First-principle band calculations of ordered 3d and 4d transition-metal alloys Fe(Rh,Pd) with the CsCl and CuAu-I type structures are carried out by a linear muffin-tin orbital method within an atomic sphere approximation, where Rh and Pd atoms are treated as virtual 4d-atoms with the atomic number averaged over the concentration. A generalized gradient correction for exchange-correlation potential is taken into account. Total energies for paramagnetic, ferromagnetic and three kinds of antiferromagnetic states are estimated as a function of lattice constants, a and c. Observed lattice constants and spin structures for the present alloys are well described by the present calculations.

Interplay between magnetism and crystal volume in itinerant electron ferromagnets is reexamined paying particular attention on effects of spin fluctuations. By introducing Grüneisen parameters, magnetovolume properties are derived from the explicit volume dependence of the free energy. As the result, we have clarified the presence of new thermal expansions, giving rise to the enhanced T-linear coefficient of the thermal expansion coefficient at low temperatures around the magnetic instability point. We have also found that the magnetovolume coupling constant is temperature dependent and vanishes at the critical point. These results are compared with experiments.

The electronic structures were systematically calculated for Heusler alloys X₂YZ (X and Y = 3d transition element, Z = IIIb, IVb, Vb element). The results show that alloys Fe₂CrZ have the high density-of-state at the Fermi energy in the majority-spin state and show high spin polarization. The effects of chemical disorder on the half-metallicity and magnetic moment are discussed on the basis of the electronic structures. Considering three types of chemical disorder, we show that among the alloys Fe₂CrZ, there are high spin polarized materials insensitive to chemical disorder and that Fe–Cr chemical disorder may occur.

Metallic manganese nitrides Mn₃AN (A=Zn, Ga, etc) are well-known for their large magnetovolume effect (MVE), i.e., a discontinuous volume expansion at the magnetic transition. However, MVE is exceptionally absent in Mn₃CuN. We found that MVE is recovered by a small amount of Ge in the Cu site. This revival seems to coincide with recovery of the cubic structure. By further Ge doping, the volume expansion becomes gradual (ΔT∼100 K) and large negative thermal expansion (NTE) is exhibited around room temperature [α=−12×10⁻⁶ K⁻¹ (α: coefficient of linear thermal expansion) for Mn₃(Cu_0.5Ge_0.5)N]. Such a large, isotropic and non-hysteretic NTE is desirable for practical applications.

The electronic structures of anti-perovskite-type intermetallic compound Ni₃AlX_y (X=B, C, H; 0<y<1) have been calculated using coherent-potential approximation (CPA) within the local-density approximation (LDA). Ferromagnetic moment in Ni₃Al rapidly decreases with increasing y for every dopant X, even though the lattice is more expanded than non-doped Ni₃Al.

Effects of pressure P on the magnetic moment M and the Curie temperature T_C have been investigated for La(Fe_xSi_1−x)₁₃ above and below the magnetic phase boundary concentration x=0.86, where the ferromagnetic-paramagnetic transition at T_C changes from the first-order (x≥0.86) to the second-order (x<0.86). The pressure coefficient of M exhibits a sluggish variation against concentration and no anomaly was observed at x=0.86, being consistent with the Landau expansion model. On the other hand, T_C for the second-order transition has a large negative pressure coefficient dT_C⁄dP and its magnitude increases with increasing x. Above x=0.86, the magnitude of dT_C⁄dP for the first-order transition increases with x, contrary to the theoretical expectation. It has been revealed that the spin-wave dispersion coefficient becomes smaller when the first-order transition becomes clear by changing the concentration and also applying pressure. Consequently, it is plausible that dT_C⁄dP above x=0.86 is enhanced by the increase of instability of the ferromagnetic state.

The Curie temperature T_C of La(Fe_xSi_1−x)₁₃ is increased by a partial substitution of Ni for Fe. On the contrary, T_C is decreased by a partial substitution of Cr or Mn. In addition, a partial substitution of Ce for La in La(Fe_xMn_ySi_1−x−y)₁₃ causes the further decrease of T_C. As a result, La_0.65Ce_0.35(Fe_0.85Mn_0.03Si_0.12)₁₃ exhibits a thermal-induced first-order transition at T_C=60 K. This result means that T_C of the La_1−zCe_z(Fe_xMn_ySi_1−x−y)₁₃ is tunable in the temperature range between 60 and 180 K by adjusting composition with keeping the itinerant-electron metamagnetic transition. In the magnetic field change from 0 to 4 T in the vicinity of T_C=60 K, the La_0.65Ce_0.35(Fe_0.85Mn_0.03Si_0.12)₁₃ shows the isothermal magnetic entropy change ΔS_m=−13 J kg⁻¹ K⁻¹ and the relative cooling power RCP=458 J kg⁻¹. Consequently, the La_1−zCe_z(Fe_xMn_ySi_1−x−y)₁₃ compounds are useful for magnetic refrigerants working in a temperature range between 60 and 180 K.

