Fe-Pd Ferromagnetic Shape Memory Alloy Research Articles

Composition-dependent properties and phase stability of Fe-Pd ferromagnetic shape memory alloys: A first-principles study

The composition-dependent properties and their correlation with the phase stability of Fe75+xPd25−x (−10.0≤x≤10.0) alloys are systematically investigated by using first-principles exact muffin-tin orbitals (EMTO)-coherent potential approximation (CPA) calculations. It is shown that the martensitic transformation (MT) from L12 to body-centered-tetragonal (bct) occurs in the ordered alloys with about −5.0≤x≤10.0. In both the L12 and bct phases, the evaluated a and c/a agree well with the available experimental data; the average magnetic moment per atom increases whereas the local magnetic moments of Fe atoms, dependent on both their positions and the structure of the alloy, decrease with increasing x. The tetragonal shear elastic constant of the L12 phase (C′) decreases whereas that of the bct phase (Cs) increases with x. The tetragonality of the martensite (|1−c/a|) increases whereas its energy relative to the austenite with a negative value decreases with Fe addition. All these effects account for the increase of MT temperature (TM) with x. The MT from L12 to bct is finally confirmed originating from the splitting of Fe 3d Eg and T2g bands upon tetragonal distortion due to the Jahn-Teller effect.

Shock wave induced martensitic transformations and morphology changes in Fe-Pd ferromagnetic shape memory alloy thin films

Combining experimental methods and classical molecular dynamics (MD) computer simulations, we explore the martensitic transformation in Fe70Pd30 ferromagnetic shape memory alloy thin films induced by laser shock peening. X-ray diffraction and scanning electron microscope measurements at shock wave pressures of up to 2.5 GPa reveal formation of martensitic variants with preferred orientation of the shorter c-axis of the tetragonal unit cell perpendicular to the surface plane. Moreover, consequential merging of growth islands on the film surface is observed. MD simulations unveil the underlying physics that are characterized by an austenite-martensite transformation with a preferential alignment of the c-axis along the propagation direction of the shock wave, resulting in flattening and in-plane expansion of surface features.

Influence of defects on the irreversible phase transition in Fe–Pd ferromagnetic shape memory alloys

In Fe–Pd ferromagnetic shape memory alloys a face-centered tetragonal (fct) martensite can be obtained when cooling the face-centered cubic austenite through the martensite transition. Nevertheless, further irreversible transformation on cooling into a body-centered tetragonal (bct) martensite needs to be prevented in order to retain the shape memory properties. Differential scanning calorimetry experiments demonstrate that high temperature thermal treatments stabilize the fct phase, reducing the fct–bct transformation temperature. A large misfit between the cell parameters of fct and bct phases was determined by neutron diffraction, pointing to the critical role of dislocations in the accommodation of both phases. The presence of dislocations and its dynamics was analyzed by mechanical spectroscopy, and a relaxation peak at ∼443K related to the dislocation movement was identified. The driving force of the relaxation process can be proposed as a dislocation dragging mechanism controlled by the migration of vacancies without break-away. Defects such as dislocations and vacancies have been shown to play an important role in changing the irreversible phase transformation temperature. A reduction in the dislocation density reduces the irreversible transformation temperature and so increases the stability range of the alloy.

Magneto-Structural Properties and Magnetic Behavior of Fe-Pd Ribbons

Ribbons of Fe-Pd ferromagnetic shape memory alloys were prepared by rapid solidification via the melt-spinning technique. The effect of moderate and high temperature heat treatments on the martensitic transformation and the related changes of the magnetic behavior was analyzed by X-ray diffraction, differential scanning calorimetry and temperature, and field-dependent magnetometry. In addition, information about magnetic easy axis and magnetically field-induced strains (MFIS) were achieved from linear thermal expansion measurements performed under cooling at different applied fields. The observed low values of MFIS in the as-prepared ribbons are due, besides to the small size and random orientation of the grains, to the high atomic disorder inside the crystalline grains. The anisotropy field is enhanced by the reduced atomic disorder, as reflected by the increased MFIS values after thermal treatments.

