Magnetostriction of Stress-Induced Martensite. Abstract

Transcription

1 Magnetostriction of Stress-Induced Martensite J. Cui and M. Wuttig Department of Materials Science and Engineering, University of Maryland, College Park, MD T. W. Shield Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN (Dated: March 16, 2004) Abstract The magnetostriction of stress induced Fe 7 Pd 3 martensite has been investigated. The striction reaches a value 1980 ppm if this martensite is induced by a compressive stress of 12 MPa in the small temperature interval M s T M s +10 C. It is proposed that the observed magnetostriction reflects the formation and response of metastable adaptive orthorhombic martensite to the externally applied magnetic field. PACS numbers: Kf, Ec Electronic address: cuijun@aem.umn.edu Also at CAESAR Research Center, Ludwig-Erhard-Allee 2, Bonn, Germany 1

2 Ferromagnetic shape memory effect (FSME) refers to the rearrangement of martensitic variants by magnetic field leading to a change of shape. Since the discovery of FSME in Ni 2 MnGa [1, 2] and Fe 7 Pd 3 [3] alloys, research has been focused on the understanding of the physics of ferromagnetic shape memory alloys (FSMAs) [4 7] and the improvement of their engineering properties such as field-induced strain, work output, bandwidth, and fatigue life. To date, the largest field induced strain of 9.5% was achieved on a mechanically de-twined Ni 2 MnGa sample at near zero stress [7]; evidence of 6% strain cycling at about 2 khz was found [8]; and 1.2% strain can be reliably reproduced at about -3 MPa [9]. In the mean time, Galfenol (FeGa solid solution) emerges as a robust magnetostrictive material that has the potential of replacing Terfenol-D [10 12]. A comparison of these materials indicates that each material has advantages and drawbacks: Terfenol-D can do work against a large load under a small applied field, but the strain and machinability are limited by the brittle nature of the material [13 15]; Galfenol has much better machinability but its field-induced strain is smaller, about 300 ppm [12]; Ni 2 MnGa can deliver large field-induced strains at a large applied field, 0.8 T [16 18], but supports only a small load. The FSMA Fe 7 Pd 3 is situated between these three alloys. It delivers a modest strain of 0.9% at equally modest applied magnetic fields and can do either medium field-induced strain against small load, or small strain against large load [19]. It is ductile and features a long fatigue life. The martensitic phase transformation temperature, M s,offe 7 Pd 3 is sensitive to stress. Due to its small latent heat of transformation [20]; its M s can be shifted as much as 0.7 deg/mpa [21]. The magneto-mechanical behavior of stress-induced martensite is interesting because it promises medium magnetically induced strains against the large load that keeps the material at its martensite state. Investigation of this possibility was carried out. Here, we report that Fe 7 Pd 3 exhibits a magnetically induced strain of 1980 ppm at -12 Mpa and 19 C, which is about 4 CabovetheM s at zero stress. Two Fe 7 Pd 3 single crystal specimens, MM4 and MM10, were used in the investigation. They were cut in close proximity to each other from the single crystal boule FePd4 grown previously, see reference [20] for more information regarding this boule. The composition of both specimens is near 29.6 at.% Pd. Both specimens are rectangular bars with 100 surfaces. The surface normals differ less than 0.7 from 100. Combined mechanical and magnetic tests were performed with a Magneto-Mechanical Test Machine (MMTM). Details of this machine can be found in reference [22]. The specimens strain is measured by a strain 2