We studied effects of heat treatment on the magnetic transition and the magnetocaloric effects (MCEs) of Mn_1+δAs_1−xSb_x with x=0.1 and 0.25. It is found that sintering at an appropriate temperature gives a sharp magnetic transition and hence giant MCEs for Mn_1+δAs_1−xSb_x. In the bulk samples slowly cooled from a liquid state, free Sb was precipitated, which gives rise to compositional deviation. Quenching from a liquid state suppresses the precipitation of Sb. Subsequent annealing is effective to improve homogeneity, as a result, the sample undergoes a sharp magnetic transition. The optimum conditions of heat treatments of the present system for a sharp magnetic transition are discussed.

We have performed the X-ray powder diffraction measurements for Mn₃Ga_0.97Al_0.03C with a perovskite-type structure in the temperature range from 8 to 320 K in magnetic fields up to 5 T, in order to investigate the structural properties affected by the magnetic field. The compound has a successive magnetic phase transition below 160 K: the ferromagnetic (F)–intermediate (I)–antiferromagnetic (AF) phases for cooling process. The lattice parameter a abruptly increases by 0.23%, accompanied by the transition from the I to AF phases at T_AF–I=130 K. In addition, we clearly observed that magnetic field induces the I phase with small a and suppresses the AF phase with large a below T_AF–I.

Precise magnetization measurements have been made on the weak itinerant electron ferromagnet CoVSb. The magnetic moment at 4.2 K and the Curie temperature T_C are 0.16 μ_B/f.u. and 45 K, respectively. Below 10 K, the decrease in the square of the spontaneous magnetization M_s(T)² is proportional to T². However, over a wide temperature range from 24 K to the Curie temperature, the decrease in M_s(T)² is proportional to T^4⁄3. The results obtained are analyzed using spin fluctuation theory.

We have carried out resistivity measurements on itinerant antiferromagnets Nb_12−xTi_xO₂₉ (x=0 and 0.2) under pressure up to 2.5 GPa in order to investigate the pressure effects on these novel magnetic and metallic behaviors. The resistivity-temperature curves for both systems show metallic behavior at lower pressure, while the resistivity increases with decreasing temperature at higher pressure. These results indicate that a metal-insulator transition is induced by the application of the pressure in both systems. We will discuss the origin of the metal-insulator transition in terms of charge ordering and pressure-induced amorphization as have been suggested in other niobium oxides.

Effect of annealing temperature on microstructure and shape memory characteristics of Ti–22Nb–6Zr(at%) biomedical alloys was investigated by using tensile tests, XRD measurement, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After severe cold-rolling, the plate was annealed at temperatures between 773 and 1173 K. The α⁄β transus temperature in this alloy was determined to be between 823 and 873 K. The specimen annealed at 823 K for 3.6 ks exhibited a fine subgrain structure. A fully recrystallized structure was observed in the specimens annealed above 873 K. The annealing temperature less affected the transformation temperature and recovery strain. However, the critical stress for slip decreased considerably with increasing annealing temperature, because the grain size increased. All specimens annealed above 823 K exhibited stable superelastic behavior at room temperature.

The effects of Sn content and aging conditions on superelasticity in Ti–Mo–Sn alloys were investigated. Martensitic transformation temperature decreased with an increasing of Sn content. A large superelastic strain of 3.0% was obtained in a solution-treated Ti–5 mol%Mo–5 mol%Sn alloy in the tensile test. The superelasticity in the Ti–5 mol%Mo–5 mol%Sn at room temperature was improved by aging at 873 K for short periods between 180 and 420 s. A specimen aged at 873 K for 300 s exhibited superelasticity with a recovery strain of 3.5% in the tensile test. A recovery strain of 3.0% was consistently achieved in cyclic tensile deformations.

