Magnetic Field-induced Phase Transformation Research Articles

An integrated exploration from microstructure to mechanics in austenite to pearlite transformation of high carbon steel under high magnetic fields

The effects of a high magnetic field (12 T) on the microstructural evolution and mechanical properties of high carbon steel during isothermal pearlitic transformation at 720 °C were investigated. The application of a high magnetic field effectively promotes pearlitic transformation and leads to a reduction in the lamellar spacing and the geometrically necessary dislocation density (GND) as well as to a slight increase in the grain size of the pearlite, which leads to a decrease in hardness. This is because the magnetic field reduces the magnetic Gibbs energy of ferrite and cementite, resulting in an increase in the driving force of phase transformation, which shortens the pearlite maturation time and reduces the nucleation barriers of pearlite, thereby accelerating the growth rate of pearlite. In addition, the interfacial energy of ferrite/cementite was found to be increased by the magnetic field from the thermodynamic point of view, and the mechanism of action of the magnetic field -induced precipitation of cementite in the form of particles and short rods was elucidated. The present work will enhance the understanding of the mechanism of magnetic field-induced phase transformation in iron-based materials.

Variation of Electrical Resistivity During Magnetic Field-Induced Martensitic Transformation in Vanadium added NiMnSnB alloys

In this study, the structural and electrical properties of Ni49-xVxMn37Sn12B2 (x = 0, 1, 2, and 3) ferromagnetic shape memory alloys were investigated. According to XRD analyzes at room temperature, the x=0 sample was in the martensite phase, the x=1 and 2 samples were in the mixture phase, and the x=3 sample was in the austenite phase. The resistivity analyses depend on temperature showed that all samples exhibited martensitic transformation and the phase transformation temperature decreased with V doping. Magnetoresistance (MR) values were calculated using ρ-T curves performed under 0 T and 1 T magnetic fields. The observed negative MR is consistent with Kataoka's s-d model. As-Af interval was determined and M-H measurements were made at constant temperatures determined in this interval. The results were attributed to the magnetic field-induced phase transformation (MFIPT). In order to examine the effects of MFIPT on the electrical resistivity, the resistivity depend on magnetic field was measured using the same thermal process. The overlapping of the curves in the high magnetic field revealed that the resistivity decreased due to the MFIPT as well as the MR.

Strain manipulation of magnetocaloric effect in a Ni39.5Co8.5Mn42Sn10 melt-spun ribbon

Heusler-type Ni-Mn-based alloys can demonstrate large magnetocaloric effect through magnetic field-induced phase transformation, but it usually occurs in a relatively narrow temperature region. In this work, we fabricate a bilayer composite with a Ni39.5Co8.5Mn42Sn10 ribbon adhered to a Ni50Ti50 substrate in order to manipulate the magnetocaloric properties by using the recovery strain of the predeformed substrate. The transformation temperatures of the ribbons can be effectively shifted to higher temperature region through the external strain transferred from the substrate, which contributes to the extended cooling temperature region. Moreover, the effective refrigeration capacity can also be improved by such strain manipulation.

Versatile Device for the Study of Coupling Properties in Multifunctional Materials. Example: Ni-(Co)-Mn-In Heusler Materials

We report on a versatile experimental setup for the study of the electro-magneto-elasto-thermal coupling for bulk materials having coupling properties. The setup enables measurement of stress and electrical resistivity in a 80-450 K temperature range under controlled uniaxial pressure in a magnetic field up to 8 T. The strong influence of uniaxial pressure and magnetic field on strain and transport properties is shown for Ni-Co-Mn-In Heusler single crystal material. Phase transformation, magnetostress and magnetic field-induced phase transformation, and elastocaloric effect can be estimated using this versatile setup. This device is very complementary to the studies focused on the modeling of thermo-electro-magneto-mechanical behavior for functional materials.

