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Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys
Journal of Alloys and Compounds 440 (2007) 199–203
Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys Ding Chen ∗ , Akira Takeuchi, Akihisa Inoue Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 16 August 2006; accepted 17 September 2006 Available online 17 October 2006
Abstract Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (Tx ) and melting temperature (Tm ) of the Gd65 Fe20 Al15 bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of Tm and Tx , Tm (=Tm − Tx ), is as small as 135 K and the reduced crystallization temperature (Tx /Tm ) is as high as 0.86. The small Tm and high Tx /Tm values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd65 Fe20 Al15 bulk amorphous alloys are investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk amorphous materials; Metallic glasses; Magnetic properties
1. Introduction Bulk amorphous alloys have received increasing interest during recent years due to the promising prospects in engineering materials. Bulk metallic glasses with sizes greater than a couple of millimeters have been obtained in a number of alloy systems such as Mg–Cu–Y [1,2], Zr–Al–TM [3,4], Ti–Zr–Al–TM–Be [5], Zr–Ti–TM–Be [6], Pd–Ni–Cu–P [7], Fe–(Al, Ga)–(P, C, B, Si) [8], Fe–(Co, Ni)–(Zr, Hf, Nb)–B [9] and RE–Al–TM [10–15] (RE = rare earth, TM = transition metal) systems. Among these bulk metallic glasses, RE–Al–TM (La-, Sm-, Nd-, Pr-, Y- and Ce-based) alloy systems have extremely large glass-forming ability (GFA) [10–15] which enables us to fabricate bulk amorphous alloys with diameters of several millimeters by copper mold casting. Furthermore, Nd–Fe–Al and Pr–Fe–Al bulk amorphous alloys exhibit unique hard magnetic behavior with a remanence (Jr ) of 0.09 T, a saturation magnetization (J1432 ) of 0.13 T and coercivity (i Hc ) around 300 kA/m [12]. Ybased bulk amorphous alloys rods with diameters over 20 mm ∗ Corresponding author at: Hunan University, School of Materials Science and Engineering, Changsha, Hunan 410082, China. Tel.: +86 0731 8821648; fax: +86 0731 8821648. E-mail address: [email protected] (D. Chen).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.064
were obtained by water quenching [14]. Ce-based bulk metallic glasses exhibit polymer-like thermoplastic behavior caused by their extraditionally low glass transition temperature, Tg , which is similar to or lower than that of many polymers [15]. Thus, it is interesting to investigate whether bulk amorphous alloys can be obtained in other rare earth-based alloys and investigate the mechanical and physical properties of these bulk amorphous alloys. Considering the phase diagram of ternary Gd–Fe–Al alloy, it is similar to that of Nd–Fe–Al alloy. Thus, gadolinium has been chosen to explore the glass-forming ability of Re–TM–Al alloy systems. Moreover, the potential excellent magnetocaloric effect of Gd-based amorphous alloys is another reason of this investigation. 2. Experimental Master alloys of Gd–Fe–Al system with the composition range of 0–80 at% Fe and 5–40 at% Al were prepared by induction-melting a mixture of pure Gd, Fe and Al metals in an argon atmosphere. The amorphous ribbons with a thickness of about 20–30 m and a width of 1 mm were produced by a single-roller melt spinning method in an argon atmosphere. On the basis of foundational data of the Gd–Fe–Al amorphous alloys ribbons, the bulk amorphous alloys of Gd60–70 Fe15–30 Al10–20 (at%) were fabricated into cylindrical samples with the length of about 50 mm and the diameters ranging from 1 to 3 mm by injection casting of the molten alloy into copper molds with cylindrical cavities. The structure of the as-cast cylindrical samples was examined by X-ray diffractometry.
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D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203 Table 1 Thermal stability of Gd–Fe–Al bulk amorphous alloys Alloys
Tx (K)
Tm (K)
Tx /Tm
Tm (K)
dmax (mm)
Gd65 Fe20 Al15 Gd65 Fe15 Al20 Gd70 Fe15 Al15
808 770 772
943 938 940
0.86 0.82 0.82
135 168 168
2 1 1
Tx , crystallization temperature; Tm , melting temperature; Tx /Tm , reduced crystallization temperature; Tm = Tm − Tx ; dmax , the maximum diameter of bulk amorphous alloys.
