Ferromagnetic Martensite Phase Research Articles

Concomitant interfacial spin fractal transformation and exchange bias in a magnetic shape memory alloy

Small-angle neutron scattering is used to study the exchange bias effect in Mn50Ni40Sn10. The martensitic transformation is found to be responsible for the nanometer-scale spin clusters (SCs) inside the low-temperature ferromagnetic (FM) martensite phase. The magnetic field dependences of the SCs and FM domains exhibit an asymmetry that characterizes the exchange bias. We find that the surface geometries of the two magnetic phases are strongly susceptible to magnetization changes. While the FM domains and SCs are dominated by diffuse and fractal surfaces around the coercive fields, this configuration is reversed in the saturation regime, where the FM and SC surfaces become fractal and diffuse, respectively. These geometry changes are driven by the spin reorientation inside the heterogeneous FM/SC domain walls, where exchange anisotropy arises. Our work reveals the microscopic mechanism underlying the exchange bias effect in magnetic shape memory alloys.

Asymmetric magnetic-field-induced phase transformation in Fe48Mn24Ga28 Heusler alloy

We report on the conceptual design and experimental realization of a ternary Heusler-based ferromagnetic martensitic phase transformation alloy Fe48Mn24Ga28 that exhibits asymmetric magnetostructural coupling between the forward austenite-to-martensite and the reverse martensite-to-austenite phase transformations. By tuning the Curie temperature of the high-temperature austenite to be within the thermal hysteresis of the first-order phase transformation, dissimilar change in magnetization can be associated with the forward and reverse phase transformations, respectively. We show that such an asymmetry leads to enlargement of the thermal hysteresis with an applied external magnetic field. Furthermore, partly due to the field-enhanced thermal hysteresis, the magnetic-field-induced martensitic phase transformation is irreversible in such a system. This work suggests the intricate correlation between magnetostructural coupling, thermal hysteresis, and magnetic-field-induced phase transformation, which can be exploited for designing magnetic phase transformation alloys that show extended functionalities.

Magnetostriction of Heusler Ferromagnetic Alloy, Ni2MnGa0.88Cu0.12, around Martensitic Transition Temperature

In this study, magnetostriction measurements were performed on the ferromagnetic Heusler alloy, Ni2MnGa0.88Cu0.12, which is characterized by the occurrence of the martensitic phase and ferromagnetic transitions at the same temperature. In the austenite and martensite phases, the alloy crystallizes in the L21 and D022-like crystal structure, respectively. As the crystal structure changes at the martensitic transition temperature (TM), a large magnetostriction due to the martensitic and ferromagnetic transitions induced by magnetic fields is expected to occur. First, magnetization (M-H) measurements are performed, and metamagnetic transitions are observed in the magnetic field of μ0H = 4 T at 344 K. This result shows that the phase transition was induced by the magnetic field under a constant temperature. Forced magnetostriction measurements (ΔL/L) are then performed under a constant temperature and atmospheric pressure (P = 0.1 MPa). Magnetostriction up to 1300 ppm is observed around TM. The magnetization results and magnetostriction measurements showed the occurrence of the magnetic-field-induced strain from the paramagnetic austenite phase to the ferromagnetic martensite phase. As a reference sample, we measure the magnetostriction of the Ni2MnGa-type (Ni50Mn30Ga20) alloy, which causes the martensite phase transition at TM = 315 K. The measurement of magnetostriction at room temperature (298 K) showed a magnetostriction of 3300 ppm. The magnetostriction of Ni2MnGa0.88Cu0.12 is observed to be one-third that of Ni50Mn30Ga20 but larger than that of Terfenol-D (800 ppm), which is renowned as the giant magnetostriction alloy.

Low hysteretic magnetostructural transformation in Cr-doped Ni-Mn-Ga Heusler alloy

Ni-Mn-Ga based alloys are widely studied due to their potential for practical applications, making use of their martensitic transformations. However, hysteresis is a long-standing drawback that reduces the chance of transferring these alloys from the laboratory to industry. In this work, we studied a Cr-doped Ni2.15Mn0.70Cr0.15Ga alloy. We arrived at this composition by integrating data obtained from the previous phase and hysteresis diagrams taken from the literature. The compound presents a magnetostructural transition at room temperature, with a ferromagnetic martensite phase, and moderate thermal hysteresis of approximately 4 K. The magnetocaloric and ferromagnetic shape memory effects were explored, showing a reversible entropy change of approximately 9 Jkg-1K-1 under 0–5 T field change, and a cyclical magnetic-field-induced deformation close to 0.81% under 0–9 T, an advance towards high reversibility for this family alloys.