Morin transition in Hematite nanoparticles analyzed by neutron diffraction

Hematite (α-Fe2O3) undergoes a first order spin reorientation transition called the Morin transition: upon cooling, the moments align antiferromagnetically along the rhombohedral axis, and the net magnetic moment goes to zero. Morin transition temperature is around to 260K in bulk materials and depends on the mean particle size. In this work, the Morin transition has been studied by neutron diffraction as function of temperature and applied magnetic field in 47 nm nanoparticles. The Rietveld analysis of the diffraction spectra around the Morin transition shows a similar behavior to that found in bulk samples. On the other side, the magnetic field induced phase transformation has been analyzed.

Magnetic field-induced martensitic phase transformation in magnetic shape memory alloys: Modeling and experiments

In this work, a continuum based model of the magnetic field induced phase transformation (FIPT) for magnetic shape memory alloys (MSMA) is developed. Hysteretic material behaviors are considered through the introduction of internal state variables. A Gibbs free energy is proposed using group invariant theory and the coupled constitutive equations are derived in a thermodynamically consistent way. An experimental procedure of FIPT in NiMnCoIn MSMA single crystals, which can operate under high blocking stress, is described. The model is then reduced to a 1-D form and the material parameter identification from the experimental results is discussed. Model predictions of magneto-thermo-mechanical loading conditions are presented and compared to experiments.

Magnetic field induced phase transformation in polycrystalline NiCoMnAl thin films

NiCoMnAl thin films were deposited onto unheated substrates using dc magnetron co-sputtering. The microstructure of as-deposited films consisted of nanocrystals in an amorphous matrix and did not exhibit a martensitic phase transformation. After heat treatment, films crystallized into a B2 austenite phase, which exhibited a magnetic field induced martensitic phase transformation. The level of the change in the martensitic transformation temperatures in the magnetic field was determined to be ∼2.1 K/T. The films exhibited non-reversible magnetic field induced martensite to austenite transformation due to the large thermal hysteresis.

Phase diagram and composition optimization for magnetic shape memory effect in Ni–Co–Mn–Sn alloys

The composition dependent phase transformation and magnetic properties of the Ni50−xCoxMn39Sn11 (0≤x≤10) alloys were systematically investigated. Based on the obtained results, the phase diagram of this alloy system was established. Considering the combination of large magnetization difference across phase transformation ΔM and small transformation entropy change ΔS, the optimal composition range of 5≤x≤8 for magnetic-field-induced phase transformation (MFIPT) and consequently magnetic shape memory effect was figured out. Furthermore, the decrease in martensitic transformation temperatures by magnetic fields and almost fully reversible MFIPT from martensite to austenite were confirmed in the alloys within this composition range.

Shape memory and pseudoelasticity response of NiMnCoIn magnetic shape memory alloy single crystals

The shape memory characteristics of Ni45Mn36.5Co5In13.5 single crystals, which exhibit magnetic field-induced phase transformation (FIPT), were investigated under compression along the [1 0 0] orientation. The effects of temperature and bias stress on the pseudoelastic response and the shape memory effect were explored. The crystals demonstrated large stress and temperature hysteresis with a fully reversible transformation strain of 5.4%, the ramifications of which on the field requirement for FIPT are discussed.

One-way shape memory effect due to stress-assisted magnetic field-induced phase transformation in Ni 2MnGa magnetic shape memory alloys

One-way shape memory effect is achieved utilizing stress-assisted field-induced martensitic phase transformation in Ni2MnGa magnetic shape memory alloy single crystals. Depending on temperature, one-way and reversible magnetic field-induced strains of up to 3.1% and 0.3% are attained under stresses as high as 110 MPa and 24 MPa, respectively. These bias stress levels are more than one order of magnitude higher than the previously reported values obtained from field-induced martensite reorientation.

Stress-assisted reversible magnetic field-induced phase transformation in Ni2MnGa magnetic shape memory alloys

Reversible magnetic field-induced martensitic phase transformation is demonstrated for the first time in Ni2MnGa magnetic shape memory alloy single crystals under constant stress levels and low field magnitudes. Actuation stress levels larger than 25 MPa were achieved providing an order of magnitude increase with respect to the values obtained from field-induced martensite reorientation. The necessary magneto-thermo-mechanical conditions for the field-induced phase transformation are discussed.