Fig. 1. X-ray diffraction patterns for (a) Gd65 Fe20 Al15 alloy with the diameter of 2 mm, (b) Gd65 Fe15 Al20 alloy and (c) Gd70 Fe15 Al15 alloy with the diameters of 1 mm. The thermal stability associated with crystallization and melting was measured at a heating rate of 0.67 K/s by differential scanning calorimetry (DSC). Magnetic properties were measured with a vibrating sample magnetometer (VSM) under an applied field of 1432 kA/m at room temperature. Compressive testing was performed with an Instron testing machine and the strain rate was 5 × 10−4 s−1 . The gauge dimension of the specimen was 2 mm in diameter and 4 mm in height, fracture face was observed by scanning electron microscopy (SEM).
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the as-cast Gd-based cylinders with different diameters. The Gd65 Fe20 Al15 cylinder with a diameter of 2 mm (Fig. 1(a)), Gd65 Fe15 Al20 and Gd70 Fe15 Al15 cylinders with a diameter of 1 mm (Fig. 1(b and c)) exhibit a typical broad halo pattern for an amorphous structure. There are no detectable crystalline peaks in the XRD patterns. Thus, it can be said that the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 alloys of Gd–Fe–Al ternary alloy systems can be fabricated in a bulk form. Fig. 2 shows the DSC curves of the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys with a diameter of 1 mm. The thermal properties of these bulk amor-
Fig. 2. DSC curves of the Gd–Fe–Al bulk amorphous alloys with a diameter of 1 mm.
phous alloys are summarized in Table 1.The measurements were carried out using fragments taken from the central region of each as-cast cylinder specimen. The DSC curves of Gd65 Fe20 Al15 alloy show a distinct exothermic reaction due to crystallization marked with Tx , followed by an endothermic reaction peak due to melting marked with Tm which are measured to be 808 and 943 K, respectively. Accordingly, the temperature interval of supercooled liquid defined by the difference between Tx and Tm , Tm (=Tm − Tx ), is as small as 135 K and the reduced crystallization temperature (Tx /Tm ) is as high as 0.86. From Fig. 2, no glass transition phenomenon is observed in the temperature range below crystallization temperature, which indicates that the glass transition temperature (Tg ) is estimated to be higher than Tx . Consequently, the Tx /Tm can be interpreted as the reduced glass transition temperature (Tg /Tm ) for this alloy. The high amorphous-forming ability of the Gd65 Fe20 Al15 alloy is, thus, supported by the small Tm and high Tx /Tm values. The extremely high Tx /Tm value and the small Tm imply a steep increase in viscosity with the decreasing temperature in the supercooled liquid of the Gd–Fe–Al alloy. Thus, Gd–Fe–Al supercooled liquid is considered to have a high random packing density in the supercooled liquid by satisfying the factors: (1) multicomponent elements; (2) significant difference in atomic size ratios and (3) large and negative heat of mixing. These factors are frequently utilized for the development of bulk metallic glasses with high glass-forming ability. It is noticed that the melting behavior of Gd65 Fe20 Al15 alloy is different from that of the other two bulk amorphous alloys. Fig. 2(b and c) shows that the Gd65 Fe15 Al20 and Gd70 Fe15 Al15 alloys exhibit two exothermic peaks and Tx of the first peaks of each alloy is lower than that of the Gd65 Fe20 Al15 alloy, although the melting temperature (Tm ) of these three bulk amorphous alloys are almost the same. Thus, the Tx /Tm value of Gd65 Fe20 Al15 is the highest in these Gd–Fe–Al bulk amorphous alloys. These results indicate that the composition of Gd65 Fe20 Al15 alloy is much closer to the eutectic reaction in the phase diagram than the other alloy compositions, and can be interpreted that Gd65 Fe20 Al15 has the highest amorphous ability in the Gd–Fe–Al bulk amorphous alloys. Fig. 3 presents the hysteresis J–H loops of the bulk (a) Gd65 Fe20 Al15 , (b) Gd65 Fe15 Al20 and (c) Gd70 Fe15 Al15 amorphous alloy cylinders with a diameter of 1 mm prepared by the copper mold casting method, together with the data of the melt-spun amorphous ribbon with the thickness of about 30 m for comparison. The Gd–Fe–Al amorphous alloys with different shapes exhibit some what different magnetic behaviors at room temperature: the amorphous ribbons exhibit paramagnetism while the bulk amorphous cylinders show peculiar magnetic
D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203
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Fig. 4. Hysteresis J–H loops of the melt-spun Gd65 Fe20 Al15 alloy amorphous ribbon at low temperature from 173 to 273 K.