Effect of Nb addition on mechanical properties and shape memory behavior of Ti–Mo–Ga alloys was investigated by cyclic loading-unloading tensile tests, tensile tests at various temperatures and Vickers hardness tests. The martensitic transformation start temperature (Ms) decreased by 20 K with 1 at% increase of Nb content in the Ti–6Mo–3Ga(at%) alloy. The shape memory effect was observed in the Ti–6Mo–3Ga(at%) alloy. The superelastic strain increased with increasing Nb content. The increase of the yield stress during the cyclic deformation decreased the strain recovery rate in the Ti–6Mo–3Ga(at%) alloy. Aging at intermediate temperatures resulted in increase in hardness of the Ti–6Mo–3Ga–(0–4)Nb(at%) alloys. The increase in hardness decreased significantly with increasing Nb content because the addition of Nb was effective to suppress the ω phase hardening. The yield stress decreased and the strain recovery rate increased with increasing number of cycle in the Ti–6Mo–3Ga–4Nb(at%) alloy.

NiTi-films were fabricated by DC magnetron sputtering from cast-melted disc targets. The obtained freestanding films revealed superelastic properties in tensile testing experiments. At 37°C superelastic properties were achieved showing a closed-loop hysteresis and a plateau of more than 5% strain. Photolithography and wet etching technology were applied in order to fabricate net-shaped devices. Achievable structure sizes range in the order of the NiTi film thickness, i.e. typically between 5 and 15 μm. Tensile testing experiments reveal a remarkable strain tolerance of these devices which summed up to a superelastic strain of up to 5%. It has been demonstrated that the deposition process can be transferred to the fabrication of NiTi tubes, which have high potential for application as vascular implants, e.g. stents.

Sputtered-deposited nickel titanium thin films are commonly amorphous when synthesized and require annealing to crystallize them. The resulting microstructures, which are governed by nucleation and growth kinetics, dictate the actuation properties. The evolution of these microstructures was studied using in situ transmission electron microscopy (TEM) heating methods. The experimentally-determined kinetic values of nucleation and growth were inserted into a mathematical expression derived from the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, which predicts the average grain size over a broad range of temperatures.

Thin film SMAs have the potential to became a primary actuating mechanism for micropumps. In this study, a micropump driven by TiNiCu shape memory thin film is designed and fabricated. The micropump is composed of a TiNiCu/Si bimorph driving membrane, a pump chamber and two inlet and outlet check valves. The property of TiNiCu films and driving capacity of TiNiCu/Si bimorph driving membrane are investigated. The film surface shows a smooth and featureless morphology without any cracks, and the hysteresis width ΔT of TiNiCu film is about 9°C. By using the recoverable force of TiNiCu thin film and biasing force of silicon membrane, the actuation diaphragm realizes reciprocating motion effectively. Experimental results show that the micropump driving by TiNiCu film has good performance, such as high pumping yield, high working frequency, stable driving capacity, and long fatigue life time.

In-situ TEM studies were conducted to reveal the crystallization features of equi-atomic TiNi amorphous thin films. The TiNi amorphous thin film crystallization procedure can be divided to be two types: the in-homogenous nucleation and growth mode in the ultra thin regions and the homogenous polymorphous mode in the thick areas. In the thin regions, the thickness controls the in-homogenous nucleation mode. The formed nano-crystallites in the thin areas are with a size of 5–20 nm while in the homogenous nucleation and growth mode, the grain size drops to the range of sub-micron level. In general, the stabilized grain size is a function of thin film thickness and can be described as G=kx, where x is the thickness in nano-meter and k is a constant related to lattice parameter. An intermediate phase forms through the crystallization procedure in the thick region. The intermediate phase possesses a cubic structure with lattice parameter of a=9.03 A. The intermediate phase transforms to the stable B2 phase when the specimen being kept above the crystallization temperature for some time. The crystallization sequence in the thick region is determined to be: TiNi amorphous → intermediate phase → B2 + Ti₃Ni₄.

Martensitic transformation behavior and shape memory properties of a Ti₅₀Ni₄₀Pt₁₀ (TiNiPt) melt-spun ribbon fabricated by a single roll melt-spinning technique were characterized. The constituent phases of the as-spun ribbon were B2 (parent phase) and B19 (martensite phase) at room temperature. The B2–B19 martensitic transformation temperatures of the as-spun ribbon were 100 K higher than those of the bulk-material with the same chemical composition. The martensitic transformation temperatures of the as-spun ribbon were decreased with increasing the temperature of the heat-treatment made after the melt-spinning. The as-spun ribbon and the heat-treated ribbons exhibited shape recovery by heating and/or pseudoelasticity. The martensitic transformation temperatures determined from the temperature dependence of the 0.2% flow stress of the pseudoelastic deformation were in good agreement with those of B2–B19 martensitic transformation determined by DSC. It was confirmed that the observed shape recovery and pseudoelasticity are shape memory effect and superelasticity due to the B2–B19 martensitic transformation. Shape memory effect and superelasticity of melt-spun TiNiPt alloy were found to appear at higher temperatures compared to those of Bulk-material with the same composition.

