Martensite-austenite Transition Research Articles

Investigation of the magnetic and structural properties in the non‐stoichiometric Heusler alloy Ni50Mn25+xSn25‐x; x = 13, 14

AbstractMemory shape magnetic alloys, especially Heusler alloys, are important materials in replacing conventional cooling with magnetic systems. In the present study off stoichiometric Heusler alloys with nominal composition Χ50Υ25+xΖ25‐x (X = Ni; Y = Mn; Z = Sn; x = 13, 14) were prepared by arc melting followed by thermal treatment. Structural properties were analyzed with X‐ray diffraction at room temperature (RT) and at elevated temperatures, above the martensite—austenite transition area, to determine the relevant crystallographic parameters and observe the transition. Martensite stabilization at RT appears to be a challenge, coexistence of martensite—austenite phases were observed and calculated for both 38–12 and 39–11 (16% and 12% austenite, respectively). Magnetization measurements versus temperature and field were recorded in the areas of interest where 1st order transitions were expected (355 K for x = 13 and 408 K for x = 14), and the magnetic entropy's changes (ΔSm) were determined [0.4 (J/kgK) for x = 13 and 0.3 J/(kgK) for x = 14; Hmax = 1 T]. The complex character of the magnetic properties and their dependence on Mn‐Sn ratio and on the distance between Mn atoms is discussed. The structure and the lattice parameters were determined using an anisotropic strain broadening model; stress and strain were detected in the structure due to crystal phase coexistence.

Investigation of martensitic transformation and magnetoresistance properties of cu-substituted Ni-Mn-Sn-B melt spun ribbons

Ni50-xCuxMn38Sn12 + B3% at (x = 0, 1, 3 and 5) melt spun ribbons have been fabricated by melt spinning technique. Structural, electrical and magnetoresistance properties of the ribbons were investigated in this study. It was identified that the austenite phase with the L21 type structure was formed in the ribbons, regardless of composition. Temperature dependence of resistance measurements exhibited the first order martensite transition in the ribbons. The largest negative magnetoresistance (~55%) was obtained at 262 K in the RB1 ribbon. Depending on the Cu-content and magnetic field, a noticeable shift of martensite transition temperature to low temperatures has been clearly observed. From magnetic field dependence of the resistance, it was seen that the resistances dropped sharply with the increase of the applied magnetic field and overlapped near 80 kOe, regardless of temperatures, which strongly indicates the magnetic field-induced martensite-austenite transition.

Thermoelastic properties across martensitic transformation of Ni[formula omitted]MnGa Heusler alloy from time-resolved photothermal mirror

We report on the thermoelastic properties of Ni2MnGa shape-memory alloys across the martensitic to austenitic phase transition using the time-resolved pump–probe photothermal mirror technique. Quantitative information on the thermo-mechanical characteristics of the sample are obtained and compared to specific heat measurements and differential scanning calorimetry (DSC) analysis. The thermal diffusivity varied remarkably within the martensitic–austenitic transition range. Both the amplitude signal and the thermal diffusivity present, in addition to the strong martensite to austenite phase transition, a clear evidence of a subtle second-order type phase transition close to 80 °C. This magnetic transition is also observed in the calorimetric analysis. The transformation temperatures suggest the non-stoichiometric composition of sample, which is also revealed by non-symmetrical shapes of DSC peaks. The possible variation of the local chemical composition could be a result of gallium loss during the sample preparation or local inhomogeneity in the martensitic transformation.

Degradation of the Magnetocaloric Effect in Ni49.3Mn40.4In10.3 in a Cyclic Magnetic Field

The direct measurements of the magnetocaloric effect ΔTad in the Heusler alloy Ni49.3Mn40.4In10.3 have been carried out in cyclic magnetic fields with amplitudes to 8 T. The maximum values of the magnetocaloric effect are 5.29 K in the martensite–austenite transition region and 3.39 K in the region of the magnetic phase transition in the austenite phase. A long-term action of a cyclic magnetic field is shown to lead to a marked decrease in the value of the magnetocaloric effect in the region of the magnetostructural martensite–austenite transition. The initial properties are recovered after the complete transition of the sample to the austenite phase. The phenomenon observed in this study restricts the application of these materials in the magnetic cooling technology.

