Ni-Fe-Ga磁致形状记忆合金中间马氏体相变

采用差示扫描量热和x射线衍射技术研究Ni-Fe-Ga磁致形状记忆合金的马氏体相变行为.结 果发现,在多晶Ni565_56.5Fe190_19.0Ga245_{2 4.5}和Ni563_56.3Fe170_17.0 Ga267_26.7合金中除马氏体相变外,还观察到一次完整的、正相变和逆相 变对应出现、单 纯由温度诱发的中间马氏体相变.该中间马氏体相变与马氏体相变均为热弹性相变. 关键词： Ni-Fe-Ga 中间马氏体相变 磁致形状记忆合金     

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni-Mn-Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using ⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni-Mn-Ga-Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.     

Stress analysis, structure and magnetic properties of sputter deposited Ni-Mn-Ga ferromagnetic shape memory thin films

The residual stress instituted in Ni-Mn-Ga thin films during deposition is a key parameter influencing their shape memory applications by affecting its structural and magnetic properties. A series of Ni-Mn-Ga thin films were prepared by dc magnetron sputtering on Si(1 0 0) and glass substrates at four different sputtering powers of 25, 45, 75 and 100 W for systematic investigation of the residual stress and its effect on structure and magnetic properties. The residual stresses in thin films were characterized by a laser scanning technique. The as-deposited films were annealed at 600 °C for 1 h in vacuum for structural and magnetic ordering. The compressive stresses observed in as-deposited films transformed into tensile stresses upon annealing. The annealed films were found to be crystalline and possess mixed phases of both austenite and martensite, exhibiting good soft magnetic properties. It was found that the increase of sputtering power induced coarsening in thin films. Typical saturation magnetization and coercivity values were found to be 330 emu/cm³ and 215 Oe, respectively. The films deposited at 75 and 100 W display both structural and magnetic transitions above room temperature.     

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L2₁ cubic austenite by Cu substituting for Ni in the Ni_50-xCu_xMn₃₁Ga₁₉ (x=2--10) alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius (k_F) is applied to evaluate the change of the martensitic transformation temperature. The increase of k_F leads to the increase of the martensitic transformation temperature.     

Magnetoresistance in ferromagnetic shape memory alloy NiMnFeGa

The magnetoresistance (MR){=[R(H)−R(0)]/R(0)} properties in ferromagnetic shape memory alloy of NiMnFeGa ribbons and single crystals, and NiFeGa ribbons have been investigated. It is found that the NiMnFeGa melt-spun ribbon exhibited GMR effect, arising from the spin-dependent scattering from magnetic inhomogeneities consisting of antiferromagnetically coupled Mn atoms in B2 structure. In the absence of these magnetic inhomogeneities, Heusler alloys seem to show a common linear MR behavior at around 0.8T_C, regardless of sample structures. This may be explained by the s-d model. At low temperatures, conventional AMR behaviors due to the spin-orbital coupling are observed. This is most likely due to the diminished MR from s-d model because of much less spin fluctuation, and is not associated with martensite phase. MR anomaly at intermediate field (ρ_⊥>ρ_||) is also observed in single crystal samples, which may be related to unique features of Heusler alloys.     

Laser beam interaction with Ni-Mn-Ga ferromagnetic shape memory alloys

Laser beam interaction with alloys belonging to the Ni-Mn-Ga ferromagnetic shape memory alloy group has been investigated in order to assess the possibilities to use laser radiation for processing these materials, as for machining, joining or surface modification. The brittleness of Ni-Mn-Ga alloys is, as expected, an important factor and the formation of cracks appears to be difficult to be avoided. The laser interaction leads to vaporization on the impact zone and the changes in the chemical composition have been analyzed with respect to the beam parameters for penetration and surface melting, respectively.     

