Premartensitic Transition Research Articles

Characteristics of the premartensitic transition strain in ferromagnetic shape memoryNi50.5Mn24.5Ga25 single crystals

By measuring the strain across the premartensitic transition without and with external dcmagnetic fields applied along the [001] and [010] directions of the parent phase,respectively, the characteristics of the premartensitic transition strain is investigated indetail. It was found that not only does the premartensitic transition temperaturepronounce a more rapid decrease, but the premartensitic transition strain also exhibits alarger change for a field applied along the [010] direction than does a field along the [001]direction. In accord with a previous model, the present results confirm that themagnetoelastic interaction is responsible for the premartensitic transition, and themagnitude of the magnetoelastic interaction in the [010] direction is stronger than that inthe [001] direction.

Detection of weak-order phase transitions in ferromagnets by ac resistometry

It is shown that ac resistometry can serve as an effective tool for the detection of phase transitions, such as spin reorientation or premartensitic phase transitions, which generally are not disclosed by dc resistivity measurement. Measurement of temperature dependence of impedance, Z(T), allows one to unmask the anomaly, corresponding to a weak-order phase transition. The appearance of such an anomaly is accounted for by a change in the effective permeability μ of a sample upon the phase transition. Moreover, frequency dependence of μ makes it possible to use the frequency of the applied ac current as an adjusting parameter in order to make this anomaly more pronounced. The applicability of this method is tested for the rare earth Gd and Heusler alloy Ni2MnGa.

Mean field and Monte Carlo simulation studies of premartensitic effects in Ni2MnGa

We have extended the degenerate BEG model[1] to include magnetic degrees of freedom in order to study the premartensitic effects in Ni 2 MnGa. The model is solved by using mean-field theory and Monte Carlo techniques [2]. The numerical simulations reveal the crucial importance of fluctuations in pretransitional effects. Moreover, we find that a large variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the coupling parameter a first-order transition line ending in a critical point appears. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling strength. In particular, the premartensitic transition occurs when such coupling is strong enough to prevent a complete softening of the involved phonon mode. The implication of the results in relation to the available experimental data is discussed.

Premartensitic transition in Ni2+xMn1-xGa Heusleralloys

The temperature dependencies of the resistivity and magnetization ofa series of Ni2+xMn1-xGa (x = 0-0.09) alloys wereinvestigated. Along with the anomalies associated withferromagnetic and martensitic transitions, well-defined anomalieswere observed at the temperature of premartensitic transformation.The premartensitic phase existing over the temperature range 200-260 Kin the stoichiometric Ni2MnGa is progressively suppressed by themartensitic phase with increasing Ni content and has vanished in theNi2.09Mn0.91Ga composition.

Phase transitions in ferromagnetic Ni2+x Mn1−x Ga alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni2+xMn1−xGa is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter.

Effect of low dc magnetic field on the premartensitic phase transitiontemperature of ferromagnetic Ni2MnGa singlecrystals

The temperature and field dependence of alternating-current (ac)susceptibility in a Ni50Mn25Ga25 single crystal was measuredusing an ac susceptometer with a dc magnetic field oriented in the [100] and[110] directions, respectively. It is found that even at very low fields thepremartensitic transition temperature decreases monotonically with theincrease of the applied field, and it decreases more rapidly with fieldapplied along the [110] direction than with field along the [100]direction. In accord with a previous model, present results confirm that themagnetoelastic interaction is responsible for the premartensitic transition,and the magnitude of the magnetoelastic interaction in the [110] directionis stronger than that in the [100] direction.

Investigation of phase transformations in Ni 2MnGa using high magnetic field low-temperature X-ray diffraction system

We present results on the magnetic field-induced martensitic phase transformation in Ni 2MnGa. The experiments were carried out in a wide temperature range from 10 to 300 K and in magnetic fields up to 5 T. Two successive anomalies in the temperature dependence of main (2 2 0) peak width, corresponding to martensitic transition temperature ( T m) and premartensitic transition temperature ( T p), respectively, are detected. It is found that T m increases and T p decreases with applying magnetic fields up to 2 T, and then the transition temperatures do not change in fields up to 5 T.