Functional NiTi grids for in situ straining in the TEM

In situ measurements are a pivotal extension of conventional transmission electron microscopy (TEM). By means of the shape memory alloy NiTi thin film Functional Grids were produced for in situ straining as alternative or at least complement of expensive commercial holders. Due to the martensite-austenite transition temperature straining effects can be observed by use of customary heating holders in the range of 50 to 100 °C. The grids can be produced in diversified designs to fit for different strain situations. Micro tensile tests were performed and compared with finite element simulations to estimate the applied forces on the sample and to predict the functionality of different grid designs. As a first example of this Functional Grid technology, we demonstrate the impact of applying a strain to a network of ZnO tetrapods.

Analytical modelling of Rolamite mechanism made of shape-memory alloy for constant force actuators

This article analyses the Rolamite architecture exploiting shape-memory alloys as power element to obtain a solid-state actuator. The Rolamite mechanism was discovered in the late 1960s, initially as precision and low friction linear bearing. The most common Rolamite configuration consists of a flexible thin metal strip and two rollers mounted between two fixed parallel guide surfaces. The system can roll back and forth without slipping guided by the plates along its so-called sensing axis. The system presents another relevant advantage in addition to low friction coefficient, which is the possibility to provide force generation in a quite simple way. In the original literature works, the force was provided, thanks to cut-outs of various shapes in the strip, although this method does not allow the Rolamite to be considered a proper actuator, but only a force generator. In this article, we developed the idea of exploiting the shape-memory alloy as Rolamite power element, and therefore, to use the shape-memory effect to change the elastic properties of the strip and to provide the actuation force. The mechanical analysis, where the martensite–austenite transition is modelled in a simplified way, shows that this application is feasible, mainly thanks to the initial precurvature of the shape-memory alloy strip. The discussion of the results highlights some important merits of this architecture such as long stroke, constant force and compactness.

Multi-scale simulations of metamagnetic martensite transition in NiCoMnIn

A multi-scale theoretical model is performed to investigate magnetic field-induced metamagnetic martensite transition in NiCoMnIn alloys. First, the total energy is calculated to demonstrate antiferromagnetic-ferromagnetic transition from first principle calculation. Second, the transition temperature from antiferromagnetic to ferromagnetic can be tuned by the external magnetic field and hydrostatic pressure in thermodynamic calculation, which is consistent with experimental results. Third, phase field simulation found that martensite variant boundaries are superposed with the 90° antiferromagnetic magnetic domain walls, revealing magneto-structural coupling between structural and magnetic domain structures. It is demonstrated that this multi-scale model is very effective for describing antiferromagnetic-ferromagnetic accompanied with martensite-austenite transitions with magnetostructural coupling.

Effect of pressure and high magnetic field on phase transitions and magnetic properties of Ni1.92Mn1.56Sn0.52 and Ni2MnSn Heusler compounds

Complex study of magnetic, magnetocaloric and structural properties of the Ni2MnSn and Ni1.92Mn1.56Sn0.52 compounds was performed. The stoichiometric single-crystal of Ni2MnSn was prepared by Czochralski method. The remarkable pressure effect on the martensitic magnetization and the martensite-austenite transition temperature TM–A was observed in the Ni1.92Mn1.56Sn0.52 compound. The coefficient dTM–A/dp reached value of 18 K/GPa. The already low value of martensite magnetization of Ni1.92Mn1.56Sn0.52 was further substantially decreased by external pressure, in contrast with pressure almost insensitive magnetization of the stoichiometric Ni2MnSn single-crystal. The pulse magnetic field of 58 T invoked the structural transition at temperature 180 K that is of about 100 K below TM–A of Ni1.92Mn1.56Sn0.52 at zero field. An anomalous increase of resistivity of the compound has been observed at temperature range below TM–A, however, it does not copy the sharp change of magnetization at TM–A. The obtained results indicate the important role of interatomic distances on the magnetic ordering and electronic structure of the studied Heusler alloys and are in agreement with the Jahn-Teller mechanism of the martensitic transition in these compounds.

The phase diagrams with influence of biquadratic exchange coupling on martensitic–austenitic transformations for core–surface nanoparticles

Recently, origin of the martensite–austenite transitions in core–surface type magnetic nanoparticles has been investigated theoretically and it has been indicated that repulsive biquadratic exchange coupling (K<0.0) causes the coexisting martensite and austenite phases. In the present paper, the phase diagrams of homogeneous and composite nanoparticles in the kBT/J0−D/J0 plane are studied for the presence and absence of attractive biquadratic exchange interaction in addition to repulsive biquadratic exchange interaction. Significant changes in the phase diagram points are discussed in the presence of martensitic and austenitic transformations. Four regions in the phase diagrams are found as second-order, martensitic–austenitic, TCid and first-order phase transition regimes.

