Magnetostriction and magnetic structure in annealed recrystallization
of strain-forged ferromagnetic shape memory Fe–Pd–Rh alloys
Yin-Chih Lin1,a兲 and Hwa-Teng Lee2 1
Department of Mechanical Engineering, National Cheng Kung University, Tainan, 701 Taiwan,
Republic of China and Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, 807 Taiwan
2
Department of Mechanical Engineering, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China
共Presented 19 January 2010; received 23 October 2009; accepted 13 January 2010; published online 12 May 2010兲
Bulk ferromagnetic shape memory Fe68– Pd30– Rh2 and Fe66– Pd30– Rh4 共at. %兲 alloys were strain-forged to produce a 35%–40% reduction in thickness. The reduced alloys were thermally annealed at 950– 1050 ° C for various times to induce recrystallization. The magnetostriction test demonstrated that the grain size reduction of recrystallization had a direct influence on the magnetic properties of the materials. The magnetostrictive strain measurements revealed that the strain-forged metal treated with thermal recrystallization to induce the fine-grained structure had a higher magnetostriction as well as a higher magnetostrictive susceptibility共⌬储s/⌬H兲. It was also found
that at room temperature, the saturation magnetostriction 共s= 77⫻10−6兲 of the fine-grained Fe– Pd–Rh alloys strain-forged with thermal recrystallization was higher than that of those without grain size reduction共s= 50– 56⫻10−6兲, where sis共2/3兲关储s−⬜s兴. In addition, with the magnetic field applied perpendicular to the sample’s longitude, the fine-grained Fe–Pd–Rh material contracted by as much as⬜s= −36⫻10−6. This value is about three times higher than that of alloys without grain size reduction. Microstructure investigation indicated that a magnetic applied field normal to the sample’s longitude caused high contraction共⬜s兲 of the fine-grained Fe–Pd–Rh alloys, which could be ascribed mainly to the grain refinement as well as deformation twins or microtwins 共transformation and transverse twins兲. The study demonstrates that the magnetostrictive strains of Fe–Pd–Rh alloys induced in the L10 martensite by the magnetic field can be attributed to the reorientation of the L10 martensite twin structures. © 2010 American Institute of Physics. 关doi:10.1063/1.3367979兴
Figure1 presents the essential linear optimal magneto-striction 共⫻10−6兲 versus magnetic applied field 共H兲−H curves of the Fe66– Pd30– Rh4 共at. %兲 alloys, measured at room temperature共RT兲 共300 K兲 of the as received specimen, sample without strain-forging with annealing at 950 °C for 1.5 h, and strain-forged 35% reduction sample thermally an-nealed at 950 ° C for 1.5 h and quenched in ice brine, where 储denotes共⌬L/L兲储with a magnetic applied field parallel to
the sample’s longitude, and ⬜ denotes 共⌬L/L兲⬜ with a magnetic applied field normal to the sample’s longitude, re-spectively. Two typical 储 and ⬜ curves as a function of magnetic applied field can be seen in Fig. 1. With the mag-netic applied field parallel to the sample’s longitude, the magnetostriction储is positive共elongation兲. Magnetostriction
⬜is negative共contraction兲 with the magnetic applied field perpendicular to the sample’s longitude. When the magnetic field共H兲⬎3.5 kOe, 储and⬜approach the saturated values
储s and⬜s, respectively. By careful analysis of Fig. 1, it is discovered that the magnetostrictive strain 共储s= 65⫻10−6; ⬜s= −10⫻10−6兲 of the non strain-forged sample with an-nealing at 950 ° C for 1.5 h is much smaller than that of the strain-forged 35% reduction specimen through annealing at
the same temperature and time 共储s= 80⫻10−6; ⬜s= −36 ⫻10−6兲, due to the grain size refinement with deformation twins, transformation twins, and transverse twins appearing in the latter specimen. This strongly suggests that the mag-netic field aligns the twin structure of the L10 transforming martensite, as well as the contribution of the reorientation of
a兲Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.:⫹886-73814526. FAX: ⫹886-62085103.