This paper describes the influence of severe plastic deformation (SPD) on the structure, phase transformations, and physical properties of melt-spun Ti₂NiCu-based and Ni₂MnGa-based shape memory intermetallic alloys. It was found that the SPD by high pressure torsion (HPT) at room temperature can be effectively used for the synthesis of bulk nanostructured states in these initially submicro-grained or amorphized alloys obtained by melt-spinning method in the form of a ribbon. The subsequent low-temperature annealing of HPT-processed alloys leads to formation of homogeneous ultrafine nano-grained structure. This is connected with a very high degree and high homogeneity of deformation at SPD in the whole volume of deformed samples.

This paper presents the fabrication condition of TiNi alloy powder by mechanical alloying and shape memory characteristics of the sintered alloy. The effect of mechanical alloying condition on the characteristics of mechanically alloyed powder (MA powder) was investigated. Also, the difference in sintering behavior between the MA powder and the elementally mixed powders by V-blender and the shape memory characteristics of the sintered alloys were also examined. The MA powder was fabricated by milling using a planetary ball mill in a rotational speed between 200 and 500 min⁻¹ for various milling times in an atmosphere of Ar gas. These two types of powders prepared in different processes were sintered using a pulse-current pressure sintering equipment at various sintering temperatures. The powder agglomerated and its particle size became larger with an increase in milling time. The mixture of Ti and Ni powders changed into an amorphous state by processing for 3.6 ks over 300 min⁻¹. The sintered alloy of the MA powder showed more uniform phase of TiNi than that of the elementally mixed powders sintered in a same manner, however, the former showed a lower density than the latter due to a larger particle size of the MA powder of before-sintering. It was found from the measurement of the transformation temperature of the sintered alloy of the MA powder using DSC that the alloy has shape memory characteristics, and the transformation temperatures of the alloy are higher than those of the alloy of the elementally mixed powders due to waste of Ni powder.

In order to fabricate the NiTi foams with greater than 80% porosity, a vacuum process applied to a slurry was developed. A mixture of elemental Ni and Ti powders was dipped into a solution of 7.5 mass% polyvinyl alcohol, and stirred to make the slurry. The lightly compacted lumps of slurry were then subjected to reduced atmospheric pressure to make foamed compacts. The green foams were debound and sintered under vacuum into NiTi sintered foams with 85% porosity. X-ray analysis showed alloying of NiTi was completed by the sintering at ≥1100°C. X-ray diffraction analysis and DSC measurement also indicated that the NiTi foams consisted of B2 austenite and B19′ martensite phases. Measurement of shape recovery strain showed the NiTi foams obtained by this process had the far excellent shape recovery characteristics compared with those of wrought NiTi alloys. Furthermore, repeated compressive deformation and heating greatly increased the shape recovery strains of these high-porosity NiTi foams.

The application of the microscopic theory of elasticity in a discrete lattice model is made on the transitional metal carbide and nitride precipitates formed in the Fe–Mn–Si–Cr shape memory alloy in conjunction with the improvement of shape memory effect (strain) and the improvement of strength. Two distinguishable methods of analysis have been established using the microscopic theory of elasticity in a discrete lattice model: One is the establishment of the description of precipitate and misfit dislocations in Fourier space. The second is the rigorous estimation of interaction energies among precipitate and misfit dislocations. The results could successfully describe the shape of the precipitate observed in the experimental investigation. It was also concluded that the elastic strain energy increases with the lattice parameter of the precipitate. Among the transition carbides and nitrides under investigations, VN, which revealed the minimum value of the elastic energy, is manifested to be the most favored one for the precipitation enhanced Fe–Mn–Si–Cr shape memory alloy. Homogeneously precipitated VN containing materials could show large deformability, higher strength by precipitation hardening and the higher shape recovery strain due to the nucleation sites of the precipitates in its reverse phase transformation.

The SME (shape memory effect) in an Fe–Mn–Si based SMA is governed by motion of Shockley partial dislocations which carry the fcc↔hcp phase transformations. The degree of SME is determined by preservation of the partial dislocations whereas the strength is determined by the internal stress opposing against the dislocation motion. The increase in the internal stress tends to induce dislocation reactions which ruin the preservation of the Shockley partials and hence to decrease the degree of SME. Nevertheless, usage of this SMA for construction of a large structural product has recently gained much attention even with sacrifice in the degree of SME. Upon application of this SMA in such a field, the optimum condition has to be searched both in the mechanical properties of the SMA itself and the type of the usage in the sense of deformation mode such as elongation, contraction, bending, twisting and so on. In this paper, the dislocation motion responsible for a good SME with high strength will be discussed on the ground of the basic knowledge of the dislocation generation, the motion and the reactions.