Strain mediated coupling in magnetron sputtered multiferroic PZT/Ni-Mn-In/Si thin film heterostructure

The strain mediated electrical and magnetic properties were investigated in PZT/Ni-Mn-In heterostructure deposited on Si (100) by dc/rf magnetron sputtering. X-ray diffraction pattern revealed that (220) orientation of Ni-Mn-In facilitate the (110) oriented tertragonal phase growth of PZT layer in PZT/Ni-Mn-In heterostructure. A distinctive peak in dielectric constant versus temperature plots around martensitic phase transformation temperature of Ni-Mn-In showed a strain mediated coupling between Ni-Mn-In and PZT layers. The ferroelectric measurement taken at different temperatures exhibits a well saturated and temperature dependent P-E loops with a highest value of Psat ∼ 55 μC/cm2 obtained during martensite-austenite transition temperature region of Ni-Mn-In. The stress induced by Ni-Mn-In layer on upper PZT film due to structural transformation from martensite to austenite resulted in temperature modulated Tunability of PZT/Ni-Mn-In heterostructure. A tunability of 42% was achieved at 290 K (structural transition region of Ni-Mn-In) in these heterostructures. I-V measurements taken at different temperatures indicated that ohmic conduction was the main conduction mechanism over a large electric field range in these heterostructures. Magnetic measurement revealed that heterostructure was ferromagnetic at room temperature with a saturation magnetization of ∼123 emu/cm3. Such multiferroic heterostructures exhibits promising applications in various microelectromechanical systems.

Magnetic properties and magnetocaloric effect in Ni–Mn–Sn alloys

Abstract Magnetic and magnetocaloric properties in Ni50Mn50−xSnx alloys with wide range of the Sn-concentration (x=0–40) were investigated. The alloys were prepared by arc-melting and subsequently annealing at 850 °C for 4 h. The X-ray diffraction analyses manifest the formation of the crystalline phases (Ni2MnSn, NiMn, Ni3Sn2, Mn3Sn, and MnSn2) in the alloys with various compositions and fabrication conditions. With increasing x, the saturation magnetization first increases from near zero (at x=10) to above 40 emu/g (at x=20) and then decreases to below 10 emu/g (at x=40) for both the as-melted and annealed cases. The martensitic–austenitic transition was observed in the alloys with a narrow range of x (13–15). The magnetic transitions in the alloy can be controlled by changing Sn-concentration. The alloy reveals both the positive and negative entropy changes with quite large magnitude (∆Sm>1 J/kg K with ∆H=12 kOe) with appropriate compositions and annealing conditions.

Structure of the Martensite–Austenite Transition Zone After a Local Pulse Heating of the Martensite

Using electron microscopy, the structure of the transition region between austenite and martensite in an austenitic-martensitic iron–chromium–nickel alloy is investigated following a local pulse heating of the martensite. It is found out that the martensite-austenite transition area has a complex structure consisting of a few regions with different phase compositions and degree of defect structure relaxation.

Vibrational properties of Ni–Mn–Ga shape memory alloy in the martensite phases

Studying the phonon dispersion of the ferromagnetic shape memory alloy system Ni–Mn–Ga gives insight into the mechanism of the martensite transition and the forces driving the transition. Transformation of austenite single crystals under uniaxial stress results in the coexistence of two martensitic variants with perpendicular modulation vector. Here we report on inelastic neutron scattering studies of martensite crystals with off-stoichiometric compositions, varying from non-modulated (NM) to five- (5M) and seven- (7M) layer modulated martensite phases. Both the 5M and 7M crystals show fully commensurate satellite peaks along [0], corresponding to the five- and seven-layer modulation. These superstructure peaks become Γ-points of the modulated structure. Due to the coexistence of two variants within the (001) plane, both new acoustic phonons reflecting the modulation vector [0] and acoustic TA2[ξξ0] phonons corresponding to the non-modulated direction are observed. The latter display a pronounced softening around ξ = 0.2–0.4 when approaching the martensite–austenite transition from above and below, i.e. this soft mode has lowest frequency at the transition temperature. Overall the phonon dispersion of the austenite and martensite phase resemble each other very much. The coexistence of two martensitic variants after uniaxial transformation explains the particular behaviour of the low-energy excitations, in contrast to previous interpretations involving charge-density waves and associated phason modes.