It is not clear yet why Gd–Fe–Al amorphous alloys exhibit the superparamagnetism-like behavior. However, similar experimental results were reported about the amorphous ribbons of NdFeAl [16], Y60 Fe30 Al10 [17] and GdFeAl [18] alloy systems, in which the origin of the superparamagnetism is due to the formation of Fe-rich magnetic clusters embedded in a paramagnetic amorphous matrix [19–21]. It can be expected that similar mechanism may play a role in the Gd–Fe–Al bulk amorphous alloys. The superparamagnetism behavior of Gd–Fe–Al bulk amorphous alloys is presumably due to the existence of the Ferich clusters with a larger size or stronger cluster interaction in the amorphous matrix, which can be proved by the HRTEM images obtained from the Gd70 Fe15 Al15 alloy cylinders shown in Fig. 5. Fig. 6 gives the compressive stress–strain curve of the bulk Gd65 Fe20 Al15 amorphous alloy cylinder with a diameter of 2 mm. The compressive fracture strength (σ f ) and Young’s modulus (E) of bulk Gd65 Fe20 Al15 amorphous alloy are 1120 MPa and 56 GPa, respectively, which are higher than those of the Lnbased bulk metallic glasses. Fig. 7 gives the SEM photographs
Fig. 3. Hysteresis J–H loops of the melt-spun amorphous ribbon and the ascast (a) Gd65 Fe20 Al15 , (b) Gd65 Fe15 Al20 and (c) Gd70 Fe15 Al15 cylinders with a diameter of 1 mm.
properties which are different from that of the conventional magnetism represented by ferromagnetism, paramagnetism and so on. The Gd–Fe–Al amorphous alloy ribbons exhibit peculiar magnetism according to the VSM measurement at the temperature lower than room temperature. As shown in Fig. 4, the shapes of the J–H loops change from paramagnetism type to another with decreasing temperature from 273 to 173 K. A part of the J–H loop of the ribbon specimen at the applied field nearly 0 kAm−1 shows partially ferromagnetism-like behavior, and is completely different magnetism from the paramagnetism. According to the J–H loops of the ribbon specimen obtained at 173 K, it can be said that superparamagnetic-like mechanism works for the Gd–Fe–Al amorphous alloy at low temperature.
Fig. 5. HRTEM images and selected-area electron diffraction patterns of the cast bulk Gd70 Fe15 Al15 alloy sample.
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D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203
Fig. 6. Compressive stress–strain curves of bulk Gd65 Fe20 Al15 amorphous alloy with a diameter of 2 mm.
of the fracture surface morphology of the Gd65 Fe20 Al15 bulk amorphous alloy rod. The fracture surface consists of a number of small fracture zones and their zone planes are tilted nearly 90◦ to the direction of applied load, which is nearly the same with the previous results of Fe-based bulk glassy alloys [22,23] and Fe-rich Fe–Nd–Al bulk amorphous alloys. However, the tilting angel of nearly 90◦ is different from the previous results for typical bulk metallic glasses where the fracture frequently occurs along the maximum shear plane at 45◦ to the direction of applied load [24–26]. The simultaneous generations of a number of small fracture zones is presumably due to the easy initiation of fracture at many sites, which may be resulted from the mechanism that the initiation of crack occurs at a high stress level and resulting in a shock wave and induces cracks at different sites. Furthermore, the fine shell pattern caused by the propagation of the cracks (Fig. 7(b)) also supported the fracture mechanism. Gd–Fe–Al amorphous alloy has a promising application prospects as a new kind of magnetic refrigeration material with high magnetocaloric effect (MCE), because Gd has the Curie temperature near room temperature (294 K) and