Magnetic and magnetostrictive properties of the ternary Fe67.5Pd30.5Ga2 ferromagnetic shape memory ribbons
Introduction
The ferromagnetic shape memory alloys (FSMA) are materials that undergo a thermoelastic and reversible martensitic transformation (MT) in the temperature range of a magnetically ordered austenite phase. The large number of applications of these alloys is related to the possibility to magnetically induce a large strain by a rearrangement of the martensite variants and to magnetically shift the MT temperature. The first issue is related to a large magnetoelastic coupling in martensite [1] and the second to the difference between the austenite and martensite magnetization. The large strains are induced by applied magnetic field in the martensitic phase by twin variants reorientation and twin boundary motion. The phenomenon differs from the ordinary magnetostriction (Joule magnetostriction), where the magnetic moments rotate to field direction without changing the unit cell orientation. The most studied FSMA is Ni2MnGa Heusler alloy that shows besides other phenomena a high magnetic field induced strain of 5% [2,3]. However, the high fragility and the relative low Curie temperature of Ni-Mn-Ga are impediments to applications. As an alternative, the Fe-Pd based FSMAs are promising materials for applications, due to the better mechanical properties, corrosion resistance and higher Curie temperature (~763 K), even though the reversible field induced strain is moderate (0.6% achieved in a Fe70Pd30 - single crystal) [4]. Moreover, biocompatibility is a significant benefit of Fe-Pd (30 at.%) over the Ni2MnGa-based Heusler alloy, allowing this material to be used for biomedical applications such as stents or pumps [5].
The Fe-Pd alloy with the atomic composition around 70-30% undergoes by cooling the thermo-elastic and reversible martensitic transformation (MT) from face-centered cubic (f.c.c.) to face-centered tetragonal (f.c.t.) structure [6,7]. A non-thermoelastic and therefore irreversible martensitic transformation (IMT), f.c.t.-b.c.t. (body-centered tetragonal) may be evidenced on further cooling [6,8]. The martensite start temperature is near room temperature, and may be tuned by changing the composition. While the f.c.c.-phase responsible for the MT in Fe-Pd is stable only at high temperature, it becomes generally difficult to stabilize it, as single phase, in a bulk material prepared by classical methods. Rapid solidification, by using a melt-spinning technique, is an effective processing route to obtain ribbons with a non-equilibrium structure, in which the high temperature f.c.c. Structure can be frozen as single phase [9]. In order to stabilize the f.c.t. Martensite, i.e. to avoid the formation of the undesirable b.c.t. Martensite, as well as to manipulate the transformation temperatures, the addition of a third alloying element (Ni, Pt, Co, Rh, Mn, Cu, In) in Fe-Pd system was considered [8,[10], [11], [12], [13], [14], [15]]. An extensive study regarding the effect of Cu, Mn and Pt as third alloying element in Fe-Pd alloy prepared as thin films using combinatorial fabrication and high-throughput investigation methods is given in Ref. [16]. In Heusler compounds, as well in Fe-based FSMAs, the martensite follows the premartensite phase characterized by share instability as result of the softening of the transversal acoustic phonon [17]. The share instability allows a large degree of twinning at nanometer scale. Hence, it generates a soft magnetic environment being also the nuclei of the tetragonal phase. As effect of lattice softening, the shear elastic modulus decreases and large strains are expected during the transformation [18,19].