FIG. 1.共Color online兲 The linear magnetostriction 共⫻10−6兲 at RT in parallel 共储兲 and normal 共⬜兲 applied field 关H 共kOe兲兴 to sample’s longitude of
Fe66– Pd30– Rh4共at. %兲 alloys for the as received specimen, without strain-forged sample with annealing at 950 ° C for 1.5 h, and the strain-strain-forged 35% reduction sample through annealing recrystallization at 950 ° C for 1.5 h. JOURNAL OF APPLIED PHYSICS 107, 09D312共2010兲
0021-8979/2010/107共9兲/09D312/3/$30.00 107, 09D312-1 © 2010 American Institute of Physics
the twin structure to the total strain, which can be confirmed
by the magnetostriction measurements and TEM
observations.1,2
The strains measured in the magnetic applied field par-allel to the sample’s longitude are observed to be substan-tially larger than those induced by the field perpendicular to the sample’s longitude, as shown in Fig.1. The difference in the field needed to initiate the strain for the two field orien-tations is a reflection of the sample shape and crystallogra-phy. The strains induced in the L10 martensite by the mag-netic field are attributed to the reorientation of the L10 martensite twin structure.3In addition, analysis of Fig.1also reveals that the grain-refined共i.e., strain-forged兲 sample has a near saturated magnetostriction with a small applied field 共H储= 2.5 kOe兲 parallel to the sample’s longitude, and has a
higher value of magnetostrictive susceptibility 共⌬储s/⌬H兲 than those of either the nonstrain-forged sample with anneal-ing treatment or the as received specimen. The lower mag-netostriction of the as received metal is due to segregation-impeded parts of the L10 twin boundary motion in realistic magnetic fields. Also, as the magnetic field was applied per-pendicular to the sample’s longitude, the fine-grained Fe66– Pd30– Rh4 material contracted by as much as ⬜s= −36⫻10−6. This value is about three times higher than that of alloys without grain size reduction of Fe66– Pd30– Rh4 metals共⬜s= −10⫻10−6兲. Figure2is the optimal linear mag-netostriction 共⫻10−6兲 versus magnetic applied field 共H兲 − H curves, measured at RT, of the Fe68– Pd30– Rh2alloys no strain-forging with annealing treatment, and the same alloys strain-forged to a 39% reduction and then annealed at 950 ° C for 3 and 6 h. Figure 2 clearly reveals an optimal magnetostriction 共储s= 86⫻10−6; ⬜s= −30⫻10−6兲 with an
optimal magnetostrictive susceptibility 共⌬储s/⌬H兲 of the
strain-forged sample annealed at 950 ° C for 3 h, which is better than that of the nonstrain-forged specimen annealed at the same temperature and time 共储s= 74⫻10−6; ⬜s= −10
⫻10−6兲, due to the grain size refinement, deformation twins, transformation twins, and transverse twins existing in the strain-forged sample treated with annealing recrystallization. In addition, Fig. 2 also reveals that the magnetostrictive
strains gradually decrease as the alloys are annealed at 950 ° C for 6 h due to grain growth with the heat treatment. A comparison of Figs. 1 and 2 indicates that the optimal saturation magnetostriction value 共s= 77⫻10−6兲 is similar in the grain-refined Fe66– Pd30– Rh4 and Fe68– Pd30– Rh2 al-loys. However, with the magnetic field applied perpendicular to the sample’s longitude, the strain-forged Fe66– Pd30– Rh4 alloys treated with annealing recrystallization had higher magnetostrictive strains 共⬜s= −36⫻10−6兲 than that of Fe68– Pd30– Rh2 alloys 共⬜s= −30⫻10−6兲. Figure 3 reveals the saturation magnetostriction 共s兲 versus magnetic field 共H兲, s− H curves of Fe66– Pd30– Rh4alloys, measured at RT 共300 K兲 of the as received specimen, sample without strain-forging with annealing at 950 ° C for 1.5 h, and strain-forged 35% reduction sample thermally annealed at 950 ° C for 1.5 h and quenched in ice brine. The saturation magnetostriction sis evaluated ass=共2/3兲关共储兲−共⬜兲兴, where 储共⬜兲 is the magnetostriction in the longitudinal direction with a mag-netic field parallel共perpendicular兲 to the sample’s longitude. It can be seen in Fig.3that the optimal saturation magneto-striction of the strain-forged sample with annealing at 950 ° C for 1.5 h 共s= 77⫻10−6兲 is higher