Nanoscale magneto-structural coupling in as-deposited and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films

Ferromagnetic shape memory alloys are characterized by strong magneto-mechanical coupling occurring at the atomic scale causing large magnetically inducible strains at the macroscopic level. Employing combined atomic and magnetic force microscopy studies at variable temperature, we systematically explore the relation between the magnetic domain pattern and the underlying structure for as-deposited and freestanding single-crystalline Fe7Pd3 thin films across the martensite–austenite transition. We find experimental evidence that magnetic domain appearance is strongly affected by the presence and absence of nanotwinning. While the martensite–austenite transition upon temperature variation of as-deposited films is clearly reflected in topography by the presence and absence of a characteristic surface corrugation pattern, the magnetic domain pattern is hardly affected. These findings are discussed considering the impact of significant thermal stresses arising in the austenite phase. Freestanding martensitic films reveal a hierarchical structure of micro- and nanotwinning. The associated domain organization appears more complex, since the dominance of magnetic energy contributors alters within this length scale regime.

Dynamical Behavior of Martensite-Austenite Transitions: Simulations and Experiments

The dynamical behavior of the reverse martensitic transformation has been numerically simulated with an atomistic model and compared with experiments in Cu-Zn-Al alloys. Starting from different configurations of the martensitic variants (varying mainly their mean size), the transformation to austenite was studied as a function of the heating speed. Both, experimental and numerical results show that at low velocities there is no dependence of the transition temperatures, whereas at higher speeds they gradually increase. Simulations allow us to have an insight of the underlying processes during the transition to austenite. They also show that a heating speed independent transition can only be obtained when a microstructure of very small variants is present.

Magnetocaloric and Critical Behaviors of ${\rm Ni}_{0.5}{\rm Mn}_{0.5-{\rm x}}{\rm Sn}_{\rm x}$ Heusler Alloys

This work presents the magnetocaloric effect (MCE) and critical behaviors of bulk Ni0.5Mn0.5-xSnx alloys (x=0.14 and 0.15) prepared by an arc-melting method. Under an external field of 50 kOe, we have observed the coexistence of the inverse MCE with a maximum magnetic entropy change (ΔSmax) of 1.39 J·kg-1·K-1 at 218 K (associated with the martensitic-austenitic transition), and normal MCE with a minimum magnetic entropy change (ΔSmin) of -4.97 J·kg-1·K-1 at 308 K (associated with the ferromagnetic-paramagnetic transition in the austenitic phase) in x=0.14 . Meanwhile, the magnetic entropy changes for x=0.15 keeps normal negative, with ΔSmin=-4.55 J·kg-1·K-1 at 315 K. Interestingly, the ferromagnetic-paramagnetic transition in the austenitic phases for both alloys is found to be second-order type. Having used the Arrott-Noakes method, the critical exponents obtained for x=0.14 are β = 0.451 and γ = 1.217, and for x=0.15 are β = 0.501 and γ = 0.963, which are close to those expected for the mean-field model (β = 0.5 and γ = 1). This reveals that a long-range-order ferromagnetic interaction dominates in the austenitic phase. A small deviation from the theoretical values, particularly for x=0.14, is assigned to magnetic inhomogeneity induced by the martensitic-austenitic transition.

Phase transitions in coated nickel titanium arch wires: A differential scanning calorimetric and X-ray diffraction analysis

Shape memory and super-elastic properties of orthodontic nickel titanium wires, which are crucial for its clinical performance are dependent on the austenitic–martensitic phase transitions in its metallic microstructure that happen as a result of temperature or stress. The objective of this study was to compare the austenitic–martensitic phase transitions in new, black oxide coated nickel titanium (0·016 inch, Black Diamond, NiTi) arch wires in the ‘as-received’ form, from the manufacturer and ‘retrieved form’ after two months of intraoral use. This was done to analyse whether the new oxide coated nickel titanium wires suffered any significant loss in shape memory and super elasticity properties at the end of two months of intra oral use, findings of which could give valuable inferences prior to its widespread application in clinical practice.Five arch wire samples in both groups were investigated for their austenitic–martensitic phase transitions in an in vitro set up, using differential scanning calorimetry (DSC), (−90° to 100°C at a rate of 10°C/min) and X-ray diffraction (XRD) analysis (10° to 90°), as a function of temperature. Martensitic–austenitic thermograms showed an intermediate rhombohedral phase in the heating cycle of both groups, but cooling cycles showed direct reversal from austenitic to martensitic phase. Lower austenitic start (\(A_{\rm \textbf{s}}\) = 10·78 ± 0·46°C) and finish (\( {A}_{\rm \textbf{f}}\) = 22·26 ± 0·24°C) temperatures of coated wires compared to the conventional wires showed (i) ability of the wire to remain in austenitic phase below oral temperature, that permits it to take up greater force during activation, (ii) increased springiness and (iii) consistent force delivery for an extended period of time. Statistical analysis with paired Student’s ‘t’ test did not show any significant difference in mean values of transition temperatures and enthalpies between the two groups which proved similar shape memory and super-elastic properties at the end of intra oral use. Black oxide coating of NiTi wires may, therefore, be effective in diversified oral conditions and may find acceptable for re-use after sterilization. Low enthalpy values (0·92–3·59 j/g) compared to conventional ones, implied complete phase transition at the atomic level that can improve performance of the material in activation and deactivation cycles of NiTi wires. X-ray diffraction analysis of the two groups demonstrated predominance of austenitic phases (A, 110, 220 and 211) with complete reversibility at the atomic level. Discrete crystallographic structure and absence of multiple phases showed complete martensitic–austenitic transition, which authenticated the differential scanning calorimetric findings. This can earn acceptance for the new product in contemporary orthodontic practice with adequate scope for indigenization.

