Characterization and properties of ferromagnetic shape memory alloys

Ni–Mn–Ga shape memory alloys are employed for applications in actuators and sensing devices. These alloys exhibit ferromagnetic shape memory effect with large reproducible strains in moderate magnetic fields. This work presents a study of the effect of composition and annealing treatment on the microstructure and magnetic properties in Mn-rich off-stoichiometric Ni–Mn–Ga alloys. Modulated martensitic structure (c/a < 1) with hierarchical twins was found at room temperature in alloys with Mn ≥ 28 at.% whereas the alloy containing higher Ga (> 22 at.%) revealed austenitic structure at room temperature. Ferromagnetic nature of the alloys was confirmed by the magnetization curves. It is demonstrated that a maximum of 400 parts per million strain was measured in the alloy with 7 M martensitic structure at room temperature.     

Effect of small γ-precipitates on the two-way shape memory effect in Cu–Zn–Al alloys

The γ-precipitates in Cu–Zn–Al alloys, trained by the stabilization of the stress induced martensite (SSIM) method, have been studied. After the SSIM treatment, it was found that small γ-precipitates in the β-austenite are ellipsoidal, with a large strain field oriented in the same direction; while in the martensite the γ-precipitates changed their shape from ellipsoid to spheroid, and relaxed their strain fields. In order to check whether the strain field of the γ-precipitates is capable of producing a thermoelastic martensitic transformation, an in-situ observation, by heating a sample holder in TEM, was performed. It was found that during heating over a temperature A_s, the γ-precipitates with a spherical shape in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ-precipitates disappeared again. It was proposed that the strain field of the γ-precipitates, trained by the SSIM method, plays an important part in the thermoelastic martensitic transformation, and presents two-way shape memory effects.     

Martensitic transformation and shape memory effect in Fe–Mn–Si based alloys

The research status of the Fe–Mn–Si based alloys is reviewed with emphasis on the recent progress in the martensitic transformation and the associated shape memory effect (SME). Particular interest is given to the fcc(γ)–hcp(ε) transformation mechanism in the alloys featured by low stacking fault energy and the approaches aiming to the enhancement of SME through alloy design including microalloying and microstructure control by introducing texture and precipitates into the parent γ matrix. Potential topics of oncoming focus are briefly highlighted.     

A new formulation for martensite start temperature of Fe–Mn–Si shape memory alloys using genetic programming

This study presents genetic programming (GP) soft computing technique as a new tool for the formulation of martensite start temperature (M_s) of Fe–Mn–Si shape memory alloys for various compositions and heat treatments. The objective of this study is to provide a different formulation to design composition at certain ranges and to verify the robustness of GP for the formulation of such characterization problems. The training and testing patterns of the proposed GP formulation is based on well established experimental results from the literature. The GP based formulation results are compared with experimental results and found to be quite reliable.     

Internal friction behaviour of Ni–Mn–Ga

The internal friction (IF) behaviour of shape memory alloys (SMA) is characterised by an IF peak and a minimum of the elastic modulus during the martensitic transformation (MT), and a higher IF value in the martensitic state than in parent phase. The IF peak is considered to be built of three contributions, the most important of them being the so-called “transient” one, existing only at non-zero temperature rate. On the other hand, the ferromagnetic Ni–Mn–Ga system alloys undergoes a MT from the L2₁ ordered parent phase to martensite, the characteristics of the transformation depending largely on the e/a ratio of the alloys. Indeed, a variety of transformation sequences, including intermediate phases between parent and martensite and intermartensitic transformations, have been observed for a wide set of studied alloys. Furthermore, the IF and modulus behaviour during cooling and heating these alloys show specific characteristics, such as modulus anomalies, strong temperature dependence of the elastic modulus, temperature dependent internal friction in martensite, and, as a general trend, a low transient contribution to the IF. In the present work, the IF and modulus behaviour of several Ni–Mn–Ga alloys will be reviewed and compared to that observed for “classical” systems like Cu- or NiTi-based shape memory alloys.     

Differently mobile twin boundaries and magnetic shape memory effect in 10 M martensite of Ni–Mn–Ga

We investigated twin boundaries with sharply different mobility or twinning stress in five-layered modulated (10 M) martensite of Ni–Mn–Ga exhibiting magnetic shape memory effect or magnetically induced reorientation. Different mobility is ascribed to the different microstructures of the macrotwin planar interface. In monoclinic approximation the mobile boundaries are of Type I and Type II with complex microstructure of adjoining variants. These boundaries respond differently to magnetic field. For Type II boundary the reorientation takes place at very low field 250 Oe.     