relative large MCE [27,28]. Recently, a series of Gd5 (Six Ge1−x )4 (0≤ x ≤5) alloys were prepared in Ames Laboratory [29,30]. These results show that the isothermal magnetic entropy change (SM ) is at least two times higher than that of Gd near the room temperature, and 2 to 10 times higher than that of the best magnetocaloric material below room temperature. Furthermore, it is reported that the Curie temperature varies from 20 to 286 K by changing the Si to Ge ratio and by introducing small alloying additions of Ga to Gd5 (Six Ge1−x )4 [31]. However, the exorbitant cost of Ge is a fateful disadvantage of this kind of magnetocaloric material. On the contrary, Gd–Fe–Al bulk amorphous alloys have lower cost than that of the Gd5 (Six Ge1−x )4 alloys. Although the investigations of the magnetocaloric effect of Gd–Fe–Al bulk amorphous alloys are not carried out yet, but the research results of magnetocaloric effect in NdFeAl amorphous ribbon by Si et al. [16] and the peak magnetocaloric effect in Al–Gd–Fe alloys by Provenzano et al. [32] have suggested the possibility of the large magnetocaloric effects of Gd–Fe–Al amorphous alloys. The former shows that the NdFeAl amorphous phase has a relatively high magnetocaloric effect around the Curie temperature of 110 K, and the latter shows that the peak in magnetocaloric effect was found in several alloys around the Al28 Gd60 Fe12 with a much larger SM peak found at temperature about 10 K. 4. Conclusion The Gd–Fe–Al bulk amorphous alloys with high amorphousforming ability were developed. The structure, thermal stability and magnetic properties were investigated primary for cast Gd60–70 Fe15–30 Al10–20 (at%) alloys. The conclusions are drawn as follows:
Fig. 7. (a and b) Fracture surface morphology of the Gd65 Fe20 Al15 bulk amorphous alloy cylinder subjected to compressive deformation test.
(1) The X-ray diffraction results show that the maximum diameters of Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys are 2, 1 and 1 mm, respectively. (2) The reduced crystallization temperatures (Tx /Tm ) of the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amor-
D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203
phous alloys with a diameter of 1 mm are about 0.82–0.86, and the Tm (=Tm − Tx ) is measured to be 135–168 K. The Gd–Fe–Al bulk amorphous alloys exhibit superparamagnetism at the room temperature, while the amorphous ribbons show the paramagnetism at the same temperature. (3) These Gd–Fe–Al bulk amorphous alloys with large amorphous-forming ability are promising for future development as new type of magnetocaloric materials. References [1] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mater. Trans., JIM 32 (1991) 609–616. [2] A. Inoue, T. Nakamura, N. Nishiyama, T. Masumoto, Mater. Trans., JIM 33 (1992) 937–945. [3] A. Inoue, T. Zhang, N. Nishiyama, K. Ohba, T. Masumoto, Mater. Trans., JIM 34 (1993) 1234–1237. [4] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 36 (1995) 391–398. [5] A. Inoue, T. Zhang, N. Nishiyama, K. Ohba, T. Masumoto, Mater. Sci. Eng., A 179–180 (1994) 210–214. [6] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342–2344. [7] A. Inoue, N. Nishiyama, T. Matusda, Mater. Trans., JIM 37 (1996) 181–184. [8] A. Inoue, Y. Shinohara, J.S. Gook, Mater. Trans., JIM 36 (1995) 1427–1433. [9] A. Inoue, T. Zhang, A. Takeuchi, Appl. Phys. Lett. 71 (1997) 464–466. [10] A. Inoue, K. Kita, T. Zhang, T. Masumoto, Mater. Trans., JIM 30 (1989) 722–725. [11] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 31 (1990) 425– 428. [12] A. Inoue, Mater. Sci. Eng., A 226–228 (1997) 357–363. [13] G.J. Fan, W. Loser, S. Roth, J. Eckert, Acta Mater. 48 (2000) 3823–3831.
