Martensitic transformation of TiNiPd high-temperature shape memory alloys: A first-principles study

Heat of formation, elastic property and electronic structure of TiNiPd high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane-wave method. The results show that the heat of formation difference between austenite and martensite plays an important role in the martensitic transformation. The effect of Pd content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. High martensitic transformation temperature can be attributed to a low shear resistance C′. Furthermore, the mechanism of the effect of Pd addition on elastic constants is explained on the basis of the electronic structure.     

Quantitative model of large magnetostrain effect in ferromagnetic shapell memory alloys

A quantitative model describing the large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar types of materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa. Received 29 September 1999     

Three-dimensional constitutive equations for Ni–Mn–Ga single crystals

This paper presents a formulation of isothermal three-dimensional (3D) quasi-static magneto-mechanical constitutive equations and 3D magnetisation constitutive equations for tetragonal martensite Ni–Mn–Ga FSMA single crystals (c/a<1)$(c/a<1)$ with both ends restrained from twin-boundary motion. The formulated 3D constitutive equations model the 3D quasi-static magnetic fields as well as the coupling between uniaxial strains and stresses, and shear strains and stresses. The constitutive equations are compared with experimental results available in the literature and are found to correlate well with the experimental results, including magnetic field reversals. Both sets of 3D constitutive equations require only macroscopic parameters that are readily obtainable from magnetisation and mechanical stress–strain curves.     

Magnetization anomalies in melt-spun Ni–Mn–Ga ribbons

Temperature and field dependencies of the magnetization of melt-spun ribbons of Ni_45.7Mn_37.2Ga_17.1 (T_C<T_A) and Ni_54.3Mn_20.5Ga_25.2 (T_C>T_A), where T_A and T_C are the reverse martensitic transformation and Curie temperatures, respectively, have been studied in broad ranges of magnetic fields (±30 kOe) and temperatures (10–350 K). It was found that the magnetic and thermomagnetic properties have a number of peculiarities and anomalies. Particularly, the magnetization values measured at both low and high magnetic fields increase significantly after annealing. Low-temperature anomalies of magnetization suggest the presence of the superparamagnetic behavior and/or phenomena related to the concurrent exchange interactions in the as-spun state. The obtained results can be explained by the influence of concentration inhomogeneities and atomic disordering in the rapidly quenched ribbons, which can be reduced by annealing.     

Modulus characteristics during the γ→ε martensitic transformation in Fe-Mn-Si-Cr-N alloys

The temperature dependences of internal friction and modulus were measured for the Fe-Mn-Si-Cr-xN alloys. A stable modulus softening of parent was newly observed to associate with the γ→ε martensitic transformation in the alloys containing certain amount (e.g. >0.086 mass%) of nitrogen, although the rather strong softening has been always recognized to occur during its reverse transformation. The enhanced strength and the increased stacking fault energy of γ matrix by nitrogen alloying are suggested to make contribution to the soft mode.     

Magnetic properties of HITPERM-type alloys at high temperature

(Fe,Co)–Zr,Hf)–Cu–B (HITPERM-type) alloys with variable Hf, Zr and Co content were isothermally crystallised at 500–650 °C for 1 h, and the optimum nanocrystallisation temperature was selected on the basis of the minimum coercive field at room temperature. The quasistatic hysteresis loops were measured at temperature from 20 to 650 °C. Subsequently, the optimally annealed alloys were subjected to long-term annealing at 500, 550 and 600 °C. Working temperature of 600°C is too high for the investigated alloys to maintain stable magnetic properties. Temperature of 550 or 500 °C permits the material to be magnetically stable for a long period. The magnetic hysteresis loops recorded for the nanocrystalline alloys, where Fe:Co ratio is close to 1 and refractory metals content is 7 at.%, prove that coercive field increases slightly with temperature, but remains in the range of 20–40 A/m (depending on the alloy composition) from 20 to 550 °C. This proves that the investigated alloys, after optimisation of chemical composition, may be suitable for high temperature use.     

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 in Ni-Mn-Ga alloys

Single crystals of different compositions in shape memory alloys Ni-Mn-Ga have been studied by electron and low temperature X-ray diffraction as well as by differential scanning calorimetry. It is shown that the cooling-induced martensitic phases are long-periodic ones modulated along the (110) directions by a transverse wave of atomic shifts with 5 and 7 atomic layers periodicity for the alloys studied exhibiting a martensitic transformation at 180 K and 446 K, respectively. The transformation heats appeared to be about 10 times different for both 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.     

Pulsed laser deposition of NiTi shape memory effect thin films

2 O₃(100) substrates. We also produced free-standing NiTi films by deposition on KBr substrates and subsequent substrate removal by immersion in water. The presence of the solid-solid phase transformation responsible for the shape memory effect has been demonstrated through temperature-dependent X-ray diffraction and four-probe resistance versus temperature measurements. On cooling the deposited film, the austenite-martensite transformation was measured at around 195 K; on heating the film the reverse transformation was around 250 K. Evidence of the shape-memory effect for free-standing films was obtained in a bending deformation-shape recovery experiment. Received: 31 July 1996/Accepted: 6 January 1997     

Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal

The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni_50.5Mn_24.5Ga₂₅ single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction.     