3 gauge glued to a polished surface (Type EA AP-120, adhesive M-200 bond, Measurement Group, Inc.). In the temperature range of the tests reported below the temperature dependence of the gauge factor leads to an error of less than 2%. The tests were designed to measure the strain induced by a cyclic magnetic field at various compressive stresses and temperatures (5 50 C). For the tests at and below 23 C, the specimen was first transformed to austenite by heating to 34 C and subsequent cooling to the measurement temperature at a rate of approximately 1 C/min. At each temperature three compressive stresses, 1.0, 6.2, and 12.4 MPa, were sequentially applied at a rate of 0.02 mm/min along the long dimension of the specimen. While holding at a particular temperature and stress, a magnetic field was applied along the loading direction at a rate of 60 Oe/sec. The field was ramped between Oe and Oe twice before the stress was increased to the next level without unloading. The three tests at one temperature were concluded with unloading the stress from -12 MPa to zero. To determine the martensitic phase transformation temperature, the specimen was held at -1 MPa while ramping the temperature from 35 to 5 C at a rate of 1 C/min. The slope of the obtained strain-temperature curve displays two abrupt changes at 15.7 C and 10.4 C, respectively, that represent the average martensitic starting, M s, and finishing, M f, temperature at 1.0 MPa. M s and M f at zero stress or -12 MPa can be obtained by using the factor -0.7 C /MPa [20]. The field-induced strains at various temperatures and stresses are summarized in FIG. 1 through 3. FIG. 1 shows that significant magnetostrictive strains in stress-induced martensite are observed in a small temperature range above M s. FIG. 2 indicates the formation of stress-induced martensite at various temperatures. The three dashed horizontal lines indicate the bias stresses supplied at 19 C to determine the full strain-magnetic field characteristic of stress induced martensite displayed in FIG. 3. The relevant features of FIG. 3 are, 1) the maxima of the magnetostriction that are observed upon increasing and decreasing the magnetic field, and 2) the difference of their magnitude. Another interesting feature of Figs.1 and 2 is the relatively large magnetostriction of about 500 ppm at bias stresses of -1.0 MPa, i.e. in the pre-martensite state. This number is about 4 times large than that of austenite. FIG. 1 and 3 indicate that the observed magnetostriction is characteristic of stress induced Fe 7 Pd 3 martensite. And, as FIG. 2 demonstrates, a large striction at -12 MPa is observed in the small temperature range M s δt M s +10 C. This is the temperature range in which 3

4 the adaptive orthorhombic phase [23 25] exists. This adaptive phase, often being referred as tweed, starts to form a few degrees above the M s temperature and continues to exist to a few degrees below the M f temperature. It is proposed that the observed large magnetostriction is related to the formation of this phase. The strain-field curve at -12 MPa in FIG. 3 has the shape of butterfly with a superimposed intermediate maximum and non-zero strain at zero field. While the shape of butterfly and hysteresis are indications of classic magnetostriction, the superimposed intermediate maximum implies a magnetically induced phase change. Premartensite Fe 7 Pd 3 is a matrix of austenite with a distribution of martensite nuclei whose tetragonality varies from zero to near that of the full-grown martensite depending on temperature and stress. The martensite nuclei have magnetic easy axes perpendicular to their crystalline short axis, which results in a positive field induced strain. The austenite matrix can be stabilized by a magnetic field: a decrease of the transformation temperature as much as 3 C in a field of 3000 Oe has been observed in Fe 7 Pd 3 [21]. The transformation to austenite beyond 2000 Oe results in a shortening of the sample and the first strain maximum (Max-A). Upon decreasing the field from 4100 Oe, the magnetically induced austenite starts to transform back to martensite nuclei. Because these nuclei are formed in a magnetic field, their magnetizations are all parallel to it, the sample starts to elongate eventually forming a new maximum at the point Max-B in FIG. 3. Since now all nuclei are aligned the strain at Max-B is higher than at Max-A where only one third is transformed. This hypothesis is supported by the relative magnitude of the strain differences between these maxima, one and one third as shown in FIG. 3. In summary, this paper reports on the magnetostriction of stress induced martensite that can reach values as large as 1800ppm. It is proposed that the effect is associated with the adaptive phase and that the interplay of stress and magnetic fields results in non-monotonous magnetostrictive behavior in the temperature range where it and the tetragonal phases can be stabilized. Acknowledgments This work was supported by the Office of Naval Research, contracts N , N and MURI N as well by the National Science Foundation, 4

