Microstructures and magnetostriction of two-phase Fe66-Pd30-Ni4 high-temperature ferromagnetic shape memory alloys

Microstructures and magnetostriction of two-phase Fe

-Pd

-Ni

high-temperature ferromagnetic shape memory alloys

1

Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan; Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China

2

Department of Mechanical and Automatic Engineering, National Kaohsiung First University of Science and Technology, Taiwan, Republic of China

3

Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China (Presented 18 November 2010; received 22 September 2010; accepted 8 November 2010; published online 24 March 2011)

The microstructures and magnetostrictive strains of ferromagnetic shape memory Fe-Pd30 alloy systems with additions of Ni elements (4 at.%) have been investigated by a magnetostriction meter, scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The research results show that the magnetostrictive strains of the Fe66-Pd30-Ni4alloys after homogenization treatment (ks_jj¼ 79  106_{) are higher than those of the as received materials}

(ks_jj¼ 55  106_{). The lower magnetostriction of the as received metal is due to segregation-impeded}

parts of the L10twin boundary motion in realistic magnetic fields. In addition, an important discovery in this study is that doping the Fe-Pd30alloy system with Ni substitution for Fe seems to prevent the decomposition of L10þ L1m twin phase into stoichiometric L10þ L1mþ abct structures when the strain-forged alloys are solution treated and recrystallization annealed, and then aged at 400C for 100 hours. The magnetostrictive strains of the 400 C/100 h aged sample are maintained with ks_jj¼ 62  106_{; k}s

?¼ 11  106, and the XRD analysis revealed a complete absence of abct phase in the aged sample. This magnetic property of the alloys is suitable for application in a high temperature and high frequency (T<400C) environment. The strain-forged samples were solution treated (ST) and recrystallization annealed, and then aged at 500700_{C for 100 hours. XRD analysis}

revealed the formation of an abctphase when the specimen was aged at 500 C for 100 hours. This abctphase occurrence causes large reductions in the magnetostrictive strains. When the samples were aged at 600700 _{C for 100 hours, overaging occurred, the martensitic stoichiometric L1}

0þ L1mþ abct lamellar structures dissolved into the matrix simultaneously, and the magnetostrictive strains gradually recovered.VC 2011 American Institute of Physics. [doi:10.1063/1.3540690]

The magnetostriction measurements were made use of strain gauge method in association with a Vishay Micro-Meas-urements Model P3 strain indicator and recorder. A sample with the dimensions of 15 mm (length) 5 mm (width)  1 mm (thickness) was performed for the magnetostriction measure-ment. A FLA-2-11-A515411 type strain gauge, purchased from TML (Tokyo Sokki Kenkyujo Co. Ltd.), was employed. The gauge was glued to the sample along its longitude using an M-Bond 200 Adhesive Kit. To cure the glue, both the gauge and the specimen were kept at room temperature (RT) for 2 days. Then the magnetostrictive strains were measured.

Figure 1presents the essential linear magnetostriction k (106) vs. magnetic applied field (H) k-H curves of the Fe66 -Pd30-Ni4 alloys, measured at RT (300 K) of the as received specimen, a sample homogenized at 1050C for 70 hours, and a sample homogenized without strain-forging and then annealed at 950 C for 6 hours and quenched in ice brine, where kjj denotes (DL=L)jj with a magnetic applied field

FIG. 1. (Color online) The linear magnetostriction ( 106_{) at RT in} parallel (k||) and normal (k\) applied field (H (kOe)) to sample’s longi-tude of Fe66-Pd30-Ni4(at.%) alloys for the as received specimen, sample homogenized at 1050C for 70 hours, and sample homogenized without strain-forging and then annealed at 950_{C for 6 hours and quenched in ice} brine.

a)_{Author to whom correspondence should be addressed. Electronic mail:} [email protected]. FAX:þ886-73835015.

0021-8979/2011/109(7)/07A912/3/$30.00 109, 07A912-1 VC2011 American Institute of Physics

JOURNAL OF APPLIED PHYSICS 109, 07A912 (2011)

parallel to the sample’s longitude, and k\ denotes (DL=L)\ with a magnetic applied field normal to the sample’s longitude, respectively. Two typical kjj and k\ curves as a function of magnetic applied field can be seen in Fig.1. By careful analy-sis of Fig.1, it is discovered that the magnetostrictive strain (ks_jj¼ 55  106_{) and magnetostrictive susceptibility (Dk}s

jj=DH)

of the as received specimen are much smaller than those of the sample homogenized at 1050C for 70 hours (ks_jj¼ 79  106_).