Inherent Magnetic Sensor for Estimation of Fatigue Damage in Type 304 Stainless Steel

Abstract In this study, the fatigue damage in type 304 stainless steel was estimated using an inherent magnetic sensor via plane-bending fatigue tests and the electromagnetic impedance method. The sensor was a magnetic composite material incorporating a ferromagnetic martensite phase generated in type 304 stainless steel by a surface finish process during the production stage. The output properties of this sensor as a function of the number of cycles were evaluated under various conditions. It was demonstrated that this sensor could detect fatigue damage starting from the zeroth cycle. The sensor output repeatability was evaluated, and the variation in the output between the inherent magnetic sensors was approximately 10% regardless of the sensor type and total strain amplitude. By using the two proposed estimation methods, the specific fatigue level and the number of cycles could be estimated with errors of 3–27%. These results indicated that the inherent magnetic sensor was suitable for use for fatigue damage estimation.

Metallurgical Phase Transformation and Behavior of Steels Under Impact Loading

This study deals with the mechanical behavior of the austenitic stainless steel, AISI 304, under quasi-static and impact loadings. In particular, the evolution of the transformation-induced plasticity (TRIP) is investigated. Several compression tests have been performed at various strain rates under both static and impact conditions (crash tests). In order to show the influence of the strain rate on the phase transformation, the microstructures of as-received and deformed specimens have been observed with a scanning electron microscope. Then, the newly formed ferromagnetic martensitic phase is analyzed with the help of the vibrating sample magnetic (VSM) method and by X-ray diffraction (XRD). The results obtained from the mechanical testing indicate that the TRIP effect is actually occurring since a significant change of the curve slope is visible during the hardening regime in the stress–strain curve. The VSM and XRD techniques, on the other hand, show that the proportion of the martensitic phase is reduced when the strain rate is increased. The quantity of martensite formed is greater when quasi-static loadings are applied to the material.

Interference between distortion and magnetization for the reentrant martensitic transition in Heusler-type alloys Co2Cr(Ga,Si)

Heusler-type ${\mathrm{Co}}_{x}{\mathrm{Cr}}_{78\ensuremath{-}x}{\mathrm{Ga}}_{11}{\mathrm{Si}}_{11}$ alloys exhibit the reentrant martensitic transition, i.e., the forward martensitic transition from the paramagnetic austenite (Para-A) phase into the paramagnetic martensite (Para-M) phase and the successive reverse martensitic transition from the Para-M phase into the ferromagnetic austenite (Ferro-A) phase with decreasing temperature $T$. In the present paper, this complicated transition is analyzed by use of phenomenological free energy, which is expanded in powers of two order parameters, i.e., the tetragonal distortion ${e}_{3}$ and the magnetization $M$. It is assumed that an interference between ${e}_{3}$ and $M$ through the repulsive magnetostructural interaction, $G{e}_{3}^{2}{M}^{2}(g0)$, in the free energy is strong enough to suppress out the ferromagnetic martensite (Ferro-M) phase with both nonvanishing ${e}_{3}$ and $M$. With the presence of this repulsive interaction, the observed phase diagram in the $T\ensuremath{-}x$ plane is found to be successfully explained. The $T$ dependencies of ${e}_{3}$ and the entropy are also calculated to see the equilibrium state. The inverse magnetic susceptibility in the martensite phase ${\ensuremath{\chi}}^{\ensuremath{-}1}$ is derived by taking into account that the term $G{e}_{3}^{2}{M}^{2}(g0)$ opposes the magnetization by an external magnetic field in the martensite phase. The calculated ${\ensuremath{\chi}}^{\ensuremath{-}1}$ exhibits a negative paramagnetic Curie temperature as was observed, which verifies directly the existence of $G{e}_{3}^{2}{M}^{2}(g0)$. An essential role of $G{e}_{3}^{2}{M}^{2}(g0)$ in the phase stabilities is also found in the calculated $T$ dependence of the elastic constant $({c}_{11}\ensuremath{-}{c}_{12})$. On the basis of the present analyses, the conditions for the appearance of the reentrant martensitic transition are described.