5 grant DMR The authors would also like to thank R. D. James for his participation in this project. 5

6 [1] A. N. Vasil ev, S. A. Klestov, R. Z. Levitin, and V. V. Snegirev, JETP 82, 524 (1996). [2] K. Ullakko, J. K. Huang, C. Kantner, R. C. O Handley, and V. V. KoKorin, Appl. Phys. Lett. 69, 1966 (1996). [3] R. D. James and M. Wuttig, Philo. Mag. A 77, 1273 (1998). [4] A. DeSimone and R. D. James, J. Mech. Phys. Solids 50, 283 (2002). [5] M. Wuttig, L. Liu, K. Tsuchiya, and R. D. James, J. Appl. Phys. 87, 4707 (2000). [6] R. C. O Handley, J. of Appl. Phys. 83, 3263 (1998). [7] K. U. A. Sozinov, A. A. Likhachev, IEEETrans. Mag. 38(5), 2814 (2002). [8] M. A. Marioni, R. C. O Handley, and S. M. Allen, Appl. Phys. Lett. 83, 3966 (2003). [9] R. Tickle, R. D. James, T. W. Shield, M. Wuttig, and V. V. Kokorin, IEEE Trans. Magn. 35, 4301 (1999). [10] M. Wuttig and J. Cullen, J. Appl. Phys. 91, 7804 (2002). [11] R. Wu, J. Appl. Phys. 91, 7358 (2002). [12] N. S. S. Guruswamy, J. Appl. Phys. 90, 5680 (2001). [13] M. Dapino, F. Calkins, A. Flatau, and D. Hall, Proc. SPIE 2717, 697 (1996). [14] M. Dapino, F. Calkins, and A. Flatau, Proc. SPIE 3041, 256 (1997). [15] R. Kellogg and A. Flatau, Proc. SPIE 3668, 184 (1999). [16] R. Tickle and R. D. James, J. Magn. Mag. Mat. 195, 627 (1999). [17] R. Tickel, Ph.D. thesis, University of Minnesota (2000). [18] L. Dai, J. Cui, and M. Wuttig, Proc. SPIE 5053, 595 (2003). [19] J. Cui, R. D. James, and T. W. Shield, Acta. Metall. Mater. (2004), submitted. [20] J. Cui, R. D. James, and T. W. Shield, Acta. Metall. Mater. 52/1, 35 (2003). [21] J. Cui, T. W. Shield, and R. D. James, J. Mech. Phys. Solids (2004), submitted. [22] T. W. Shield, Rev. Sci. Instrum. 74, 4077 (2003). [23] S. Muto, S. Takeda, R. Oshima, and F. E. Fujita, Jpn. J. Appl. Phys. 27, L1387 (1990). [24] A. Khachaturyan, S. M. Shapiro, and S. Semenovskaya, Mats. Trans., JIM 33, 278 (1992). [25] H. Seto, Y. Noda, and Y. Yamada, J. Phys. Soc. Jpn. 59, 965 (1990). 6

7 MPa -6 MPa -12 MPa Field Induced Strain, ppm MPa M f 0MPa M s -12MPa M f -12MPa M s Temperature, o C FIG. 1: Magnetic field-induced strain of sample MM10 at various temperatures and stresses. The two dotted lines represent the M s and M f at zero stress. 7

8 0 10 o C 5 o C Stress, MPa o C 17 o C 19 o C 50 o C 40 o C 25 o C 23 o C 21 o C Strain, ppm FIG. 2: Stress-strain curves of sample MM10 at various temperatures. The magnetic field-induced strains are measured at 1.0, 6.2 and 12.4 MPa represented by the three dashed lines. 8

9 Field Induced Strain, ppm MPa -6 MPa -1 MPa Max-B δ δ Max-A δ Max-O Magnetic field, Oe FIG. 3: Strain-field curves at 19 C. 9