Acoustic Emission at the Premartensitic and Martensitic Transitions of Ni<sub>2</sub>MnGa Shape Memory Alloy

An Acoustic Emission (AE) study of the martensitic and premartensitic transitions in nearly stoichiometric Ni-Mn-Ga alloy is reported. By comparing the AE with calorimetric results, it is shown that AE is generated at both transitions. The AE intensity at the premartensitic transition is extremely weak, in accordance with its weakly first-order character.The amplitude distribution of the AE signals recorded during the martensitic transformation is a power law, with an exponent α = 2.2 ± 0.1. This value is compared with recent results obtained in other shape-memory alloys.

Premartensitic phase transformation in the Ni2MnGa shape memory alloy

Abstract We present a phenomenological model for the premartensitic phase transition in ferromagnetic NiMnGa alloys. The model contemplates the effects of an applied mechanical stress on the premartensitic transition. It is shown that the premartensitic transition is driven by a magnetoelastic coupling. The first order character of this transition is enhanced by the application of a mechanical stress. Experimental data are also presented which support the predictions of the proposed model.

Electronic Structure and Martensitic Transformation in Ni2MnGa Heusler Alloy

The ferromagnetic Ni2MnGa Heusler alloy is known to exhibit a martensitic transformation from cubic to tetragonal phase. The neutron studies [1, 2] established the existence of a weakly first-order structural phase transition at about 265 K. A number of studies have been concentrated on premartensitic transition. The results of elastic, thermal, and electron microscopy measurements clearly show the formation of premartensitic phase existing in the temperature range of 30-60 K above the martensitic transformation temperature [3]. The fact of the existence of premartensitic state is confirmed by the behaviour of the elastic constants [4, 5]: The elastic constant C' associated with the transverse acoustic (TA2) branch plays an important role in the structural transformation. The basic feature of this kind of transition is the instability of the phonon system, which leads to the crystal lattice rearrangement. The nearly complete softening m the (q, q, 0) TA 2 branch results in a premartensitic first-order transition which involves a commensurate periodic distortion of the parent structure Ni2MnGa with the periodicity equal to that of the soft mode [2]. The specific heat, elastic constants, and magnetic susceptibility in a ferromagnetic Ni2MnGa alloy exhibit anomalous behaviour [6, 7] at the temperature of condensation of the [110] TA2 phonon at the q = 0.33. But the phonon anomaly is not related to the magnetic ordering.

Modeling premartensitic effects inNi2MnGa: A mean-field and Monte Carlo simulation study

The degenerate Blume-Emery-Griffiths model for martensitic transformations is extended by including both structural and magnetic degrees of freedom in order to elucidate premartensitic effects. Special attention is paied to the effect of the magnetoelastic coupling in Ni2MnGa. The microscopic model is constructed and justified based on the analysis of the experimentally observed strain variables and precursor phenomena. The description includes the (local) tetragonal distortion, the amplitude of the plane-modulating strain, and the magnetization. The model is solved by means of mean-field theory and Monte Carlo simulations. The results show that a variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the magnetoelastic coupling parameter we find a premartensitic first-order transition line ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition occurs when such coupling is strong enough to freeze the involved mode phonon.