The effect of magnetic stress and stiffness modulus on resonant characteristics of Ni-Mn-Ga ferromagnetic shape memory alloy actuators

The prospect of using ferromagnetic shape memory alloys (FSMAs) is promising for a resonant actuator that requires large strain output and a drive frequency below 1 kHz. In this investigation, three FSMA actuators, equipped with tetragonal off-stoichiometric Ni₂MnGa single crystals, were developed to study their frequency response and resonant characteristics. The first actuator, labeled as A1, was constructed with low-k bias springs and one Ni-Mn-Ga single crystal. The second actuator, labeled as A2, was constructed with high-k bias springs and one Ni-Mn-Ga crystal. The third actuator, labeled as A3, was constructed with high-k bias springs and two Ni-Mn-Ga crystals connected in parallel. The three actuators were magnetically driven over the frequency range of 10 Hz-1 kHz under 2 and 3.5 kOe magnetic-field amplitudes. The field amplitude of 2 kOe is insufficient to generate significant strain output from all three actuators; the maximum magnetic-field-induced strain (MFIS) at resonance is 2%. The resonant MFIS output improves to 5% under 3.5-kOe amplitude. The frequency responses of all three actuators show a strong effect of the spring k constant and the Ni-Mn-Ga modulus stiffness on the resonant frequencies. The resonant frequency of the Ni-Mn-Ga actuator was raised from 450 to 650 Hz by increasing bias spring k constant and/or the number of Ni-Mn-Ga crystals. The higher number of the Ni-Mn-Ga crystals not only increases the magnetic force output but also raises the total stiffness of the actuator resulting in a higher resonant frequency. The effective modulus of the Ni-Mn-Ga is calculated from the measured resonant frequencies using the mass-spring equation; the calculated modulus values for the three actuators fall in the range of 50-60 MPa. The calculated effective modulus appears to be close to the average modulus value between the low twinning modulus and high elastic modulus of the untwined Ni-Mn-Ga crystal.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Composition, structure and magnetic properties of sputter deposited Ni-Mn-Ga ferromagnetic shape memory thin films

Polycrystalline Ni-Mn-Ga thin films were deposited by the d.c. magnetron sputtering on well-cleaned substrates of Si(1 0 0) and glass at a constant sputtering power of 36 W. We report the influence of sputtering pressure on the composition, structure and magnetic properties of the sputtered thin films. These films display ferromagnetic behaviour only after annealing at an elevated temperature and a maximum saturation magnetization of 335 emu/cc was obtained for the films investigated. Evolution of martensitic microstructure was observed in the annealed thin films with the increase of sputtering pressure. The thermo-magnetic curves exhibited only magnetic transition in the temperature range of 339-374 K. The thin film deposited at high sputtering pressure of 0.025 mbar was found to be ordered L2₁ austenitic phase.     

Combined experimental and theoretical study on the effect of Nb content on martensitic transformation of NbRu shape memory alloys

The effect of Nb content on the martensitic transformation of NbRu high-temperature shape memory alloys is investigated by experiments and first-principles calculations. We calculate the lattice parameters, density of states, charge density, and heats of formation of Nb_50+xRu_50-x β phase. The results show that an increase in Nb content increases the stability of Nb_50+xRu_50-x β phase, leading to a significant decrease of the β to β' martensitic transformation temperature. In addition, the mechanism of the effects of Nb content on phase stability and martensitic transformation temperature is studied on the basis of electronic structure.     

冲击诱发NiTi形状记忆合金相变行为研究

NiTi形状记忆合金的高应变动态响应特性在军事、航空等领域具有重要应用.为研究NiTi合金在动态力学诱导下的相变行为,在不同温区不同冲击速率下,通过轻气炮装置对NiTi合金进行了动态加载实验.利用差示扫描量热仪(DSC),综合物性测量系统分析了冲击波残余效应对NiTi合金相变行为的影响.研究发现:受冲击的样品在第一次DSC热循环中观察到了三个马氏体吸热峰,表现为三步逆马氏体相变,而在第二次热循环中其中两个应力诱发马氏体吸热峰因变形恢复消失.形成两个应力诱发马氏体吸热峰的原因可能是晶粒内部与晶界处的相变过程不同步.受冲击后样品DSC放热峰上出现了一小肩峰,表明可能因中间相(R相)的出现而发生了两步相变,结合电阻测量曲线进一步确认R相的存在,且发现奥氏体相向R相转变以及R相向马氏体相转变这两种相变过程在某一温度范围内可同时进行.同时,文中也具体讨论了不同的冲击加载条件对相变过程的影响.     