Exchange bias and spin glass transition in quaternary MnCuNiSn Heusler alloy

We have systematically investigated the exchange bias (EB) effect and low-temperature magnetic state transition in Mn42Cu6Ni42Sn10 Heusler alloy by means of AC and DC magnetization measurements. Experimental results suggest that the martensitic phase is in the state of superparamagnetic (SPM), following that the particles turn into the ferromagnetic (FM) state at the martensitic Curie temperature, and the moments are frozen as the temperature is below 150K. In addition, EB effect was observed in the phase separated martensitic state, which can achieves about 536Oe under the field of 500Oe after field-cooled (FC) to 10K. First-principle investigations indicate that such a small value is originated from nonmagnetic Cu atoms substitution. The appearance of EB effect can be ascribed to the competition between the FM martensitic phase and spin glass (SG) phase due to the impact of Mn atoms, whereas the nonmagnetic Cu atoms doping would hinder the magnetic aggregation, resulting the small EB value eventually.

Comparisons of damage-induced magnetizations between austenitic stainless and carbon steel

To explore the mechanism of damage-induced magnetization for paramagnetic austenitic stainless steel, the differ- ences of damage-induced magnetizations between an austenitic stainless steel and a carbon steel are experimentally studied. Various plastic strains are introduced into the test-pieces of the austenitic stainless steel of 304 type and S185 carbon steel material. The residual plastic strains and the damage-induced magnetic fields nearby the test-pieces, the volume fraction of the damage-induced ferromagnetic martensitic phase in austenitic stainless steel test-pieces are measured. The relationships be- tween the residual plastic strain and the damage-induced magnetic field of two materials are investigated and their differences are compared. In addition, by establishing the relationships of the damage-induced magnetic field with the damage-induced martensitic phase, the effect of the phase transformation on the damage-induced magnetization of austenitic stainless steel is analyzed.

Magnetic phase diagram of ferromagnetic shape memory alloys [formula omitted

Permeability and differential scanning calorimetry measurements were carried out on Ni2MnGa1-xFex(0⩽x⩽0.40). On the basis of the experimental results, the phase diagram in the temperature–concentration plane was determined. The characteristics of the determined phase diagram are similar to those of the phase diagrams of Ni2+xMn1-xGa,Ni2Mn1-xCuxGa and Ni2MnGa1-xCux systems, where the magnetostructural transition between the paramagnetic austenite phase (Para-A) and ferromagnetic martensite phase (Ferro-M) occurs. Furthermore, the concentration dependence of the magnetic moment per formula unit, μs, at 5K for Ni2MnGa1-xFex(0⩽x⩽0.375) is also reported.

Martensitic Transformation of Austenitic Stainless Steel Cruciform Geometry Sample by Biaxially Fatigued Cycling

A specifically designed cruciform-shaped austenitic stainless steel AISI 321 sample was subjected to ex-situ biaxial tension-compression cycling to establish ferromagnetic martensitic phase conversion under the action of plastic deformation. The time-of-flight neutron diffraction technique was employed for in-plane residual stress determination in this sample for both the austenitic and martensitic phases. The 2D data enabled determination of the macro-, micro-, hydro- and deviatoric contributions to the total phase stresses.

Magneto-optical spectroscopy of the martensitic transition in Fe48Mn24Ga28 Heusler alloys

Magneto-optical spectra of polycrystalline samples of the Fe48Mn24Ga28 Heusler alloy undergoing martensitic transformation from the high-temperature paramagnetic austenitic to ferromagnetic martensitic phase have been studied at 50–320 K in the transversal Kerr effect geometry. A comparison of magnetooptical spectra with data obtained in magnetic measurements has demonstrated that the martensitic transition on the surface of a sample and in its bulk takes place in the same temperature interval. Magnetic anisotropy has been found in the martensitic phase driven by large multidomain inclusions of martensite in austenite. The magneto-optical signal of Fe48Mn24Ga28 differs strongly in spectral shape from that measured in Ni-Mn-Ga.