Effect of Small γ Precipitates on the Two-way Shape Memory Effect in a Cu–Zn–Al Alloy

After the treatment for the Stabilization of Stress-Induced Martensite (SSIM) in Cu–Zn–Al alloys, it was found that the small γ precipitates in the β austenite are ellipsoidal with a large strain field oriented in the same direction, while in the martensite, the γ precipitates changed their shape from ellipsoidal to spheroidal, which relaxed the strain field. To check whether the strain field of the γ precipitates is available to produce thermoelastic martensitic transformation, in situ observations with a heating sample holder in TME were performed. It was found that after heating above the A_s temperature, the spherical γ precipitates in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ precipitates disappeared again. This means that the strain fields of the γ precipitates trained by the SSIM method play an important part in the thermoelastic martensitic transformation that presents the two-way shape memory effect.     

Phase transition and mechanical properties of constraint-aged Ni–Mn–Ga–Ti magnetic shape memory alloy

Ni–Mn–Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni–Mn–Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni–Mn–Ga–Ti alloy and about 900 MPa in Ni–Mn–Ga–Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni–Mn–Ga–Ti alloy.     

Microstructure characterization and effect of thermal cycling and ageing on vanadium-doped Cu–Al–Ni–Mn high-temperature shape memory alloy

The effect of vanadium addition on the microstructure of Cu–Al–Ni–Mn high-temperature shape memory alloy (SMA) and its thermal cycling and ageing behaviour has been investigated. Using scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction analysis, the morphology, distribution and structure of secondary phase, induced by vanadium addition, have been identified. The effect of secondary phase on grain refining of Cu–Al–Ni–Mn has also been revealed. Differential scanning calorimetry measurement was used to investigate the effect of thermal cycling and ageing on the transformation temperature. It has been found that thermal cycling has a strong influence on the transformation temperature of the present Cu–Al–Ni–Mn–V high-temperature SMA. Ageing also caused an apparent change of the transformation temperature. It has been suggested that this was mainly due to the precipitation of secondary phase, because the sample was heated to a rather high temperature in both thermal cycling and the ageing process. The experiment showed that the transformation temperature could be maintained stable in the thermal cycling process by pre-ageing the sample at a suitable temperature.     

Phase transformation,thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni₅₀Mn₃₀Ga_20−xCu_x alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement.     

Texture evolution analysis of warm-rolled Fe–28Mn–6Si–5Cr shape memory alloy

Since texture control tends to be a promising way to improve the shape memory effect (SME) of polycrystalline Fe–Mn–Si shape memory alloys, rolling texture evolution of an Fe–28Mn–6Si–5Cr shape memory alloy was systematically investigated with orientation distribution functions (ODFs) and electron backscattering diffraction (EBSD) analysis. At the rolling temperature of 873 K, Copper-type texture components, including D, S, Goss, as well as a weak Brass, obviously develop before 44% rolling reduction. With increased rolling reduction to 57%, D orientation abruptly disappears, which indicates a texture transition has occurred. S orientation and α fiber texture except the Goss orientation undergo a decrease accompanying the intensification of γ fiber texture. In the whole deformation processes, Goss orientation is the dominant texture component while no pronounced Brass component is observed. The dominant Goss component can be attributed to the preferred Goss orientation both in shear bands and in matrix. When the rolling temperature is decreased to 573 K, even at the early deformation stage, 42% rolling reduction, both D and Brass orientations are not observed. EBSD analysis confirms that the texture evolution is promoted to the early deformation stages at lower rolling temperature.     

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co₄₉Ni₂₁Ga₃₀ alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.     

Hot stage nanoindentation in multi-component Al–Ni–Si alloys: Experiment and simulation

The mechanical properties of individually pure and intermetallic phases of typical Al–Ni–Si piston alloys are investigated at different temperatures using hot stage nanoindentation. The hardness and the indentation modulus of a number of phases are determined at room temperature, 500 K and 650 K. Both, hardness and reduced modulus drop with increasing temperature in different ratios for the various phases. Increasing Ni content in the grains improves the mechanical stability of the material at elevated temperatures in general. The indentation patterns are studied using atomic force microscopy with particular reference to the indentation depths and pile-up effects. Site-specific samples from the material surrounding the nanoindents are prepared using a focussed ion beam field emission gun for examination in the transmission electron microscope. This allows direct observation of material changes as a result of the indentation process in the different phases within the alloy system.Corresponding linked atomistic finite element calculations have been carried out for Si and Ni–Al systems as a function of increasing Ni content at various temperatures. The results show only a small difference in the mechanical behaviour of Si between 300 K and 650 K as observed in the experiments. Large differences for Al at both temperatures studied result in an increase of plasticity with rising temperature and atomic motion that changes from slip in well-defined planes to a viscous fluid-like behaviour. The formation of dislocations and slip bands during indentation for the Ni–Al systems is studied.     

Interaction of Cu–Al–Ni shape memory alloys particles with molten In and In + Sn matrices

Metal matrix composites for high-damping application were produced by embedding soft metallic matrices (pure In, In–10 wt.% Sn and In + Sn eutectic alloys) in powders of Cu–Al–Ni shape memory alloys (SMAs). During the composite production, the shape memory alloy particles interact with the molten matrices giving place to Cu dissolution from the shape memory alloy particles to the matrices, grain boundary penetration, and formation of intermetallic compounds. Adhesion, wetting and interfacial reaction are crucial for the final composites properties. Preliminary results on microstructural investigations performed applying optical and electron microscopy are presented in this contribution. The influence of thermal treatments on the microstructure of one composite is also discussed.     