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[14] G.J. Fan, S.J. Poon, G.J. Shiflet, Appl. Phys. Lett. 83 (2003) 2575–2577. [15] B. Zhang, D.Q. Zhao, M.X. Pan, W.H. Wang, A.L. Greer, Phys. Rev. Lett. 94 (2005) 205–502. [16] L. Si, J. Ding, L. Wang, Y. Li, H. Tan, B. Yao, J. Alloys Compd. 316 (2000) 260–263. [17] Y. Li, J. Ding, X.Z. Wang, Phys. Status Solidi A 172 (1999) 461–468. [18] H.Z. Kong, J. Ding, L. Wang, Y. Li, IEEE Trans. Magn. 37 (2001) 2500–2502. [19] L. Wang, J. Ding, Y. Li, Y.P. Feng, N.X. Phuc, N.H. Dan, J. Appl. Phys. 89 (2001) 8046–8053. [20] L. Wang, J. Ding, Y. Li, Y.P. Feng, N.X. Phuc, N.H. Dan, J. Magn. Magn. Mater. 226–230 (2001) 1504–1506. [21] L. Wang, J. Ding, Y. Li, H.Z. Kong, Y.P. Feng, X.Z. Wang, J. Phys.: Condens. Matter 12 (2000) 4253–4256. [22] A. Inoue, B.L. Shen, A.R. Yavari, A.L. Greer, J. Mater. Res. 18 (2003) 1487–1492. [23] A. Inoue, B.L. Shen, C.T. Chang, Acta Mater. 52 (2004) 4093–4099. [24] A. Inoue, T. Zhang, Mater. Trans., JIM 36 (1995) 1184–1187. [25] A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, Acta Mater. 49 (2001) 2645–2652. [26] T. Zhang, A. Inoue, Mater. Trans., JIM 43 (2002) 708–711. [27] V.K. Pecharsky Jr., K.A. Gschneidner, J. Magn. Magn. Mater. 200 (1999) 44–56. [28] B.F. Yu, Q. Gao, B. Zhang, X.Z. Meng, Z. Chen, Int. J. Refrig. 26 (2003) 622–636. [29] V.K. Pecharsky Jr., K.A. Gschneidner, Appl. Phys. Lett. 70 (1997) 3299–3301. [30] V.K. Pecharsky Jr., K.A. Gschneidner, Phys. Rev. Lett. 78 (1997) 4494–4497. [31] V.K. Pecharsky Jr., K.A. Gschneidner, Adv. Mater. 13 (2001) 683–686. [32] V. Provenzano, A.J. Shapiro, R.D. Shull, T. King, E. Canavan, P. Shirron, M. Dipirro, J. Appl. Phys. 95 (2004) 6909–6911.
Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys Ding Chen ∗ , Akira Takeuchi, Akihisa Inoue Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 16 August 2006; accepted 17 September 2006 Available online 17 October 2006
Abstract Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (Tx ) and melting temperature (Tm ) of the Gd65 Fe20 Al15 bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of Tm and Tx , Tm (=Tm − Tx ), is as small as 135 K and the reduced crystallization temperature (Tx /Tm ) is as high as 0.86. The small Tm and high Tx /Tm values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd65 Fe20 Al15 bulk amorphous alloys are investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk amorphous materials; Metallic glasses; Magnetic properties
1. Introduction Bulk amorphous alloys have received increasing interest during recent years due to the promising prospects in engineering materials. Bulk metallic glasses with sizes greater than a couple of millimeters have been obtained in a number of alloy systems such as Mg–Cu–Y [1,2], Zr–Al–TM [3,4], Ti–Zr–Al–TM–Be [5], Zr–Ti–TM–Be [6], Pd–Ni–Cu–P [7], Fe–(Al, Ga)–(P, C, B, Si) [8], Fe–(Co, Ni)–(Zr, Hf, Nb)–B [9] and RE–Al–TM [10–15] (RE = rare earth, TM = transition metal) systems. Among these bulk metallic glasses, RE–Al–TM (La-, Sm-, Nd-, Pr-, Y- and Ce-based) alloy systems have extremely large glass-forming ability (GFA) [10–15] which enables us to fabricate bulk amorphous alloys with diameters of several millimeters by copper mold casting. Furthermore, Nd–Fe–Al and Pr–Fe–Al bulk amorphous alloys exhibit unique hard magnetic behavior with a remanence (Jr ) of 0.09 T, a saturation magnetization (J1432 ) of 0.13 T and coercivity (i Hc ) around 300 kA/m [12]. Ybased bulk amorphous alloys rods with diameters over 20 mm ∗ Corresponding author at: Hunan University, School of Materials Science and Engineering, Changsha, Hunan 410082, China. Tel.: +86 0731 8821648; fax: +86 0731 8821648. E-mail address: [email protected] (D. Chen).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.064
were obtained by water quenching [14]. Ce-based bulk metallic glasses exhibit polymer-like thermoplastic behavior caused by their extraditionally low glass transition temperature, Tg , which is similar to or lower than that of many polymers [15]. Thus, it is interesting to investigate whether bulk amorphous alloys can be obtained in other rare earth-based alloys and investigate the mechanical and physical properties of these bulk amorphous alloys. Considering the phase diagram of ternary Gd–Fe–Al alloy, it is similar to that of Nd–Fe–Al alloy. Thus, gadolinium has been chosen to explore the glass-forming ability of Re–TM–Al alloy systems. Moreover, the potential excellent magnetocaloric effect of Gd-based amorphous alloys is another reason of this investigation. 2. Experimental Master alloys of Gd–Fe–Al system with the composition range of 0–80 at% Fe and 5–40 at% Al were prepared by induction-melting a mixture of pure Gd, Fe and Al metals in an argon atmosphere. The amorphous ribbons with a thickness of about 20–30 m and a width of 1 mm were produced by a single-roller melt spinning method in an argon atmosphere. On the basis of foundational data of the Gd–Fe–Al amorphous alloys ribbons, the bulk amorphous alloys of Gd60–70 Fe15–30 Al10–20 (at%) were fabricated into cylindrical samples with the length of about 50 mm and the diameters ranging from 1 to 3 mm by injection casting of the molten alloy into copper molds with cylindrical cavities. The structure of the as-cast cylindrical samples was examined by X-ray diffractometry.