A macro-mechanical constitutive model of shape memory alloys

It is of practical interest to establish a precise constitutive model which includes the equations describing the phase transformation behaviors and thermo-mechanical processes of shape memory alloy (SMA). The microscopic mechanism of super elasticity and shape memory effect of SMA is explained based on the concept of shape memory factor defined by the author of this paper. The conventional super elasticity and shape memory effect of SMA are further unified as shape memory effect. Shape memory factor is redefined in order to make clear its physical meaning. A new shape memory evolution equation is developed to predict the phase transformation behaviors of SMA based on the differential relationship between martensitic volume fraction and phase transformation free energy and the results of DSC test. It overcomes the limitations that the previous shape memory evolution equations or phase transformation equations fail to express the influences of the phase transformation peak temperatures on the phase transformation behaviors and the transformation from twinned martensite to detwinned martensite occurring in SMA. A new macro-mechanical constitutive equation is established to predict the thermo-mechanical processes realizing the shape memory effect of SMA from the expression of Gibbs free energy. It is expanded from one-dimension to three-dimension with assuming SMA as isotropic material. All material constants in the new constitutive equation can be determined from macroscopic experiments, which makes it more easily used in practical applications. Supported by the National Natural Science Foundation of China (Grant No. 95505010), the National High Technology Research and Development Program of China (Grant No. 2006AA03Z109), the China Postdoctoral Science Foundation (Grant No. 20080430933), the Open Foundation of Institute of Engineering Mechanics of National Seism Bureau of China (Grant No. 2007B02), and the Harbin Talent Foundation of Scientific and Technical Innovation (Grant No. RC2009QN-017046)     

Successive phase transformation in ferromagnetic shape memory alloys Co37Ni34Al39 melt-spun ribbons

The martensitic transformation in Co37Ni34Al29 ribbon is characterized in detail by means of in-situ thermostatic x-ray diffraction and magnetic measurements.The results show a structural transition from the body-centred cubic to martensite with a tetragonal structure during cooling.Comparison between the results of the diffraction intensity with the magnetic susceptibility measurements indicates that the martensitic transformation takes place in several different steps during cooling from 273 to 163 K.During heating from 313 to 873 K,the peak width becomes very wide and the intensity turns very low.The γ-phase(face-centred cubic structure) emerges and increases gradually with temperature increasing from 873 to 1073 K.     

Photoemission study of the (1 0 0) surface of Ni2MnGa and Mn2NiGa ferromagnetic shape memory alloys

The (1 0 0) surface of Ni₂MnGa and Mn₂NiGa ferromagnetic shape memory alloys have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). It is shown that by sputtering and annealing, it is possible to obtain a clean, ordered and stoichiometric surface that shows a four-fold 1 × 1 LEED pattern at room temperature. For both Ni₂MnGa and Mn₂NiGa, the surface becomes Ni-rich and Mn deficient after sputtering. However, as the annealing temperature is increased Mn segregates to the surface and at sufficiently high annealing temperature the Mn deficiency caused by sputtering is compensated. The (1 0 0) surface of Ni₂MnGa is found to have Mn-Ga termination. The valence band spectra of both Ni₂MnGa and Mn₂NiGa exhibits modifications with surface composition. For the stoichiometric surface, the origin of the spectral shape of the valence band is explained by calculations based on first principles density functional theory.     