新型磁驱动形状记忆合金研究进展

磁驱动形状记忆合金是一种新型功能材料,由于兼具大的输出应变和高响应频率等综合特性,成为智能材料领域的研究热点之一.本研究首先总结了Ni-Mn-Ga合金在相变和磁致应变性能方面的特点,然后着重介绍了Co-Ni-Ga和Ni-Fe-Ga两类新型磁驱动记忆合金在结构、相变、形状记忆效应、磁性能等方面的研究进展,并对其中存在的问题进行了讨论.     

Strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel

The strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel was investigated through the modified Crussard–Jaoul (C–J) analysis and microstructural observations. The strain hardening rate obtained by modified C–J analysis was high up to the critical strain of 37% and then greatly decreased with further strain. The electron backscatter diffraction (EBSD) observation showed that the deformation twinning rate is greatly decreased beyond about 34% strain, indicating that the reduced strain hardening rate at the large strain region is attributed to the deceleration of deformation twinning rate. The volume fraction of twinned region was increased with tensile strain due to the increase in the number of deformation twins not to the lateral growth of each deformation twin.     

Strain localisation in solution heat treated Al–Zn–Mg alloys

Two Al–Zn–Mg alloys with recrystallised and fibrous grain morphologies are studied with regards to the microstructure after solution heat treatment, cold water quenching and immediate room temperature deformation. It was found that the dislocation movement was localised in narrow slip bands cutting through the dislocation tangles. This observation is related to dynamic strain ageing and to macroscopic shear bands frequently observed in these alloys.     

磁驱动相变材料研究进展

磁驱动相变材料利用外磁控制下铁弹马氏体变体重排或磁诱导一级相变产生的形状记忆效应来捕获应变,兼具铁弹形状记忆与磁致伸缩功效特征。Heusler型Ni-Mn-X(X=Ga或In)系磁驱动相变合金材料具有磁感生应变大、能量密度高、反应速度快等优点,是未来重要磁传感器和磁驱动器研制的关键。主要介绍了国内外Ni-Mn-Ga、Ni-Co-Mn-In、反铁磁体等磁驱动相变材料的研究进展,以及本课题组利用高能X射线衍射和中子散射技术对磁驱动相变材料的原位研究。最后,展望了磁驱动相变合金材料的发展趋势。     

Key Properties of Ni?Mn?Ga Based Single Crystals Grown with the SLARE Technique

Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of Ni? Mn? Ga based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of Ni? Mn? Ga, Ni? Mn? Ga? Fe, and Ni? Mn? Ga? Co of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. Ni? Mn? Ga alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as Ni? Mn? Ga with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.     

Molecular dynamics study of surface effect on martensitic cubic-to-tetragonal transformation in Ni–Al alloy

Molecular dynamics (MD) study of martensitic transformation (MT) in nickel and aluminum alloy is performed. The behavior focused on is transformation between crystalline structures from B2 cubic cell to body-centered tetragonal cell, which is simply realized by uniaxial tensile loading. The potential function used is Finnis–Sinclair type having only single energy minimum where B2 structure exists. The availability of this specific many-body potential for stress-induced MT phenomena under uniaxial loading is fully discussed. In MD simulations, martensite phase is induced by tensile stress or strain in the atomic system, as predicted by a potential energy map. It is understood that the characteristic of the potential energy function with regard to deformation is crucial for MT studies and investigating energy-strain or stress–strain map is worthwhile. The MT behavior in the atomic system occurs during a plateau region of stress–strain (S–S) curve of the whole specimen, that is typical for experimental superelastic or shape-memory alloys under uniaxial loading. It is found that, during each MT event, large jump of atomic strain is observed. Owing to single energy minimum, the atomic system shows almost perfect recovery in S–S curve, where the graph comes completely back to initial state after unloaded. Besides, the present paper focuses on surface effect for MT behavior. Since the surface effect is dominant in MT phenomena especially in microscopic specimens, a novel computational scheme for stabilizing condition in which uniaxial loading is always applied together with arbitrary periodic boundary condition(s) is devised. By comparing one-, two-, and three-dimensional models under uniaxial loading, it is recognized that the nucleation behavior depends strongly on the existence of free surface region (including corner edge). When there is no surface, a chaotic nucleation of martensite is observed. On the other hand, the free surface induces first martensite because of less constraint in tensile deformation of unit cells. It is confirmed that the tendency toward MT nucleation corresponds to yield stress or strain of the specimen. In order to define and detect martensite structure as for each atom, an atomic strain measure (ASM) with our own formation is introduced. It is shown that the ASM is very effective to distinguish martensite bct unit structure from others.