V.A. Chernenko

Large cyclic magnetic-field-induced deformation in orthorhombic (14M) Ni–Mn–Ga martensite

Magnetomechanical experiments were performed with a ferromagnetic Ni–Mn–Ga single crystal consisting of thermoelastic orthorhombic martensitic phase at room temperature. The crystal was deformed in uniaxial compression along 〈100〉 with an orthogonal magnetic field and without a magnetic field. The sample deforms due to motion of twin boundaries. When compressed with a magnetic field, twinning occurs at higher stress than without a magnetic field, and the twinning is reversible upon unloading. The stress–strain curves exhibit two plateaus which are related to two different twinning systems, namely (110)[110] and (011)[011]. During cyclic experiments in a rotating magnetic field, the magnetic-field-induced strain increases from initially 6% to 9.7%. The latter value was repeatedly measured upon more than 1000 rotations of the field. The increase of magnetic-field-induced strain during magnetomechanical cycling is related to a transition from combined partial (110)[110] and (011)[011] twinning to complet...

Stress-induced twin rearrangement resulting in change of magnetization in a Ni-Mn-Ga ferromagnetic martensite

Abstract Magnetoplasticity is the large magnetic-field-induced deformation of ferromagnetic martensite. This effect is caused by a twin rearrangement. A strain-induced change of magnetization is the inverse effect. This effect is studied experimentally on a Ni–Mn–Ga single crystal. The deformation-induced change of magnetization is linear to the compressive strain and shows only small hysteresis in a loading–unloading cycle performed in an orthogonal magnetic field.

A microscopic approach to the magnetic-field-induced deformation of martensite (magnetoplasticity)

Abstract Deformation experiments were performed in uniaxial compression with a Ni–Mn–Ga single crystal subjected to a magnetic field perpendicular to the stress axis. Depending on the field strength, different stress–strain curves for loading and unloading were obtained. The magnetic-field-induced stress (magneto-stress) and the work done by the corresponding magnetic force were evaluated. In order to understand the relationship between the magneto-mechanical properties and the microstructure, the microscopic processes occurring during magnetic-field-induced deformation are discussed in detail. It turns out that the magnetic work per unit volume and, to some extent, the macroscopic magneto-stress depend on the microstructure, i.e. the spatial distribution of martensite domains. The magnetic threshold field required for triggering magnetoplasticity depends on the twin thickness and is controlled by the mutual interaction of twinning dislocations and their interaction with interfaces. The threshold field can be entirely described within this microscopic approach, taking into account the elementary carrier of magnetoplasticity, which is the twinning dislocation.

Superelasticity in high-temperature Ni–Mn–Ga alloys

The stress–strain behavior of two single crystalline Ni–Mn–Ga alloys with martensitic transformation temperatures above the Curie temperature has been studied during uniaxial compression along the [001] and [110] axes. The superelastic effect associated to the reversible stress-induced martensitic transformation has been obtained in these high-temperature shape memory alloys. The experimental values of the critical stress to induce the martensitic transformation, σc, depend linearly on the test temperature. This dependence is described well by the thermodynamics of the stress-induced martensitic transformation through a Clausius–Clapeyron-type equation. However, the experimental values of slopes dσc/dT obtained in the two compression directions can only be interpreted well if it is assumed that different martensitic phases are formed for each compression axis. Good agreement with the thermodynamic model exists if tetragonal martensites with c/a 1 are induced in the [001] and [110] axes, respectively.

INTERNAL FRICTION ASSOCIATED WITH THE STRUCTURAL PHASE TRANSFORMATIONS IN Ni-Mn-Ga ALLOYS

Abstract A double peak in the temperature dependence of internal friction (IF) and elastic modulus in some off-stoichiometric Ni 2 MnGa shape memory alloys has been observed both on cooling and heating between the parent and martensite phases. Transmission electron microscopy (TEM) observations have shown that these anomalies are related to structural transformations from the parent cubic phase (P) to an intermediate cubic modulated phase (I) and from the I phase to the martensitic one (M). As for the IF results, the I to M transformation has the characteristics of a first-order phase transition, whereas the P to I transformation shows some distinctive features, such as no temperature-rate dependence.

Long-period martensitic structures of Ni-Mn-Ga alloys studied by high-resolution transmission electron microscopy

The layered martensitic structures formed in Ni-Mn-Ga alloys have been investigated by high-resolution transmission electron microscopy (including image simulations) in order to discern from two structural models reported in the literature. For the seven-layer martensite, the observations discard the model based on shuffling of the atomic positions (modulation) by a function with a period of seven planes and confirm its nanotwinned nature, which is inherent to the description as stacking of nearly close-packed planes derived from {110}aust with (52¯) sequence, that is, the so-called 14M structure. The observed stacking sequence, however, is notably distorted [the perfect (52¯) sequence is present only in small areas], although it preserves a predominant periodicity of seven planes. For another alloy composition, a nanotwinned martensite with periodicity of twelve planes and (75¯) average stacking sequence has also been observed. In case of the five-layer martensite, the image simulations and atom projecti...

Pre-martensitic state in Ni - Mn - Ga alloys

The pre-martensitic anomalies observed by elastic, thermal and electron microscopy measurements in the group of Ni - Mn - Ga alloys with K have been attributed to the formation of the intermediate phase existing in the temperature range 30 - 60 K above the martensitic transformation temperature. The results are discussed in terms of the soft-mode condensation in this alloy system.

Transformation and ageing behaviour of melt-spun Ni–Mn–Ga shape memory alloys

Abstract The effects of rapid quenching on the transformation behaviour of three melt-spun ribbons of Ni–Mn–Ga alloys has been studied by X-ray and electron diffraction, calorimetry, thermomechanical and magnetisation tests. One step martensitic transformation (MT) has been detected for each ribbon accompanied by the formation of modulated martensitic crystal structures with 7- or 10-layer periods, which differ from those observed for single crystals of the same compositions. The MT temperatures can be raised or lowered by ageing in the parent phase, depending on composition. The ageing also causes a significant increase of both Curie temperature and magnetization in all alloys. These effects indicate the occurrence of some atomic rearrangement processes leading to a more uniform element distribution and, probably, change of atomic order. The activation energies for these processes have been estimated to be between 0.2 and 0.7 eV.

Magnetic moment and chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys

Recent studies have shown that the total magnetic moment in off-stoichiometric Ni–Mn–Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44–52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn–Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.

Effect of atomic order on the martensitic and magnetic transformations in Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of long-range L2(1) atomic order on the martensitic and magnetic transformations of Ni-Mn-Ga shape memory alloys has been investigated. In order to correlate the structural and magnetic transformation temperatures with the atomic order, calorimetric, magnetic and neutron diffraction measurements have been performed on polycrystalline and single-crystalline alloys subjected to different thermal treatments. It is found that both transformation temperatures increase with increasing atomic order, showing exactly the same linear dependence on the degree of L2(1) atomic order. A quantitative correlation between atomic order and transformation temperatures has been established, from which the effect of atomic order on the relative stability between the structural phases has been quantified. On the other hand, the kinetics of the post-quench ordering process taking place in these alloys has been studied. It is shown that the activation energy of the ordering process agrees quite well with the activation energy of the Mn self-diffusion process.