Phase diagram and magnetic moment of Ni50+Mn27−Ga23 ferromagnetic shape memory alloys

Abstract Permeability, magnetization, differential scanning calorimetry and X-ray powder diffraction measurements were carried out on the off-stoichiometric ferromagnetic shape memory alloys Ni 50+ x Mn 27− x Ga 23 (−25 ⩽ x ⩽ 6) in order to establish the phase diagram over a wide range of concentration. The phase diagram was found to comprise of four regions, a paramagnetic austenite phase (Para-A), a paramagnetic martensite phase (Para-M), a ferromagnetic austenite phase (Ferro-A) and a ferromagnetic martensite phase (Ferro-M). Furthermore the martensitic transformation and the Curie temperatures crossed over at around x = 2.5. The martensitic transformation was found to occur over a wide Mn-rich region. A linear increase in the martensitic transformation temperature with increasing number of valence electrons per atom, e / a , was limited to a narrow concentration range around x = 0.0. Overall the martensitic transformation temperature exhibited a concave behavior with a minimum occurring at approximately x = −8.

Magnetic Field-Induced Transition in Co-Doped Ni<sub>41</sub>Co<sub>9</sub>Mn<sub>31.5</sub>Ga<sub>18.5</sub> Heusler Alloy

Thermal strain, magnetostriction and magnetization measurements of Ni41Co9Mn31.5Ga18.5 polycrystalline ferromagnetic shape memory alloy (FSMA) were performed across the martensitic transition temperature, TM, and the reverse martensitic transition temperature, TR ,a t atmospheric pressure. When cooling from the austenite phase, a steep decrease in thermal expansion due to the martensitic transition at TM was found. When heating from the martensitic phase, a steep increase in the thermal expansion due to the reverse martensitic transition at TR was observed. These transition temperatures decreased gradually with increasing magnetic field. The field dependence of the martensitic transition temperature, dTM/dB ,i s¹4.2K/T and that of the reverse martensitic transition temperature, dTR/dB ,i s¹7.9K/T. The metamagnetic transition appeared between 330 and 390K. The results of thermal strain and magnetization measurements indicate that a magneto-structural transition occurred at TM. The region above TM or TR is the ferromagnetic austenite phase and that below TM or TR is the paramagnetic or weak ferromagnetic martensitic phase. At constant temperature, a magnetic field-induced strain was observed with a value of 1.0 © 10 ¹3 , which indicates that this alloy is sensitive to magnetic fields. Strong magneto-structural coupling was revealed by the magnetic properties and phase transitions. [doi:10.2320/matertrans.M2012289]

Magnetic entropy and magnetocaloric effect of ferromagnetic Heusler alloys Ni2Mn0.75Cu0.25Ga and Ni2MnGa0.88Cu0.12

Abstract The magneto-structural transformation and the associated magnetic entropy change were studied in the ferromagnetic shape memory alloys Ni 2 Mn 0.75 Cu 0.25 Ga and Ni 2 MnGa 0.88 Cu 0.12 by using magnetization measurements performed around the martensite transformation temperature T M and the Curie temperature T C . The direct transition from the paramagnetic austenite phase (PM phase) to the ferromagnetic martensite phase (FM phase) was observed in both alloys. In a magnetic field of 2.0 T, the Ni 2 Mn 0.75 Cu 0.25 Ga and Ni 2 MnGa 0.88 Cu 0.12 alloys showed large magnetic entropy changes of −Δ S M = 15.2 J/kg K at 302 K and −7.5 J/kg K at 333 K, respectively. The magnetic entropy change of Ni 2 Mn 0.75 Cu 0.25 Ga is larger than that of the polycrystalline Ni 2.19 Mn 0.81 Ga alloy. Theoretical calculations were also performed, and they agreed well with the experimental determinations of −Δ S M .