This article reports the mechanical properties of concrete prestressed by the Fe–Mn–Si-based shape memory alloys containing NbC that exhibit an excellent shape memory effect without the so-called ‘training’ treatment. A thermomechanically treated Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloy was used for this purpose. Four square bars of the alloy were embedded in mortar, and heated above their reverse martensitic transformation start temperature after hardening of the mortar matrix. Three-point bending tests were performed for the mechanical property characterization. It was found that prestressing by the shape memory alloys increased the bending strength and cracking stress of the mortar.

A novel process for synthesizing composite particles, named the high temperature molten salts method, is discussed in this paper. The molten salts are a reaction medium that do not take part in the chemical reaction and can be easily dissolved by water washing. By this method, composite particles were prepared in molten salts at 680–850°C. The heat released from the chemical reaction was found playing an important role to obtain the desired composite particles. The reverse martensitic transformation of the NiTi particles is confirmed in these composite particles by differential scanning calorimetry (DSC).

The shape-memory alloy AuZn transforms martensitically (T_M=65 K) from a cubic B2 (Pm\\bar3m) to a rhombohedral R-phase (P3) through softening of the TA₂[110] phonon branch. We report elastic constants, specific heat, and thermal expansivity measurements through the transition. A large elastic anisotropy, A=9 at T_M, associated with softening of the TA₂[110] phonon branch accompanied with a volume change ΔV⁄V of 0.25% characterize the transition. We find that specific heat and thermal expansion are well represented by adding low-energy Einstein modes to the harmonic lattice. On the basis of these measurements, combined with established group-theoretical symmetry criteria, the free-energy density is presented with atomic shuffle displacements as the primary order parameter, Q. We use this free-energy model to explain the atomic displacements in the R-phase.

We have calculated electronic structure of B2-type Ti–(50−x)Ni–xFe (0≤x≤28) alloys in order to understand the concentration dependence of the phase stability of the B2-type structure in this system. The Fermi surface of each alloy shows a nesting with a sharp peak of generalized susceptibility χ(q) at a nesting vector of q=[ζζ0]2π⁄a. The value of ζ at the peak position decreases linearly as the Fe content increases. On the contrary, the peak height of χ(q) does not change monotonically but shows a maximum value for Ti–44Ni–6Fe alloy. This result is consistent with the experimental results obtained by resistivity and specific heat measurements. In addition, we found that although the χ(q) shows a local maximum near 1⁄3[110]2π⁄a in Ti–44Ni–6Fe alloy, it shows a saddle point near 1⁄3[110]2π⁄a in TiNi.

The electronic structure of the full- and half-Heusler alloys have been studied by ab-initio calculations using full potential augmented plane-wave-method (FLAPW). It was shown that obtained equilibrium lattice parameters and magnetic moments agree well with available experimental data. The influence of vacancies on the electronic structure and magnetic properties of Ni_2−xMnGa and Co_2−xZrSn is analyzed.

The kinetics of the martensitic transformation in a CuAlMn shape memory alloy (SMA) has been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite as a function of time and temperature can be derived from the measured AE power. The fraction data obtained can be described by the Koistinen and Marburger (Acta Metall. 7 (1959) 59) equation with high accuracy, which indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The martensite-start temperature determined from the measured AE data is in good agreement with the value found by differential scanning calorimetry (DSC). Furthermore, the results of AE experiments on the SMA are compared with optical Confocal Laser Scanning Microscopy (CLSM) observations of the surface of the SMA. The observations show that both small and large martensite plates are formed both at the beginning and at the end of the transformation, which is in agreement with the assumption of a constant average volume of martensite crystals used in the Koistinen and Marburger model.

Magnetic properties and martensite structures of Ni₅₀Mn₂₈Ga₂₂ ferromagnetic shape memory alloy (FSMA) haven been investigated by physical properties measurement system (PPMS), magnetic force microscope (MFM) and transmission electron microscope (TEM) in the present paper. Magnetization-magnetic field (M–H) curves and MFM images show that the martensite has stronger magnetic anisotropy than the parent phase. The typical martensite with twin substructures is observed in Ni₅₀Mn₂₈Ga₂₂ alloy, while another type martensite substructure showing stripe like morphology is also found. It is assumed that such martensite structure may affect the magnetic field induced strain (MFIS) of Ni₅₀Mn₂₈Ga₂₂ alloy.