Phonon dispersion and martensitic phase transition in ordered alloys

In a previous neutron scattering study, we had observed that the TA1[ξξ0] phonon softening in L12-ordered ferromagnetic Fe72Pt28 Invar is pronounced at the zone boundary M-point and leads to an antiferrodistortive phase transition at low temperatures. Here, we report on similar neutron scattering investigations on two ordered crystals with higher Fe content to investigate the relation between the TA1[ξξ0] phonon softening and the martensitic transformation, which occurs in Fe-rich ordered Fe-Pt. We find that the TA1[ξξ0] phonon softening, especially at the M-point zone boundary, does not depend on the composition of the investigated crystals. In Fe74.5Pt25.5, however, the antiferrodistortive phase transition temperature is enhanced due to tetragonal strain preceding the martensitic transition. In Fe77Pt23 a precursor driven premartensitic phase transition is not observed. The structure of the martensite is, however, influenced by the soft mode lattice instability of the austenite. This would explain the origin of structural details found previously for Fe3Pt thermoelastic martensite.

Premartensitic and martensitic phase transitions in ferromagneticNi2MnGa

We present an experimental study of the premartensitic and martensitic phase transitions in a Ni 2MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model @A. Planes et al., Phys. Rev. Lett. 79, 3926 ~1997!# that incorporates a magnetoelastic coupling as a key ingredient. @S0163-1829~99!01834-2#

Magnetic and transport properties of the alloys

Magnetization and transport measurements have been performed to study the martensitic and pre-martensitic transitions for a series of ferromagnetic Heusler alloys. Both magnetization and resistivity measurements show a clear jump at the martensitic transition and a discontinuous slope change at the pre-martensitic transition. The characteristic temperatures correspond well with those derived from previous direct structural results from neutron scattering, electron microscopy and ultrasonic studies. The martensitic transition temperature decreases with increasing Mn concentration.

Magnetic properties of the premartensitic transition inNi2MnGaalloys

Magnetization as a function of field and temperature for a ferromagnetic Heusler alloy Ni2MnGa is reported. Magnetization above the Martensitic transition displays a field-dependent peak effect, a direct magnetic evidence of premartensitic phase. At low fields, the peak effect occurs at a temperature consistent with the observations of the micromodulated structure transition seen from neutron scattering, electron microscopy, and ultrasonic studies in this compound. At high fields, the peak effect is suppressed. The strong field dependence of the peak temperature suggests a large magnetoelastic interaction in the intermediate phase. @S0163-1829~98!09537-X# The ferromagnetic Heusler alloy Ni22xMn11xGa with x 50 was first studied in the early 1980’s. 1 For the stoichiometric Ni2MnGa, the alloy was found to be ferromagnetic with a Curie temperature of 376 K. A martensitic phase transition from a cubic structure to a complex tetragonal structure at 202 K on cooling was observed from microscopy and neutron-scattering measurements, with a corresponding jump in magnetization at the same temperature. Later studies on alloys with nonstoichiometric compositions show that both the Curie temperature and the Martensitic transition can be varied with x. 2‐4 Recent interest in the Ni22xMn11xGa alloys as shape memory materials has lead to much more careful studies of the structural transition. It has been reported that the Martensitic transition is proceeded by a premartensitic transition as observed from several experiments such as x-ray, electron- and neutron-scattering, and ultrasound measurements. 5‐11 However, it is generally believed that there is no magnetic anomaly corresponding to the premartensitic transition. 2‐4 In this paper, we report direct magnetic characterizations of the premartensitic transition for the stoichiometric Ni2MnGa alloy. Magnetization M as a function of temperature T at various applied field H shows clear evidence of a premartensitic transition. The premartensitic transition is characterized by a peak in M(T) well above the Martensitic transition temperature Tm . The premartensitic transition temperature T p is found dependent on the applied field. The field dependence of T p demonstrates a large magnetoelastic effect in the premartensitic or intermediate state. Samples are prepared with the conventional arc-melt process with the stoichiometric composition of starting materials. 1 Structural analysis confirms the single phase,

Premartensitic Transition Driven by Magnetoelastic Interaction in bcc FerromagneticNi2MnGa

We show that the magnetoelastic coupling between the magnetization and the amplitude of a short wavelength phonon enables the existence of a first order premartensitic transition from a bcc to a micromodulated phase in $Ni_{2}MnGa$. Such a magnetoelastic coupling has been experimentally evidenced by AC susceptibility and ultrasonic measurements under applied magnetic field. A latent heat around 9 J/mol has been measured using a highly sensitive calorimeter. This value is in very good agreement with the value predicted by a proposed model.