Structural defects in Fe–Pd-based ferromagnetic shape memory alloys: tuning transformation properties by ion irradiation and severe plastic deformation

Fe–Pd-based ferromagnetic shape memory alloys constitute an exciting class of magnetically switchable smart materials that reveal excellent mechanical properties and biocompatibility. However, their application is severely hampered by a lack of understanding of the physics at the atomic scale. A many-body potential is presented that matched ab inito calculations and can account for the energetics of martensite austenite transition along the Bain path and relative phase stabilities in the ordered and disordered phases of Fe–Pd. Employed in massively parallel classical molecular dynamics simulations, the impact of order/disorder, point defects and severe plastic deformation in the presence of single- and polycrystalline microstructures are explored as a function of temperature. The model predictions are in agreement with experiments on phase changes induced by ion irradiation, cold rolling and hammering, which are also presented.

Processing and properties of Ni–Mn–Ga magnetic shape memory alloy based hybrid materials

Magnetic shape memory (MSM) materials based on non-stoichiometric Ni–Mn–Ga (NMG) alloys have attracted extensive interest over the last decade because of their actuating, sensing, and damping properties. In addition to MSM phenomenon, NMG alloys have shown conventional shape memory effect, traditional and magnetic-field-assisted superelasticity, magnetocaloric, and special transport properties. The multifunctionality of MSM alloys may be utilized for actuators, sensors, dampers, and perhaps also for energy harvesting. The dominant mechanism behind the MSM phenomenon is the twin boundary movement in the martensitic phase, and in order to take the full advantage of above mentioned properties single crystals are usually desired. However, growth of Ni–Mn–Ga single crystals is quite tedious and high quality crystals are only limitedly available at a considerably high prize. Therefore, alternative material solutions based on hybrid concepts have attained increasing interest. NMG-polymer hybrids can be tailored for a particular application by a proper combination of NMG alloy and polymer and prepared in various forms including embedded NMG particles, ribbons, or sheets. These materials are especially interesting for applications in vibration damping, actuation, and sensing. Polymer selection is critical for the functionality of NMG-polymer hybrids, and the applied polymer should be selected based on the martensite type of the NMG alloy. E.g., the transition temperatures of both components should be adjusted carefully considering the application. The contact between NMG and polymer and the stiffness of the polymer are important. Excellent damping performance is obtained in the NMG-soft epoxy matrix below the martensite–austenite transition temperature region when compared to that of the pure polymer. The relative damping capacity is found to be better than that of any other known material.

Magnetic studies of Fe-doped martensitic Ni2Mn1.44Sn0.56-type Heusler alloy

Localization and magnetism of Fe replacing either Mn or Ni in the Ni2Mn1.44Sn0.56-type Heusler alloy have been systematically investigated using magnetization, scanning electron microscopy and Mössbauer spectroscopy. It has been shown that the addition of Fe either in Mn or in Ni sites reduces the fraction of the Mn-rich NiMnSn-type Heusler alloys that has short-range antiferromagnetic interactions; consequently it reduces the martensitic–austenitic transition temperature and increases the thermal hysteresis width due to an increase in atomic disorder caused by Fe replacements. The Fe atoms in Mn sites have two magnetic configurations with magnetic moments of 0.8μB/Fe and 1.4μB/Fe in the martensitic orthorhombic structure, while Fe in Ni sites have magnetic moments smaller than 0.1μB/Fe. These results indicate that the Fe atoms are distinctly substituting either Mn or Ni and the decrease in the martensitic phase transition temperature for increasing Fe content can be mainly attributed to the Fe atoms in the Mn sites in both cases.