 100 K, suggesting that the entropy change becomes zero, which is considered to cause kinetic arrest behavior.     

Investigation of the role of Ti oxide layer in the size-dependent superelasticity of NiTi pillars: Modeling and simulation

Recent compression tests of NiTi pillars of a wide range of diameters have shown significant size dependency in the strain recovered upon unloading. In this paper, we propose a numerical model supporting the previously proposed explanation that the external Ti oxide layer may be responsible for the loss of superelasticity in the small pillars. The shape memory alloy at the center of the pillar is described using a nonlocal superelastic model, whereas the Ti oxide layer is modeled as elastoplastic. Voigt average analysis and finite element calculations are compared to experiments for the available range of pillar sizes. The simulation results also suggest a size-dependent strain hardening due to the constraint on the phase transformation effected by the confining Ti oxide layer.     

高熵形状记忆合金相变行为研究现状

随着先进工程技术对形状记忆合金性能要求的不断提高,传统形状记忆合金愈发难以满足要求,高熵形状记忆合金应运而生,引起了研究者的极大关注。高熵形状记忆合金相较于传统形状记忆合金具有显著提高的可回复应变、屈服强度、高温物相稳定性和超弹性等特性,具有广阔的应用前景。本文对高熵形状记忆合金的研究现状、马氏体相变和形状记忆效应等进行了简要的论述,并对高熵形状记忆合金目前存在的不足和未来的发展进行了展望。     

Effect of atomic order on the martensitic transformation of Ni–Fe–Ga alloys

The effect of atomic order on martensitic transformation (MT) temperatures has been studied by differential scanning calorimetry in three polycrystalline alloys with compositions Ni_53.5+x–Fe_19.5−x–Ga_27.0 (x = 0, 0.5 and 1.5). Samples water quenched from different temperatures (470–1070 K) exhibit higher MT temperatures than ones slow cooled from the same temperature. This effect has been ascribed to a decrease of the L2₁ degree of order of the austenitic phase, which promotes an increase in the MT temperatures in these Ni–Fe–Ga alloys. The differences in ordering with cooling rate have been qualitative confirmed by electron diffraction patterns.     

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to B19′ during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack.     

Shape memory behavior and tension-compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.     

Origin of 2-stage R-phase transformation in low-temperature aged Ni-rich Ti–Ni alloys

After aging at intermediate temperatures (400–500 °C), Ni-rich Ti–Ni alloys undergo an abnormal 3-stage martensitic transformation behavior (1-stage R and 2-stage B19′), which stems from a preferential Ti₃Ni₄ precipitation around grain boundary. On the other hand, if aged at low-temperatures (250–300 °C), they undergo 2-stage R-phase transformation, but the origin of this strange phenomenon is unclear. In the present study, we made a systematic study of this phenomenon by considering the grain boundary effect and composition effect. We found that all single crystals undergo 1-stage R-phase transformation; in contrast, the transformation behavior of polycrystals is dependent on Ni content: low-Ni (50.6Ni, 51Ni) polycrystals undergo 2-stage R-phase transformation while high-Ni (52Ni) polycrystals undergo 1-stage R-phase transformation. The abnormal 2-stage R-phase transformation is attributed to a large-scale compositional heterogeneity in B2 matrix between grain boundary region and grain interior, due to the heterogeneity in precipitate density between the grain boundary and grain interior. But for high-Ni polycrystals, precipitates are essentially homogeneously distributed across the whole grain and this leads to normal 1-stage R-phase transformation. The different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the grain boundary; (2) for homogeneous precipitation across the whole grain with high-Ni content. The difference between the effect of intermediate-temperature and low-temperature aging lies in the difference in the ability for long-range diffusion of Ni (from the grain interior to the grain boundary), which results in whether or not Ti₃Ni₄ precipitates can form in the grain interior. Our results lead to a unified explanation for different transformation behaviors of both low-temperature and intermediate-temperature aged alloys in terms of the kinetics of precipitation in supersaturated polycrystals.     

Shape memory behavior of high strength NiTiHfPd polycrystalline alloys

Systematic characterization of the shape memory properties of a quaternary Ni_45.3–Ti_29.7–Hf₂₀–Pd₅ (at.%) polycrystalline alloy was performed in compression after selected aging treatments. Precipitation characteristics were revealed by transmission electron microscopy. The effects of aging temperature and time on transformation temperatures, recoverable and residual strains, and temperature and stress hystereses were determined by differential scanning calorimetry, constant-load thermal cycling experiments and isothermal strain cycling (superelasticity) tests. The crystal structure and lattice parameters of the transforming phases were determined from X-ray diffraction analysis. It was revealed that precipitation hardening significantly improved the shape memory properties of the NiTiHfPd alloy. Under optimum aging conditions, shape memory strains of up to 4% under 1 GPa were possible, and superelasticity experiments resulted in full strain recovery without any plastic deformation, even at stress levels as high as 2 GPa. The NiTiHfPd polycrystalline alloy exhibited very high damping capacity/absorbed energy (30–34 J cm^?3) and work output (30–35 J cm^?3), which were attributed to the ability to operate at high stress levels without significant plastic deformation and to a high mechanical hysteresis (>900 MPa) at temperatures ranging from 20 °C to 80 °C.     