Thermal Cycling Effect on the Premartensitic and Martensitic Transition in a Ti Rich NiTi Alloy

The study was carried on a Ti-rich NiTi shape memory alloy. The electrical resistance and the elastic modulus were determined versus temperature. The aim of this work is to study the thermal cycling effect on the transformation features as the temperatures or the phase succession as well as the structural modifications. The major results of this work concern the R-phase occurrence and its evolution. We have shown that the premartensitic phase could exist in a narrow range of temperature on cooling and is revealed only after a few cycles. In fact, its presence and stability is directly linked to the diminishing of Ms. The main condition for the austenite to be transform in a rhombohedral structure is that Ms becomes low enough to be below Tr. Interrupted cooling alms also show that contrarily to the austenite-martensite transition, the austenite-R-phase transition doesn't exhibit any hysteretic behaviour.

Uniaxial stress dependence of the [ζ ζ 0]-TA 2 anomalous phonon branch in Ni 2MnGa

Results of neutron scattering experiments on single crystals of Ni 2MnGa as a function of uniaxial stress and temperature are reported. Emphasis is placed on the behavior of the low-energy part of the [ζ ζ 0] TA 2 phonon branch and the associated premartensitic phase transformation. The dip in the phonon dispersion curve is stress-dependent and shifts from ζ = 0.33 for zero stress to ζ = 0.36 for an applied stress of 95 MPa. The energy of the soft phonon decreases with increasing stress. The premartensitic transition temperature is increased from 265 K in the unstrained sample to ≈ 300 K at 95 MPa.

Phase transition features of NiTi orthodontic wires subjected to constant bending strains

A resistometric investigation of premartensitic and martensitic phase transitions in NiTi orthodontic wires has been carried out as a function of temperature between −20 and +60°C, during cooling and heating. Transformation features of two types of commercial wires were examined in either their undeformed state or under application of a constant bending strain. For the undeformed case the transformation behavior was also characterized by differential scanning calorimetry (DSC). The start temperature (Ms) for the formation of martensite was appreciably different for the two types of material, while that (Rs) for the formation of the R-phase was about the same. For one material (type 1) the enthalpies ΔH∗ for the A → R and R → M transitions were deduced from DSC data and used for calculating the Clausius-Clapeyron relationships Rs(σ) and Ms(σ). For the other material (type 2), due to the exceedingly low value of Ms, the Ms(σ) relationship could not be determined. Above room temperature electrical resistivity data taken under bending did not give clear indications of stress-induced martensite for material of type 2, which, however, displayed good superelastic behavior. This lack of clarity arises from complex alterations taking place in the structure (lamellar) and proportions of the different phases (A, R and M) present under inhomogeneous bending strains when the temperature is changed. To fully clarify this point stress-strain tensile experiments were carried out, preliminary results of which are also presented in this paper.

Pseudoelasticity and Transformation Features of some NiTi Orthodontic Commercial Wires

Premartensitic and martensitic phase transitions have been investigated between -20 and +60 °C in two types of NiTi orthodontic wires subjected to constant bending strains. The electrical resistivity clearly shows a shift towards higher temperatures of the austenite/R-phase (A-R) transition, indicating pseudoelastic behaviour due to stress-induced R-phase. Pseudoelasticity associated with stress-induced martensite was not clearly displayed by resistometric measurements, due to the inhomogeneity of the applied strain field. Stress-strain tensile experiments, on the contrary, clearly show that pseudoelasticity mostly arises, as expected, from stress-induced martensite. The Clausius-Clapeyron relationships have been deduced for martensite induced by stress from both the austenite and R-phase and they appear to differ one another. Appreciable recoverable strains could be identified for both types of material at deformation levels beyond the plateau region.