Surface characteristics and nanoindentation study of Ni-Mn-Ga ferromagnetic shape memory sputtered thin films

In present paper, the off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloy thin films are fabricated using radio frequency magnetron sputtering method. The compositions, microstructures and mechanical properties of the thin films are characterized by energy dispersive X-ray spectrum (EDAX), X-ray photoelectron spectroscopy (XPS), scanning electronic microscope (SEM), atomic force microscope (AFM) and nanoindentation test, respectively. The results show that there is a thinner layer of oxides consisting of NiO, Ga₂O₃ and an unspecified manganese oxidation (Mn_xO_y) at the surface, whereas a small amount of MnO precipitates exist in internal layers of post-annealed Ni-Mn-Ga thin films. The hardness and elastic modulus decrease with increasing film thickness. Nanoindentation tests reveal that the hardness and elastic modulus of the films can be up to 5.5 and 155 GPa, respectively. The Ni-Mn-Ga thin films have remarkably improved the ductility of Ni-Mn-Ga ferromagnetic shape memory alloys bulk materials.     

The effect of Fe on the martensitic transformation of TaRu high-temperature shape memory alloys:A first-principles study

The effect of Fe on the martensitic transformation of TaRu high-temperature shape memory alloys has been investigated using first-principles calculations. The site preference of Fe in TaRu alloys has been clarified for the first time, and the results show that Fe is predicted to occupy Ru sites. The addition of Fe increases the stability of the Ta₅₀Ru_50-xFe_x β phase, leading to a significant decrease in the β to β′ martensitic transformation temperature. In addition, the mechanism of the Fe alloying effect is explained on the basis of the electronic structure.     

Influence of the atomic order on the magnetic characteristics of a Ni–Mn–Ga ferromagnetic shape memory alloy

The influence of the atomic order on the magnetic properties has been analyzed in a polycrystalline Ni_49.5Mn_28.5Ga₂₂ ferromagnetic shape memory alloy prepared by arc melting under Ar atmosphere. Different thermal treatments have been performed to modify the order degree of the alloy. The effect of the different thermal treatments on the magnetic and structural characteristics has been analyzed by superconducting quantum interference device (SQUID) and differential scanning calorimetry (DSC) measurements. The magnetic and structural properties of the alloys are modified as a consequence of the atomic order change. The martensitic transformation temperatures increase as long as the order degree increases. On the other side, the Curie temperature and magnetization saturation also reflect the order degree of the alloy but seems to be linked to the particular order of the Mn sub-lattice.     

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.     

Martensitic transformation and related magnetic effects in Ni-Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni-Mn-based alloys are reviewed based on our recent research results.     

Martensitic transformation and magnetic properties of ferromagnetic shape memory Fe66Pd30Rh4 alloys

This study mainly shows that in the Fe₆₆Pd₃₀Rh₄ (at%) alloys, the L1₀ phase plays an important role in magnetostriction due to the interplay of L1₀ martensitic twins with magnetic domains. The L1₀ martensitic twin structure exhibits a strong magnetocrystalline anisotropy energy constant (K_u=1.27-2.84×10⁶ (ergs/cm³)) along the tetragonal c axis direction. In addition, the L1₀ tetragonal martensitic twin structure shows both a perfect shape memory and a reversible shape memory effect; therefore, it is expected to be applicable in magneto-mechanical applications (such as microactuators or springs). However, in this study, we discover that solution treatment (ST) and aging heat treatments of Fe₆₆Pd₃₀Rh₄ ferromagnetic shape memory alloys influence the behavior of the martensitic transition, which is associated with the change in magnetic properties. The process of a thermoelastic L1₀+L1_m twin phase decomposition→non-thermoelastic L1₀+L1_m+α_bct structure in Fe₆₆Pd₃₀Rh₄ alloys during solution treatment and aging at 400-550 °C for various times is studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The relation of phase separation morphology to the magnetic property change is examined with a superconducting quantum interference device (SQUID) magnetometer, and magnetostriction measurement is performed with a strain gage method and magnetostrictive meter setup. The results indicate that the process of martensitic transformation during aging leads to an increase in coercivity and a decrease in magnetostriction, simultaneously.