Comparative study of R-phase martensitic transformations in TiNi-based shape memory alloys induced by point defects and precipitates

We studied the influence of point defects (Fe) and precipitates (Ti₃Ni₄) on the characteristics of R-phase martensitic transformation by comparing the transport and thermal properties of as-quenched Ti₅₀Ni₄₆Fe₄ and annealed Ti_48.7Ni_51.3 shape memory alloys. Both alloys undergo a weak first-order R-phase transformation with a small thermal hysteresis (less than 7 K) and non-zero transformation strain, suggesting the introduction of point defects and precipitates lead to a stable R-phase in these alloys due to the defects induced local lattice deformations. Furthermore, our study revealed that the transition temperature, transformation width, and transformation strain of the investigated R-phase TiNi-based alloys are strongly affected by the induced defects. As a result, the annealed Ti_48.7Ni_51.3 has a higher transition temperature than that of Ti₅₀Ni₄₆Fe₄, as expected.     

The thermochemical behavior of some binary shape memory alloys by high temperature direct synthesis calorimetry

The standard enthalpies of formation of some shape memory alloys have been measured by high temperature direct synthesis calorimetry at 1373 K. The following results (in kJ/mol of atoms) are reported: CoCr (−0.3 ± 2.9); CuMn (−3.7 ± 3.2); Cu₃Sn (−10.4 ± 3.1); Fe₂Tb (−5.5 ± 2.4); Fe₂Dy (−1.6 ± 2.9); Fe₁₇Tb₂ (−2.1 ± 3.1); Fe₁₇Dy₂ (−5.3 ± 1.7); FePd₃ (−16.0 ± 2.7); FePt (−23.0 ± 1.9); FePt₃ (−20.7 ± 2.3); NiMn (−24.9 ± 2.6); TiNi (−32.7 ± 1.0); TiPd (−60.3 ± 2.5). The results are compared with some earlier experimental values obtained by calorimetry and by EMF technique. They are also compared with predicted values on the basis of the semi empirical model of Miedema and co-workers and with ab initio calculations when available. We will also assess the available information regarding the structures of these alloys.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

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.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

Combining indentation and diffusion couple techniques for combinatorial discovery of high temperature shape memory alloys

We demonstrate the possibility of accelerated identification of potential compositions for high-temperature shape memory alloys (SMAs) through a combinatorial material synthesis and analysis approach, wherein we employ the combination of diffusion couple and indentation techniques. The former was utilized to generate smooth and compositionally graded inter-diffusion zones (IDZs) in the Ni–Ti–Pd ternary alloy system of varying IDZ thickness, depending on the annealing time at high temperature. The IDZs thus produced were then impressed with an indenter with a spherical tip so as to inscribe a predetermined indentation strain. Subsequent annealing of the indented samples at various elevated temperatures, T_a, ranging between 150 and 550 °C allows for partial to full relaxation of the strain imposed due to the shape memory effect. If T_a is above the austenite finish temperature, A_f, the relaxation will be complete. By measuring the depth recovery, which serves as a proxy for the shape recovery characteristic of the SMA, a three-dimensional map in the recovery–temperature–composition space is constructed. A comparison of the published A_f data for different compositions with the T_a data shows good agreement when the depth recovery is between 70% and 80%, indicating that the methodology proposed in this paper can be utilized for the identification of promising compositions. Advantages and further possibilities of this methodology are discussed.     

高熵形状记忆合金的研究进展

高熵形状记忆合金是在等原子比NiTi合金的基础上,结合高熵合金的概念,逐渐发展起来的一种新型高温形状记忆合金.近年来,已开发出了综合性能优异的(TiZrHf)50(NiCoCu)50系和(TiZrHf)50(NiCuPd)50系高熵形状记忆合金,引起了广泛的关注和研究兴趣.本文从物相组成、微观组织、马氏体相变行为、形状...     

Nanoindentation response of substrate-attached and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory thin films around the martensite transition: The impact of constraints and beyond

Ferromagnetic shape memory alloys offer great potential in the fields of engineering and medical sciences as integrated actuators or sensors. However, their physical properties, when miniaturized and connected to a substrate or mounted as active elements, are still insufficiently understood. The present work explores the impact of miniaturization and external boundaries on one of the most central features, namely twin boundary mobility. By measuring the nanoindentation response of substrate-attached films and freestanding foils around the austenite ? martensite transformation temperature in classical indentation, as well as dynamical quasi-continuous stiffness-measurement mode, dramatic softening and increasing recovery after film lift-off are discovered. The atomistics of these findings are explored with the help of classical multimillion-atom molecular dynamics simulations on indentation into martensite and austenite films on rigid or flexible substrates, as well as freestanding or mounted thin foils. They clearly demonstrate how substrate or lateral constrains hinder twin boundary motion, while complete untwinning only prevails in the presence of a flexible substrate or completely free foils. Experimentally observed pop-in events can be rationalized as local austenite → martensite transitions. Surface softness, which is observed by low indentation moduli, when compared to predictions from the bulk elastic constants, might indicate a more fundamental scenario close to the martensite transformation.     

Transformation behavior and shape memory effect of Ti50-xNi48Fe2Nbx alloys by aging treatment

The influence of aging treatment on transformation behavior and shape memory of the Ti50-xNi48Fe2Nbx(x=0,0.6,0.8,1.0,and 1.2)alloys was investigated using differential scanning calorimeter(DSC),mechanical drawing machine,and microhardness tester in this paper.It is indicated that the aging treatment has a significant effect on the phase transformation temperatures(Ms,Mf,Mp,As,Af,and Ap)and microhardness of the samples.The phase transformation temperatures are found to decrease initially with the increasing aging temperature from 300 to 500 ℃ and increase with further increase of the aging temperature.The aging treatment at intermediate temperature between 400 and 500 ℃ results in an improved shape memory effect.In addition,the highest microhardness value is also obtained.