Reported data on the magnetostriction effect of martensite in Fe-Pd alloy are mostly focused on single crystals [4,20,21], where huge values were reported when the ordinary magnetostriction is combined with the effect of variant reorientation. Recently, an anomalous large, ordinary magnetostriction of 500 ppm measured at the MT on a Fe68.8Pd31.2 single crystal was reported [22]. It has been explained that the large number of premartensitic magnetoelastic twin groups reduces internal elastic energy and magnetic anisotropy. Hence, the magnetic field causes the movement of magnetic and twin domains that relax the system. In polycrystals, magnetostriction takes in general moderate values [9,23] (less than 100 ppm), the variants reorientation being generally impeded by the grain boundaries [24]. In a study regarding the magnetostriction in Fe-30.5 at%Pd alloy Yosuda et al. [25] have shown that the magnetostriction values decrease in polycrystals as the grain size decreases and is largely influenced by the microstructure. The theoretical prediction given by Opahle et al. [26] points out that increasing the minority spin density of states (DOS) at the Fermi level in Fe-Pd alloys should destabilize the f.c.c. Structure and induces a premature formation of the f.c.t. Martensite, so higher MT temperatures. Further on, Kauffmann-Weiss et al. [16] have calculated the effect of a third element on the destabilization of the f.c.c structure (austenite) due to electronic instabilities. They indicate that larger local relaxation process within cubic phase is achieved when the additional elements present larger size (Pt, Ge or Ga). By the use of a phenomenological model (see, e.g. Ref. [1,27]) and employing the equivalence between magnetic and strain-induced stress one can, in principle, determine the magnetoelastic constants specific to various alloys, and the stored magnetic energy.
The present work aims to study ferromagnetic shape memory characteristics of the ternary Fe67.5Pd30.5Ga2 alloy prepared as ribbons by a rapid quenching method with focus on magnetic and magnetostrictive properties. After a short presentation regarding crystalline structure and microstructure, the shift of the martensite transformation temperature by the applied magnetic field is evaluated from thermomagnetic measurements. The result is compared with calorimetry data interpreted in the frame of Clausius-Clapeyron equation. Finally, magnetostriction measurements were carried out in a narrow temperature range around the transformation temperature. To our knowledge, this is the first report related to the magnetostrictive effect around the martensitic transformation in a polycrystalline Fe-Pd-Ga alloy.
Section snippets
Experimental
Alloy ingots of Fe67.5Pd30.5Ga2 were prepared under Ar protective atmosphere, by arc melting high purity elements. The procedure was repeated several times to ensure homogeneity. Afterwards the ingots were melt-spun into very flexible ribbons, with 3 mm wideness, 20-30 cm lengthiness and 25-30 μm thickness (Copper wheel velocity of 20 m/s, 50 kPa Ar overpressure, crucible nozzle diameter of 0.5 mm were considered). The martensitic transformation was characterized by Differential Scanning
Thermal and structural characterization
In order to check for the MT existence in the prepared samples, a first DSC measurement was performed, on cooling-heating cycle between 400 K and 90 K. Apart from the thermoelastic transformation, no evidence for any irreversible one was found for both AP and TT samples. A second scan was done, in the temperature range of interest, to establish the MT parameters. Fig. 1 shows the corresponding DSC runs for Ga-AP and Ga-TT ribbons. Due to the atomic disorder and quench in strains occurring after
Conclusions
Single phase Fe67.5Pd30.5Ga2 with textured f.c.c. crystalline structure at room temperature and martensitic transformation slightly below room temperature has been obtained by rapid quenching using melt spinning technique. The shift of the MT temperature due to an applied magnetic field of (0.87 ± 0.05) K/T was determined from the thermomagnetic measurements. A similar result was found from magnetization curves exploited in frame of the Clausius-Clapeyron equation. The magnetostriction
CRediT authorship contribution statement
Mihaela Sofronie: Conceptualization, Writing - original draft. Felicia Tolea: Conceptualization, Writing - original draft. Mugurel Tolea: Formal analysis, Writing - original draft. Bogdan Popescu: Conceptualization, Writing - original draft. Mihaela Valeanu: Conceptualization, Writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank PhD. M. Enculescu for assistance with the SEM facility and PhD. V. Kuncser for useful discussion. This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI-UEFISCDI, project number PN–III–P1-1.2-PCCDI-2017-0062, contract no.58, within PNCDI III and Core Program PN 18-11.
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