Ferromagnetic Fe–Pd alloy is a magneto-thermoelastic actuator material that has a large magnetostriction and the shape memory effect. In order to use the Fe–Pd alloy for a micromachine, we investigated the behavior of the shape memory effect for rapidly solidified Fe–29.6 at% Pd alloy ribbons. From the results, the ribbon exhibited a reversible two-way shape memory effect (TWSME) in the temperature range of 273 to 403 K, where the transformation from the martensite phase to austinite phase is found. On the basis of the development of an actuator of rapidly solidified Fe–29.6 at% Pd ribbon, a small simple-structured micromachine system was fabricated. A wireless micromachine is controlled remotely by an alternating magnetic field: it is able to swim in a fine liquid pipe with the aid of the gripping motion of a small ball. The ball is released by heating it to 340–350 K. This unique fishlike swimming micromachine will be applicable to medical curing devices in the body and as a nondestructive investigation tools for industrial machines and structures.

The martensitic transformation and microstructure of Ni–Mn–Ga films deposited on an alumina substrate and annealed at 1073 K for 36 ks are studied. Electrical resistivity and calorimetry measurements reveal a non-monotonous thickness dependence of the martensitic start temperature, T_ms, at submicron film thickness. Focused Ion Beam (FIB) and standard SEM techniques are used to clarify the film microstructure. A martensitic morphology of films is confirmed by the FIB imaging to be a laminated twin structure aligned almost parallel to the film plane in each crystallite as a consequence of {110}-type crystallographic texture. A thermodynamic model based on the Landau formalism taking into account the substructure of the film and the elastic interaction between film and substrate describes the essential features of the thickness dependence of T_ms.

The shape memory effect (SME) induced by the magnetic field is interesting and important for physics and application. The ferromagnetic Ni₂MnGa films with various compositions were deposited on a poly-vinyl alcohol (PVA) substrate with a radio-frequency (RF) magnetron sputtering apparatus using four kinds of Ni–Mn–Ga targets. After separating from the substrate, the films were heat-treated at 1073 K for 3.6 ks for homogenization and aged at 673 K for 3.6 ks in a constraint condition. The reversible two-way SME by the thermal change was confirmed for the constraint-aged films with various compositions. The gradient of the strain–temperature curve, the amount of strain accompanied by the two-way SME and the width of thermal hysteresis were dependent on the composition of the films. The strain–temperature curve shifted to a high temperature region and the martensitic phase was stabilized by the applied magnetic field. Furthermore, the two-way SME by the magnetic field was observed around the martensitic transformation temperature on cooling for the constraint-aged film, which showed the large gradient and small thermal hysteresis in the strain–temperature curve.

The magnetic emission signals, accompanying the martensitic transition in Ni₂MnGa magnetic shape memory alloy, are studied. The width (duration) and height (amplitude) distributions of the signals exhibit a power-law behaviour, with exponents α and β respectively. The values of α and β are characteristic of the magnetic noise originating from the transition itself and both have the value of 3.0±0.15.

An effect of magnetic field on the martensitic transformation (MT) temperature in ferromagnetic shape memory Ni_49.4Mn_27.7Ga_22.9 single crystal with the MT temperature 285 K is studied by measuring and theoretical treatment of the magnetization versus temperature dependencies in a wide range of the high magnetic fields higher than saturating one. In this way, a linear approximation of the field dependence of MT temperature was proved for the high-field range. The linear increase of transformation temperature is characterized by the slope of 3.5×10⁻² K/kOe. This value is essentially larger than the value 2×10⁻² K/kOe reported by Gonzales-Comas et al. [Phys. Rev. B 60 (1999) 7085–7090] for the Ni_49.5Mn_25.4Ga_25.1 alloy with a low MT temperature. This difference is in an agreement with the predictions of the Landau theory.

The technologies for fabrication, micromachining and integration of Ni–Mn–Ga thin films are developed in order to create novel microactuators and sensors. These devices simultaneously make use of the electrical, thermoelastic and ferromagnetic properties of the thin films allowing a new level of multifunctionality and, as a consequence, particularly compact designs. By adjusting the Ni-content of the thin films, the martensitic and ferromagnetic transformation temperatures are tuned close to each other above 373 K, which has important consequences on the device performance such as actuation stroke and response time. This article focuses on the mechanisms, fabrication technologies as well as typical performance characteristics of Ni–Mn–Ga microvalves and microscanners. The present state-of-the-art of FSMA microactuators is highlighted.