Magnetic properties on shape memory alloys Ni2Mn1+xIn1−x

X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni₂Mn_1+xIn_1−x. On the basis of the results, the magnetic phase diagram was determined for Ni₂Mn_1+xIn_1−x alloys. Magnetization measurements make clear that the excess Mn atoms, which substitute for In sites, are coupled ferromagnetically to the ferromagnetic manganese sublattices. A magnetic phase diagram of Ni₂Mn_1+xIn_1−x alloys is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.     

Development of an engineering model for ferromagnetic shape memory alloys

This paper presents a relationship among stress, temperature and magnetic properties of a ferromagnetic shape memory alloy. In order to derive an engineering model of ferromagnetic shape memory alloys, we have developed a measuring system of the relationship among stress, temperature and magnetic properties. The samples used in this measurement are Fe68–Ni10–Cr9–Mn7–Si6 wt% ferromagnetic shape memory alloy. They are thin ribbons made by rapid cooling in air. In the measurement, the ribbon sample is inserted into a sample holder winding consisting of the B-coil and compensation coils, and magnetized in an open solenoid coil. The ribbon is stressed with attachment weights and heated with a heating wire. The specific susceptibility was increased by applying tension, and slightly increased by heating below the Curie temperature.     

Thickness-shear modes and magnetoelastic waves in a longitudinally magnetized ferromagnetic plate

Magnetoelastic (ME) waves and thickness-shear modes in the ferromagnetic plate are studied. Coupled vibrations of magnetization and shear elastic deformations excited simultaneously by a variable magnetic field propagate in two mutually perpendicular directions: parallel and normal to a surface. For parameters characteristic of isotropic ferromagnet with the sample magnetization and Zeeman field parallel to the surface, resonant frequencies of shear modes are computed and their dispersion law is examined. It is shown that the dependence of dimensional resonances frequencies on wave number k_z of ME wave propagating along saturating field direction occurs. The possibility of excitation of ME waves with different k_z explains multimode character of thickness ME resonances.     

Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

Structural and magnetic transition temperatures of ferromagnetic shape memory alloys present a strong dependence on slight departures from the stoichiometry, as does the mobility of twin boundaries responsible for the large magnetic field induced strains. In this work we study four non stoichiometric Ni-Mn-Ga polycrystalline alloys with compositions of 43–52 at.% nickel, excess manganese and deficient in gallium, and a single crystal of composition Ni₅₂Mn₂₆Ga₂₂. Those compounds are of technical interest due to the observed large room temperature magnetic field induced strains. Calorimetric and magnetic measurements determined the martensitic transition and Curie temperatures of the alloys (A_S = 331 K and T_Curie = 366 K for 52 at.% nickel alloy). Nickel defective alloys present a martensitic transition region broader than excess nickel ones. Neutron powder diffraction analysis confirmed orthorhombic martensitic structures for nickel defective alloys, and tetragonal for excess nickel ones. In the 52 atomic % nickel alloys case the crystallographic structure of the martensitic phase was also obtained on a single crystal with the same composition, trained to get a single variant in agreement with determined in the powder sample.     

Parametric amplification of ultrasound in ferromagnetic piezoelectric composites at the vicinity of the orientational phase transition

Direct parametrical amplification of ultrasound was theoretically considered for the ferromagnetic/piezoelectric thin-layered structure composites, for which within the long-wavelength approach some, material specific, yet homogeneous bulk properties could be assumed. Having chosen appropriate homogeneous components, it is possible to construct a desirable composite with such properties that are absent in the starting-up “building blocks” (components).Sound frequencies are assumed to be such that only the dispersion due to magnetic subsystem is thought relevant; the piezoelectric components are set to have a nonlinear dielectric response. Presence of orientational phase transition for the ferromagnetic subsystem allows to vary frequency of a ferromagnetic resonance, and consequently to alter the dispersion at the chosen frequency of sound.While solving coupled equations describing dynamics of mechanical, magnetic and piezoelectric subsystems, an effective sound amplification for realistic values of physical parameters is shown possible.     

Magnetoimpedance effect in Nanoperm alloys

The influence of isothermal annealing (1 h at 600 °C in Ar atmosphere) on the soft magnetic properties and magnetoimpedance (MI) effect has been studied in ribbons of the following Nanoperm alloys: Fe₉₁Zr₇B₂, Fe₈₈Zr₈B₄, Fe₈₇Zr₆B₆Cu₁ and Fe₈₀Zr₁₀B₁₀. A maximum MI ratio of about 27% was measured for the nanocrystalline alloy Fe₈₇Zr₆B₆Cu₁ at a driving frequency of 0.2 MHz. The thermal annealing led to magnetic softening for this alloy, while a hardening is observed for the Fe₈₀Zr₁₀B₁₀ alloy.     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

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.