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D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203 Table 1 Thermal stability of Gd–Fe–Al bulk amorphous alloys Alloys
Tx (K)
Tm (K)
Tx /Tm
Tm (K)
dmax (mm)
Gd65 Fe20 Al15 Gd65 Fe15 Al20 Gd70 Fe15 Al15
808 770 772
943 938 940
0.86 0.82 0.82
135 168 168
2 1 1
Tx , crystallization temperature; Tm , melting temperature; Tx /Tm , reduced crystallization temperature; Tm = Tm − Tx ; dmax , the maximum diameter of bulk amorphous alloys.
Fig. 1. X-ray diffraction patterns for (a) Gd65 Fe20 Al15 alloy with the diameter of 2 mm, (b) Gd65 Fe15 Al20 alloy and (c) Gd70 Fe15 Al15 alloy with the diameters of 1 mm. The thermal stability associated with crystallization and melting was measured at a heating rate of 0.67 K/s by differential scanning calorimetry (DSC). Magnetic properties were measured with a vibrating sample magnetometer (VSM) under an applied field of 1432 kA/m at room temperature. Compressive testing was performed with an Instron testing machine and the strain rate was 5 × 10−4 s−1 . The gauge dimension of the specimen was 2 mm in diameter and 4 mm in height, fracture face was observed by scanning electron microscopy (SEM).
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the as-cast Gd-based cylinders with different diameters. The Gd65 Fe20 Al15 cylinder with a diameter of 2 mm (Fig. 1(a)), Gd65 Fe15 Al20 and Gd70 Fe15 Al15 cylinders with a diameter of 1 mm (Fig. 1(b and c)) exhibit a typical broad halo pattern for an amorphous structure. There are no detectable crystalline peaks in the XRD patterns. Thus, it can be said that the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 alloys of Gd–Fe–Al ternary alloy systems can be fabricated in a bulk form. Fig. 2 shows the DSC curves of the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys with a diameter of 1 mm. The thermal properties of these bulk amor-
Fig. 2. DSC curves of the Gd–Fe–Al bulk amorphous alloys with a diameter of 1 mm.