The present study systematically investigates the effect of heat treatment atmosphere on the multistage R-phase transformation (MRT) in an aged Ti–51.0 at%Ni alloy. No MRT occurs when the heat treatments were completed under the regulated atmosphere. On the other hand, the MRT is observed in the specimen heat treated under the unregulated atmosphere. It is apparent from transmission electron microscope observations that the first and the second transformations take place around the grain boundary and at the grain interior, respectively. We conclude that the MRT is extrinsic, and is an artifact during the heat treatment, rather than intrinsic in nature.

To develop high-temperature shape memory alloys, Ti–50(Pt,Ir)mol% compounds are noted because of their martensitic transformation from B2 to B19(2H) or 4H(4O) structures above 1273 K. A thermal expansion measurement and loading-unloading compression test were performed for Ti–50(Pt,Ir) to determine if the shape memory effect or superelasticity was shown. The thermal expansion measurement indicated the shape recovery in some of the compounds. The maximum shape recovery was about 4% by reheating at the above martensite transformation temperature after a loading-unloading compression test. Superelasticity was also observed in ternary compounds. The potential of Ti–50(Pt,Ir) as a high-temperature shape memory alloy is discussed.

The present work aims to investigate transformation and damping characteristics of a Ni₅₁Ti/Ni_50.2Ti alloy synthesized by explosive welding. The DSC results showed that the two endothermic peaks of the unprestrained specimen corresponded to the reverse transformation of each NiTi component. The reverse transformation temperature of Ni_50.2Ti increased with increasing prestrain level, whereas the reverse transformation peak of Ni₅₁Ti was split up into two independent endothermic peaks. Meanwhile, the internal friction results showed that the temperatures of the internal friction peaks of the Ni₅₁Ti/Ni_50.2Ti alloy and the range of reverse transformation temperature increased with increasing of prestrain level, which is consistent with the DSC results. Explosive welding is confirmed an effective method to fabricate chemical heterogeneous shape memory materials.

In the present study we investigate vacuum induction melting (VIM) of prominent ternary NiTiX (X=Cu, Fe, Hf, Zr) shape memory alloys using graphite crucibles. We apply a melting procedure which was recently developed for binary NiTi alloys and which keeps the carbon pick-up during melting at a minimum. We investigate the microstructures of the as-cast and homogenized alloys using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). Differential scanning calorimetry (DSC) was performed to study the phase transformation temperatures of the as-cast and homogenized materials. The results show that VIM processing in graphite crucibles provides ternary NiTiX shape memory alloys with good chemical homogeneity and acceptable impurity contents.

Properties and characteristics of superelastic deformation behavior based on Lüders-Like phase transformation bands in TiNi shape memory alloy (SMA) are presented. Temperature distributions accompanying the stress-induced phase transformations in the SMA are found using the infrared technique and employed for the investigation into nucleation and further development of the bands of martensitic and reverse transformations. Based on the temperature and the relevant mechanical characteristics it is noticed that just after crossing a certain threshold stress, narrow bands of considerably higher temperature, about 8 K, corresponding to the martensitic phase, appear starting from the central part of the specimen and developing towards the both specimen borders. A few such bands parallel to each other occur at higher stresses and move towards the specimen grips, as well as their next generation, developing in almost perpendicular direction. The heterogeneous field of the temperature distribution was observed also during the unloading process, while the reverse transformation occurred, also inhomogeneous and related to the significant temperature decrease. Based on the tests carried out with various strain rates, an influence of the strain rate on the mechanical behavior was presented. Thermomechanical aspects of the martensitic and the reverse transformations were discussed.

Using of MoK_α radiation, on a single crystal of a titanium nickelide intensities of 15 structural and 11 superstructural reflections of the B2-phase are measured. Structural factors of scattering for these reflections are calculated, and root-mean-square displacements of atoms of nickel and atoms of titanium from positions of equilibrium are determined. The mean square of displacements of atoms of nickel is equal ⟨u²⟩_Ni=(8.7±0.6)×10⁻⁴ nm², atoms of titanium −⟨u²⟩_Ti=(3.9±0.3)×10⁻⁴ nm².

The necessary physical properties for CuAlBe and, also tentatively, for NiTi alloys are analyzed at mesoscopic scale via static and dynamic contributions. The long time scale of the civil engineering (more than 10 or 20 years) requires analysis of the diffusion effects acting on microscopic scales. Simplified models for CuAlBe and NiTi dampers are built inside the ANSYS software scheme ensuring faster simulation for a three arch (or portico) in a family house. The dynamic simulation using accelerations of actual quakes (i.e., El Centro) shows that the SMA dampers reduce the amplitude of free oscillations by, at least, a factor two.