The lower magnetostriction and magnetostrictive susceptibil-ity of the as received metal is due to segregation-impeded parts of the L10twin boundary motion in realistic magnetic fields.1,2

The alloys were strain-forged to a 33% reduction, solution treated (ST) and recrystallization annealed, and then aged at 400–700C for 100 hours. The most important discovery of this study is shown in Fig.2. The magnetostrictive strains ks_jj and ks_?are plotted as a function of applied field (H) at RT (300 K) for the various aged specimens, indicating: ks_jj¼ 62  106_;

ks_?¼ 11  106 _{for 400} _{C/100 hours aged sample,}

ks_jj¼ 22  106_{; k}s

?¼ 4  106for 500C=100 hours aged

sample, ks_jj¼ 40  106_{; k}s

?¼ 7  106 for 600 C/100

hours aged sample, and ks_jj¼ 63  106_{; k}s

?¼ 6  106for

700C/100 hours aged sample, respectively. The magnetostric-tive strains of the 400C/100 hours aged specimen were main-tained with ks_jj¼ 62  106_{; k}s

?¼ 11  106, for the XRD

analysis revealed a complete absence of the abctphase, and the decomposition of L10þ L1m twin phase into stoichiometric L10þ L1mþ abctstructures did not occur in this aged sample. Please see Fig.4(b)and Fig.3(b). This magnetic property of the alloys is suitable for application in a high temperature and high frequency (T<400C) environment.3–5However, the magneto-strictive strains of the 500C/100 hours aged sample decreased to ks_jj¼ 22  106_{; k}s

?¼ 4  106. The cause was the abct phase as well as the appearance of stoichiometric L10þ L1mþ abct structures in the 500 C/100 hours aged sample. Please see Fig.4(c)and Fig.3(c).

Figure 3(a)shows an SEM micrograph of the alloy ho-mogenized and strain-forged to a 33% reduction in thickness and then ST and recrystallization annealed at 950C for 3 hours. The fine grain (0.5-2 lm) microstructures developed by

recrystallization are clearly demonstrated. Figure 3(b) is the strain-forged 33% sample ST and aged at 400 C for 100 hours. The grains have coalesced into a large grain (0.8-8 lm), but no phase decomposition of stoichiometric L10þ L1mþ abctstructures can be found in the SEM image. Shown in Fig. 3(c)is the SEM image of a strain-forged 33% sample ST and aged at 500C for 100 hours. The decomposi-tion of the L10þ L1m twin phase into stoichiometric L10þ L1mþ abctstructures is apparent to this aged sample. Fig.3(d) is the strain-forged 33% sample ST and aged at 700 C for 100 hours. The dissolution of the stoichiometric L10þ L1mþ abctstructures into the matrix is clearly revealed in this SEM micrograph.

FIG. 2. (Color online) The linear magnetostriction ( 106_{) at RT in parallel (k}

jj) and normal (k\) applied field (H (kOe)) to sample’s longitude of Fe66-Pd30-Ni4 alloys for the strain-forged 33% reduction specimens ST and recrystallization annealed, and then aged at 400C, 500C, 600C, and 700_{C for 100 hours, respectively.}

FIG. 3. SEM micrographs taken from: (a) the alloys homogenized and strain-forged to a 33% reduction in thickness, then ST and annealed recrys-tallization at 950C for 3 hours, and the strain-forged sample ST then aged at (b) 400_{C for 100 hours, (c) 500}_{C for 100 hours, and (d) 700}_{C for 100} hours, respectively.

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Figures4(a),(b), and(c)represent a series of XRD pat-terns of the alloys strain-forged to a 33% reduction then ST at 950 C for 3 hours, and the strain-forged alloys ST and then aged at 400C for 100 hours, and 500C for 100 hours, respectively. The XRD pattern of the ST alloys is shown in Fig. 4(a), in which the reflections are comprised of two phases (L10þ L1m). The results of investigation by XRD and TEM analysis are perfectly consistent. In Fig.4(a), the reflection of (111)L10 and (101)L1m with diffraction angle 2h¼ 41.66 _{(d spacing¼ 2.167 A˚ ) is the main diffraction}