than that of nonstrain-forged specimen with thermal annealing at same temperature and time共s= 50⫻10−6兲. The saturation magne-tostriction of polycrystalline Fe70– Pd30 共at. %兲 alloys was studied by Schmidt and Berger.4 They showed that at T = 300 K 共RT兲 saturation magnetostriction 共s兲 of the quenched Fe70Pd30 is 45⫻10−6. It is obvious that Fe70Pd30 alloys with Rh additions can substantially improve the satu-ration magnetostriction of the materials at RT 共300 K兲. An scanning electron microscope 共SEM兲 image of the Fe66– Pd30– Rh4 alloys strain-forged to a ⬃35% reduction and recrystallized through thermal annealing at 950 ° C for 1.5 h is shown in Fig.4共a兲, which clearly demonstrates that the ultra fine grain sizes共grains of about 0.5–3.5 m兲 were obtained. The fine grain structure with a corresponding opti-mal magnetostriction is shown in Figs. 1 and 3. An SEM image of nonstrain-forged metals annealed treatment at same temperature and time is shown in Fig. 4共b兲, which reveals FIG. 2.共Color online兲 The linear magnetostriction 共⫻10−6兲 at RT in parallel
共储兲 and normal 共⬜兲 applied field 关H 共kOe兲兴 to sample’s longitude of Fe68– Pd30– Rh2共at. %兲 alloys for the strain-forged 39% reduction specimen annealing at 950 °C for 3 h and 6h, and the nonstrain-forged sample with annealing at same temperature and time, respectively.
FIG. 3. 共Color online兲 The linear saturation magnetostriction s共⫻10−6兲 vs magnetic field共H兲s− H curves measured at RT of the Fe66– Pd30– Rh4 al-loys; as received specimen, sample without strain-forging with annealing at 950 ° C for 1.5 h and strain-forged 35% reduction sample thermally an-nealed at 950 ° C for 1.5 h.
09D312-2 Y.-C. Lin and H.-T. Lee J. Appl. Phys. 107, 09D312共2010兲
coarse grains with a corresponding lower magnetostriction, also presented in Figs.1and3, for comparison.
The TEM micrographs of the optimal magnetostriction of the strain-forged Fe68– Pd30– Rh2 alloys thermally recrys-tallized at 950 ° C for 3 h are shown in Figs. 5共a兲–5共d兲. Shown in Fig. 5共a兲is the zone axis 关114¯兴L10 储关573¯兴L1m 共hkl denotes tetragonal L10structure and the ordered L10 marten-sitic structure with lattice parameters of a = 3.869 Å, c = 3.696 Å, and c/a=0.955; hkl denotes L1m monoclinic martensitic phase with lattice parameters of a = 3.191 Å, b = 3.696 Å, c = 3.128 Å, and = 91.834°兲.5 Figure 5共b兲 is a dark field共DF兲 image using the 共311兲L10−2 reflection corre-sponding to Fig. 5共a兲. It is found that the L10 deformation twins 共bright contrast兲 are coarse and irregular. A DF image using the superlattice 共ST兲L10−T reflection corresponding to Fig.5共a兲is shown in Fig.5共c兲in which the bright contrast is first-ordered transformation twin L10 structures. In contrast with the deformation twins in Fig. 5共b兲, this morphology consists of parallel L10 martensite plates, each containing a regular and fine array of transformation twins. From Fig. 5共a兲, the type of the twinning mode is identified as兵1¯10其L10.6 Figure 5共d兲 is a bright field 共BF兲 image with the transverse twin indicated by arrow. The TEM images of the same alloys without strain-forging and annealed at 950 ° C for 3 h are shown in Figs. 6共a兲–6共d兲. Figure 6共a兲 is an SADP with a zone axis of关001兴L10 储关3¯14兴L1m. Figure 6共b兲 is a BF image. In this BF image, the tetragonal g共02¯0兲L10 structures and
monoclinic g共11¯1兲L1m twinning plates seem to be aligned alternately forming an lamellar twin structures. From Fig. 6共a兲, it reveals g共02¯0兲L10 储g共11¯1兲L1mwith a deviation of near 5 deg. This morphology is similar to 0.8% eutectoid plain-carbon steel. Eutectoid steel is cooled to just below the eu-tectoid temperature, causing the entire structure to transform from austenite to a lamellar structure of alternating plates of
␣ ferrite 共bcc兲 and cementite 共orthorhombic兲, known as pearlite. In this case, the similar lamellar twin structures are comprised of alternating plates of L10phase共tetragonal兲 and L1m structure 共monoclinic兲. The DF micrographs of Figs. 6共c兲 and 6共d兲 were obtained with diffraction vectors g =关221兴L1m and g =关020兴L10 corresponding to Fig. 6共a兲, re-spectively.