Mechanical properties and twin boundary drag in Fe–Pd ferromagnetic shape memory foils—experiments and ab initio modeling

We report on vibrating reed measurements combined with density functional theory-based calculations to assess the elastic and damping properties of Fe–Pd ferromagnetic shape memory alloy splats. While the austenite–martensite phase transformation is generally accompanied by lattice softening, a severe modulus defect and elevated damping behavior are characteristic of the martensitic state. We interpret the latter in terms of twin boundary motion between pinning defects via partial ‘twinning’ dislocations. Energy dissipation is governed by twin boundary drag, primarily due to lattice imperfections, as concluded from the temperature dependence of damping and related activation enthalpies.

Effect of Co and Mn Doping on the Martensitic Transformations and Magnetic Properties of Fe-Pd Ferromagnetic Shape Memory Alloys

The effect of the partial substitution of Fe by Co and Mn on the structural and magnetic properties of Fe-Pd alloys has been investigated by means of calorimetric, magnetic and neutron diffraction measurements. The addition of Mn increases both the reversible and irreversible MT temperatures whereas the addition of Co causes the opposite effect, which points out that that the compositional dependence of the transformation temperatures of the Fe-Pd alloys is not related to the Pd amount but to the e/a concentration. Furthermore, it is shown that the appearance of the BCT phase is directly related to the tetragonality of the FCT phase. On the other hand, the amount of FCT martensite that irreversibly transforms into the undesirable BCT phase is considerably reduced by both the Co and Mn doping. The substitution of Fe by Co seems to be especially interesting for magnetic field-induce strains applications since both the magnetic anisotropy and the saturation magnetization of the Fe-Pd FCT martensite are considerably increased with the Co addition.

Effect of Mn addition on the structural and magnetic properties of Fe–Pd ferromagnetic shape memory alloys

The effect of Mn addition on the structural and magnetic properties of Fe–Pd ferromagnetic shape memory alloys is investigated. In particular, a complete characterization of the influence of the partial substitution of Fe by Mn has been performed on Fe 69.4− x Pd 30.6Mn x ( x = 0, 1, 2.5 and 5) alloys. The substitution of 1% Fe by Mn fully inhibits the undesirable irreversible face-centered tetragonal to body-centered tetragonal transformation without decreasing the face-centered cubic to face-centered tetragonal temperature. In addition, the substitution of 2.5% Fe by Mn gives rise to the highest thermoelastic transformation temperature observed to date in the Fe–Pd system, probably due to an increase in the valence electron concentration. The magnetocaloric effect has been evaluated in this alloy system for the first time. Nevertheless, the low values obtained suggest that the Fe–Pd alloys are not good candidates for magnetic refrigeration applications.

Superelastic Properties of Rapidly Solidified Fe-Pd Ribbons

The Fe-Pd ferromagnetic shape memory alloy is a multifunctional material. We investigated the behavior of the superelastic properties of rapidly solidified Fe-Xat%Pd (X ¼ 29:3, 29.6, 30.2 and 30.4) alloy ribbons to develop biomedical materials applicable to orthodontics and medical instrumentation. The appearance of a stress-induced martensite phase under the loading stress was confirmed by X-ray diffraction measurements. Stress-strain curves were drawn from results obtained using a tensile testing machine at temperature of 290 and 313 K. The FeXat%Pd (X ¼ 29:6, 30.2, 30.4) ribbons exhibited superelastic strain of 3.0–3.5%. Moreover, when these Fe-Pd alloy ribbons were treated in Hanks’ solution for 150 days, they exhibited good biocorrosion resistance. [doi:10.2320/matertrans.MRA2007216]