peak. When the alloys were ST and aged at 400C for 100 hours, the main diffraction peak was the same reflection of (111)L10 and (101)L1m with a diffraction angle 2h¼ 41.67 (d spacing¼ 2.166 A˚ ). Meanwhile, the (101)abctpeak cannot be found in the 400C/ 100 hours aged sample. A compari-son of Fig.4(a)and4(b)reveals no significant difference in the XRD patterns between the ST specimen and the ST sam-ple aged at 400C for 100 hours. Figure 4(c) presents the XRD patterns of the alloys ST and aged at 500C for 100 hours. It is found that the (101)abct peak appears in diffrac-tion angle 2h¼ 44.52 _{(d spacing¼ 2.034 A˚ ), along with}

many new diffraction peaks revealed in the diffraction pat-tern. This phenomenon demonstrates that the decomposition of L10þ L1mtwin phase into stoichiometric L10þ L1mþ abct structures has occurred in the 500C/100 hours aged speci-men, which leads to low magnetostriction, as shown in Fig.2. The TEM images of the strain-forged Fe66-Pd30-Ni4 alloys ST and recrystallization annealed at 950C for 3 hours are shown in Figs.5(a),(b),(c), and(d). Shown in Fig.5(a)is

the zone axis [310]L10//[561]L1m (hkl denotes tetragonal L10structure and the ordered L10 martensitic structure with lattice parameters of a¼ 3.807 A˚ , c ¼ 3.657 A˚, and c/a ¼ 0.96; hkl denotes L1mmonoclinic martensitic phase with lattice pa-rameters of a¼ 3.147 A˚ , b ¼ 3.807 A˚, c ¼ 3.113 A˚, and b¼ 92.318). Figure5(b)is a bright field (BF) image. Shown in Fig. 5(c) is a dark field (DF) image using the ð32 3ÞL1m

reflection corresponding to Fig. 5(a). It is found that the L1m twins (bright contrast) are coarse and irregular, and this twin-ning mode is identified asf111gL1m. The DF micrographs of

Fig.5(d)were obtained with the diffraction vector g¼ ½002L10

or g¼ ½111L1m corresponding to Fig. 5(a). They reveal

gð111ÞL1m==gð002ÞL10. In this DF image, the tetragonal

gð002ÞL10 and monoclinic gð111ÞL1m twinning plates seem to

be aligned alternately, forming lamellar twin structures.

The Fe66-Pd30-Ni4alloys strain-forged to a 33% reduction and then ST and aged at 400 C for 100 hours can inhibit the decomposition of L10þ L1m twin phase into stoichiometric L10þ L1mþ abctstructures. This magnetic property of the alloys is suitable for application in a high temperature and high fre-quency (T<400C) environment. As the alloys were ST and aged at 500C for 100 hours, the L10þ L1mtwin phase decomposed into stoichiometric L10þ L1mþ abct structures, leading to a decrease in magnetostrictive strains. When the alloys were ST and aged at 700C for 100 hours, the stoichiometric L10þ L1mþ abct lamellar structures dissolved into the matrix simultaneously, and the magnetostrictive strains gradually recovered.

The authors would like to express their sincere apprecia-tion to the Naapprecia-tional Science Council of the Republic of China for supporting this study (NSC-99-2221-E-151-017).

1_{Y. C. Lin and H. T. Lee,J. Magn. Magn. Mater.322, 197 (2010).} 2_{Y. C. Lin and H. T. Lee,J. Appl. Phys.107, 09D312 (2010).} 3

Y. Li,et al.Acta Mater.58, 3655 (2010). 4

P. K. Kumar and D. C. Lagoudas,Acta Mater.58, 1618 (2010). 5

Y. Xin, Y. Li, and Zongde Liu,Scr. Mater.63, 35 (2010). FIG. 4. XRD patterns of the Fe66-Pd30-Ni4 alloys: (a) homogenized and

strain-forged to a 33% reduction, then ST at 950_{C for 3 h, and sample ST} then aged at (b) 400_{C for 100 h, (c) 500}_{C for 100 h. (41.66}_denotes dif-fraction angle 2h¼ 41.66_).

FIG. 5. TEM micrographs of the Fe66-Pd30-Ni4 alloys strain-forged to a 33% reduction in thickness, ST and recrystallization annealed at 950C for 3 hours: (a) selected area diffraction pattern (SADP) of zone axis [310]L10// [561]L1m(hkl denotes tetragonal L10reflection; and hkl denotes L1m mono-clinic structure), (b) BF image, (c) DF image of g¼ ½323L1m, and (d) DF image of g¼ ½002_L10.

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