The Fe–Pd–Rh alloys strain-forged to a 35%–40% re-duction in thickness and recrystallized through thermal an-nealing at 950– 1050 ° C for proper times show improve-ments in: 共a兲 magnetostrictive strains, 共b兲 magnetostrictive susceptibility 共⌬储s/⌬H兲, 共c兲 perpendicular saturation mag-netostriction共⬜s兲, and 共d兲 ductility. In the present study, the optimal magnetostriction is found in Fe66– Pd30– Rh4 alloys strain-forged to a 35%–40% reduction in thickness and re-crystallized through thermal annealing at 950 ° C for 1.5 h,
while the optimal magnetostriction is found in
Fe68– Pd30– Rh2 alloys strain-forged to a 35%–40% reduc-tion in thickness and recrystallized through thermal anneal-ing at 950 ° C for 3 h. SEM and TEM investigations indicate that the high magnetostriction of the fine-grained Fe–Pd–Rh alloys can mainly be ascribed to the grain refinement as well as deformation twins, transformation twins, and transverse twins.
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2K. Ullakko, J. K. Huang, C. Kantner, R. C. O. Handley, and V. V. Koko-rin,Appl. Phys. Lett.69, 1966共1996兲.
3H. E. Karaca, I. Karaman, B. Basaran, Y. Ren, Y. I. Chumlyakov, and H. J. Maier,Adv. Funct. Mater.19, 1共2009兲.
4J. E. Schmidt and L. Berger,J. Appl. Phys.55, 1073共1984兲. 5Y. C. Lin and H. T. Lee,J. Magn. Magn. Mater.322, 197共2010兲. 6J. G. Speer and D. V. Edmonds,Acta Metall.36, 1015共1988兲. FIG. 4. SEM images taken from the Fe66– Pd30– Rh4alloys:共a兲 strain-forged
to a⬃35% reduction in thickness, then thermally annealed at 950 °C for 1.5 h; and共b兲 nonstrain-forged sample annealed at 950 °C for 1.5 h.
FIG. 5. TEM images of the Fe68– Pd30– Rh2alloys strain-forged to a⬃39% reduction in thickness and annealed at 950 ° C for 3 h:共a兲 SADP of zone axis关114¯兴L10 储关573¯兴L1m共hkl denotes tetragonal L10reflection; and hkl de-notes L1mmonoclinic phase兲, 共b兲 dark field 共DF兲 image of 共311兲L10−2 defor-mation twin reflection corresponding to共a兲, 共c兲 DF image of superlattice 共ST兲L10−Ttransformation twin reflection corresponding to共a兲, and 共d兲 bright filed共BF兲 image—the transverse twin indicated by arrow.
FIG. 6. TEM images of the Fe68– Pd30– Rh2 alloys nonstrain-forged and annealed at 950 ° C for 3 h:共a兲 SADP of zone axis 关001兴L10 储关3¯14兴L1m共hkl denotes tetragonal L10reflection; and hkl denotes L1mmonoclinic phase兲, 共b兲 BF image, 共c兲 DF image of g=关221兴L1m, and 共d兲 DF image of g =关020兴L10.
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