Mechanical Properties of Fe-Pd Ferromagnetic Shape Memory Alloy Thin Films Prepared by Dual Source DC Sputtering

The purpose of this work is to quantitatively clarify the shape memory behavior of Fe-Pd films containing ~30at%Pd by thermal cycling testing under various constant stresses. Fe-Pd films (4 $m thick) were deposited onto Si wafers with thermally formed 1$m-thick SiO2 layer using a dual-source dc magnetron sputtering apparatus. The deposited films were all annealed at 900°C for 60 min followed by iced water quenching. Perfect shape recovery was observed for Fe-30.0at%Pd film when the applied stress was lower than 300 MPa. The maximum recoverable strain was ~0.6%. Fe-29.2at%Pd film, on the other hand, showed unrecovered strain after thermal cycling even if the applied stress was 40 MPa. XRD measurements of the Fe-29.2at% Pd film before and after thermal cycling revealed irreversible fcc-bct martensitic transformation that occurred during cooling process at a temperature around -80°C. The critical stress of Fe-Pd films, at which plastic deformation commences to occur, is higher for films with 30 at% Pd than for films with 29.2 at% Pd, which is practically advantageous. The Ms temperature of these films is lower than room temperature when no bias stress is applied, while it becomes higher than room temperature when appropriate bias stress is applied, obeying Clapeyron-Clausius law.

215 強磁性形状記憶合金Fe-Pdの磁場印加によるアクチュエーション(OS9(2) 形状記憶合金の特性と応用)

215 強磁性形状記憶合金Fe-Pdの磁場印加によるアクチュエーション(OS9(2) 形状記憶合金の特性と応用)

Phenomenological model of variant change during magnetization of martensite

The present study deals with the straining of ferromagnetic martensite in a magnetizing field. A phenomenological model of the movement of the interface between martensitic variants is developed for calculation of the strength of the magnetic field required for a variant change. The effect of uniaxial stress on interface movement is also examined. The results are applied to the martensite phases of NiMnGa and FePd ferromagnetic shape-memory alloys.

Deposition of Fe-Pd Ferromagnetic Shape Memory Alloy Thin Films by Sputtering and Their Shape Memory Behavior

Deposition of Fe-Pd Ferromagnetic Shape Memory Alloy Thin Films by Sputtering and Their Shape Memory Behavior

Effect of Co and Ni on Martensitic Transformation and Magnetic Properties in Fe-Pd Ferromagnetic Shape Memory Alloys

Effects of Co and Ni addition on fcc-fct transformation temperature and magnetic properties have been investigated for Fe-Pd ferromagnetic shape memory alloys. The addition of Co shifts the fcc-fct transformation temperature slightly higher while the Ni addition shifts it lower. The results suggest that the relative stability between fcc and fct phases is significantly affected by the electronic structure. Saturation magnetization values for Fe-Pd-Co and Fe-Pd-Ni were found to be slightly higher than those for binary Fe-Pd.

Microstructure control for developing Fe-Pd ferromagnetic shape memory alloys

The effect of microstructure on the ferromagnetic shape memory effect in Fe–Pd alloys was examined focusing on the orientation distribution, the grain boundary character and the grain size. Fe-29.7–31.0 at.%Pd polycrystals composed of columnar or equiaxed grains were prepared and their magnetostriction was measured under a magnetic field up to 10 kOe. The magnetostriction of the columnar crystals was much larger than that of the equiaxed ones due to the development of 〈100〉 fibre texture and (100) tilt grain boundary. In particular, a large magnetostriction of 4.5 × 10-4 was obtained even in the polycrystals. The grain boundaries of the columnar crystals also influenced the variant selection of martensites, resulting in strong dependence of the magnetostriction on the direction of magnetic field. Larger grain size did not always result in larger magnetostriction in Fe-Pd alloys. An increase in the grain size of the alloys was sometimes accompanied by the development of several sets of twin variants within a grain, resulting in a decrease in the magnetostriction.