Phase diagram of the ferromagnetic shape memory alloys Ni2MnGa1−xCox

Permeability, magnetization, and differential scanning calorimetry measurements are carried out on the ferromagnetic shape memory alloys Ni${}_{2}$MnGa${}_{1\ensuremath{-}x}$Co${}_{x}$ (0 \ensuremath{\le}$\phantom{\rule{0.28em}{0ex}}x$ \ensuremath{\le} 0.20). On the basis of the experimental results, the phase diagram in the temperature-concentration plane is determined. The determined phase diagram is spanned by a paramagnetic austenite (Para-A) phase, paramagnetic martensite phase, ferromagnetic austenite phase, ferromagnetic martensite (Ferro-M) phase, and a premartensite phase. The measurements show that a magnetostructural transition between the Para-A and Ferro-M phases does not appear in this alloy system. The phase diagram is discussed using phenomenological Landau theory.

Residual stresses in biaxially fatigued austenitic stainless steel sample of cruciform geometry

A specifically designed cruciform-shaped austenitic stainless steel AISI 321 sample was subjected to ex-situ biaxial tension-compression cycling to establish ferromagnetic martensitic phase conversion under the action of plastic deformation. The time-of-flight neutron diffraction technique was employed for in-plane residual stress determination in this sample for both the austenitic and martensitic phases. The 2D data enabled determination of macro-, micro-, hydro- and deviatoric contributions to the total phase stresses.

Coexistence of reentrant-spin-glass and ferromagnetic martensitic phases in the Mn2Ni1.6Sn0.4 Heusler alloy

A giant exchange bias field of up to 1170 Oe was observed in the Mn2Ni1.6Sn0.4 Heusler alloy. A reentrant spin glass phase and a ferromagnetic martensitic phase coexist below the blocking temperature as confirmed by dc magnetization and ac susceptibility measurements. Exchange bias in Mn2Ni1.6Sn0.4 is thought to originate from the interface exchange interaction between the reentrant spin glass phase and the ferromagnetic martensitic phase. X-ray diffraction and selected area electron diffraction results demonstrate that excess Mn atoms occupy Ni and Sn sites randomly. In this way, Mn-Mn clusters are formed and constitute the reentrant-spin-glass phase.

Thermal expansion and magnetization studies of the novel ferromagnetic shape memory alloy Ni2MnGa0.88Cu0.12 in a magnetic field

Thermal expansion, permeability and magnetization measurements of the ferromagnetic shape memory alloys, Ni2MnGa0.88Cu0.12, were carried out across the martensitic transition temperature TM and the reverse martensitic transition temperature TR at atmospheric pressure. When cooling from the austenite phase, a steep decrease in the thermal expansion due to the martensitic transition was found. The permeability indicates a sharp peak around TM and ferromagnetism below TM. Considering the permeability and magnetization results of Ni2MnGa0.88Cu0.12, the region above TM or TR is the paramagnetic-austenite phase and the region below TM or TR is the ferromagnetic-martensite phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. TM and TR increased gradually with increasing magnetic field. The shift of TM in magnetic fields (B) around zero magnetic fields was estimated as dTM/dB=1.3 K T−1. The shift of TM indicates that magnetization influences the martensitic transition, and the increase of TM with magnetic field is proportional to the difference in the magnetization between the austenite and martensite phases.

Magnetic Phase Diagram of the Ferromagnetic Shape Memory Alloys Ni<sub>2</sub>MnGa<sub>1-x</sub>Cu<sub>x</sub>

X-ray powder diffraction, permeability, magnetization and differential scanning calorimetry measurements were carried out on the magnetic shape memory alloys Ni2MnGa1−xCux (0 ≤ x ≤ 0.25). On the basis of the experimental results, the phase diagram in the temperature– concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (Para-A), paramagnetic martensite phase (Para-M), ferromagnetic austenite phase (Ferro-A), ferromagnetic martensite phase (Ferro-M) and the premartensite phase. It was found that the magnetostructural transition between the phases Para-A and Ferro-M can occur in the concentration region 0.12 < x ≤ 0.14 and that Ni2MnGa1−xCux has the characteristics of the phase diagram similar to those of the phase diagrams of Ni2+xMn1−xGa and Ni2Mn1−xCuxGa. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and magnetization was constructed and analyzed.