Diamond-like carbon (DLC) films are fabricated on the NiTi alloys at room temperature using plasma immersion ion implantation and deposition (PIIID). The effects of the substrate bias on the characteristics of the DLC films are systematically examined to correlate to the blood compatibility. The results show both the I_D⁄I_G ratio (inverse trend in the sp³⁄sp² ratio) and the G peak position first decrease and then increase, while the nano-hardness first increases and then decreases with the increase of the substrate bias, and the blood compatibility of the coated sample is better than that of the uncoated sample and the DLC films on the NiTi alloys deposited at the substrate bias of 25 kV possesses better blood compatibility than the films deposited at other substrate bias. It can be concluded that the blood compatibility of the DLC films on the NiTi alloys is influenced by the sp³⁄sp² ratio.

The nanostructured TiNi-based shape-memory alloys were synthesized by severe plastic deformation (SPD), including high pressure torsion, equal-channel angular pressing, and multi-step SPD deformations (SPD plus cold rolling or drawing). It is found that the SPD processing changed the morphology of the martensite and temperature of martensite transformation. Also, we found that the mechanical and shape memory properties can be enhanced by forming nanostructures in these alloys. SPD processing renders higher strength, higher yield dislocation strength and in results—higher recovery stress (up to 1.5 GPa) and maximum reverse strain (up to 10%) of shape memory, which are desirable in various practical application.

Single-phase nickel–titanium alloys were successfully synthesized by using a powder metallurgical process based on the use of a calcium reductant source during sintering in argon atmosphere (VPCR process). This process allows avoiding secondary phase formation during the NiTi compound forming reaction. The experimental results show that both heating rate and sintering temperature play a significant role on the final porosity. Different sintering stages at temperatures below T_E(Ti₂Ni), between T_E(Ti₂Ni) and T_E(Ni₃Ti), and above T_E(Ni₃Ti) were investigated in order to elucidate the influence of the two liquid eutectics on the densification. By choosing a slow heating rate of 0.5 K/min and a long time sintering at 1193 K, an almost dense single-phase NiTi compact was obtained with austenite ↔ martensite transformation heats comparable to those found in melt-cast NiTi alloys.

The modeling of the transformation and deformation behavior of a shape memory alloy has been investigated by many researchers. However, there are few reports that investigate plastic deformation of shape memory alloys. To design an actual product, the modeling in consideration of plastic deformation is indispensable. In this work, plastic deformation after pre-deformation is investigated using the volume fraction of slip-deformed martensite. New kinetics and constitutive equations are proposed for the reverse transformation process. The material constants in the proposed equations are determined from the results of tensile and heating/cooling tests on Ti–50 at%Ni alloy. The calculated results describe well the deformation and transformation behavior affected by pre-strain.

The low temperature relaxation peak appearing around 200 K in Ti_49.8Ni_50.2 shape memory alloy is a multiple relaxation process with activation energy Q=0.39 eV and frequency factor f₀=6.2×10⁹ s⁻¹ and is associated with the interaction of dislocations with pinning vacancies. Due to the increase of dislocation density and the annihilation of quenched-in vacancies after thermal cycling, the height of relaxation peak P_R decreases with increasing the number of thermal cycling. Higher amounts of dislocation-vacancy reaction cause a higher relaxation damping under the condition of higher quenching temperature. The quenched-in vacancies can have a significant effect on the transformation rate, and thus the heights of transformation peaks P_H1 and P_C1 decrease with increasing quenching temperature. Dislocations introduced by both thermal cycling and quenching from high temperature will depress the martensitic transformation, and hence decrease the peak temperatures of P_H1 and P_C1.

In order to study the effect of Cerium on martensitic transformation and microstructure of Ti_49.3Ni_50.7 alloy, a group of TiNiCe alloys with different content of Ce addition were prepared. The microstructure and the martensitic transformation behavior of TiNiCe ternary alloys were investigated by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of Ti_49.3Ni_50.7 alloy is changed obviously by Ce addition, and there are many Ce-rich phases dispersing in the TiNi matrix. One step martensitic transformation due to B2↔B19′ transformation occurs during the cooling and heating processes of the TiNiCe ternary alloys. The phase transformation temperatures increase remarkably when the content of Ce is less than 2 at%. However, when the content of Ce is further increasing, the phase transformation temperatures increase slowly and tend to stable.

The martensitic phase transformation (MT) start-strain of NiTi shape memory alloy under uniaxial tension is predicted theoretically on the basis of micromechanics. Results directly show that the martensitic transformation start-strain depends on three kinds of parameters: lattice correspondence strain (Bain strain), mechanical constants of Nitinol (Young’s module, Poisson ratio) and chemical energy difference between austenite and martensite phase (temperature dependence parameters). Comparison of this new prediction with other methods and experimental data is also given.