phous alloys are summarized in Table 1.The measurements were carried out using fragments taken from the central region of each as-cast cylinder specimen. The DSC curves of Gd65 Fe20 Al15 alloy show a distinct exothermic reaction due to crystallization marked with Tx , followed by an endothermic reaction peak due to melting marked with Tm which are measured to be 808 and 943 K, respectively. Accordingly, the temperature interval of supercooled liquid defined by the difference between Tx and Tm , Tm (=Tm − Tx ), is as small as 135 K and the reduced crystallization temperature (Tx /Tm ) is as high as 0.86. From Fig. 2, no glass transition phenomenon is observed in the temperature range below crystallization temperature, which indicates that the glass transition temperature (Tg ) is estimated to be higher than Tx . Consequently, the Tx /Tm can be interpreted as the reduced glass transition temperature (Tg /Tm ) for this alloy. The high amorphous-forming ability of the Gd65 Fe20 Al15 alloy is, thus, supported by the small Tm and high Tx /Tm values. The extremely high Tx /Tm value and the small Tm imply a steep increase in viscosity with the decreasing temperature in the supercooled liquid of the Gd–Fe–Al alloy. Thus, Gd–Fe–Al supercooled liquid is considered to have a high random packing density in the supercooled liquid by satisfying the factors: (1) multicomponent elements; (2) significant difference in atomic size ratios and (3) large and negative heat of mixing. These factors are frequently utilized for the development of bulk metallic glasses with high glass-forming ability. It is noticed that the melting behavior of Gd65 Fe20 Al15 alloy is different from that of the other two bulk amorphous alloys. Fig. 2(b and c) shows that the Gd65 Fe15 Al20 and Gd70 Fe15 Al15 alloys exhibit two exothermic peaks and Tx of the first peaks of each alloy is lower than that of the Gd65 Fe20 Al15 alloy, although the melting temperature (Tm ) of these three bulk amorphous alloys are almost the same. Thus, the Tx /Tm value of Gd65 Fe20 Al15 is the highest in these Gd–Fe–Al bulk amorphous alloys. These results indicate that the composition of Gd65 Fe20 Al15 alloy is much closer to the eutectic reaction in the phase diagram than the other alloy compositions, and can be interpreted that Gd65 Fe20 Al15 has the highest amorphous ability in the Gd–Fe–Al bulk amorphous alloys. Fig. 3 presents the hysteresis J–H loops of the bulk (a) Gd65 Fe20 Al15 , (b) Gd65 Fe15 Al20 and (c) Gd70 Fe15 Al15 amorphous alloy cylinders with a diameter of 1 mm prepared by the copper mold casting method, together with the data of the melt-spun amorphous ribbon with the thickness of about 30 m for comparison. The Gd–Fe–Al amorphous alloys with different shapes exhibit some what different magnetic behaviors at room temperature: the amorphous ribbons exhibit paramagnetism while the bulk amorphous cylinders show peculiar magnetic
D. Chen et al. / Journal of Alloys and Compounds 440 (2007) 199–203
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Fig. 4. Hysteresis J–H loops of the melt-spun Gd65 Fe20 Al15 alloy amorphous ribbon at low temperature from 173 to 273 K.
It is not clear yet why Gd–Fe–Al amorphous alloys exhibit the superparamagnetism-like behavior. However, similar experimental results were reported about the amorphous ribbons of NdFeAl [16], Y60 Fe30 Al10 [17] and GdFeAl [18] alloy systems, in which the origin of the superparamagnetism is due to the formation of Fe-rich magnetic clusters embedded in a paramagnetic amorphous matrix [19–21]. It can be expected that similar mechanism may play a role in the Gd–Fe–Al bulk amorphous alloys. The superparamagnetism behavior of Gd–Fe–Al bulk amorphous alloys is presumably due to the existence of the Ferich clusters with a larger size or stronger cluster interaction in the amorphous matrix, which can be proved by the HRTEM images obtained from the Gd70 Fe15 Al15 alloy cylinders shown in Fig. 5. Fig. 6 gives the compressive stress–strain curve of the bulk Gd65 Fe20 Al15 amorphous alloy cylinder with a diameter of 2 mm. The compressive fracture strength (σ f ) and Young’s modulus (E) of bulk Gd65 Fe20 Al15 amorphous alloy are 1120 MPa and 56 GPa, respectively, which are higher than those of the Lnbased bulk metallic glasses. Fig. 7 gives the SEM photographs
Fig. 3. Hysteresis J–H loops of the melt-spun amorphous ribbon and the ascast (a) Gd65 Fe20 Al15 , (b) Gd65 Fe15 Al20 and (c) Gd70 Fe15 Al15 cylinders with a diameter of 1 mm.
properties which are different from that of the conventional magnetism represented by ferromagnetism, paramagnetism and so on. The Gd–Fe–Al amorphous alloy ribbons exhibit peculiar magnetism according to the VSM measurement at the temperature lower than room temperature. As shown in Fig. 4, the shapes of the J–H loops change from paramagnetism type to another with decreasing temperature from 273 to 173 K. A part of the J–H loop of the ribbon specimen at the applied field nearly 0 kAm−1 shows partially ferromagnetism-like behavior, and is completely different magnetism from the paramagnetism. According to the J–H loops of the ribbon specimen obtained at 173 K, it can be said that superparamagnetic-like mechanism works for the Gd–Fe–Al amorphous alloy at low temperature.
Fig. 5. HRTEM images and selected-area electron diffraction patterns of the cast bulk Gd70 Fe15 Al15 alloy sample.
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Fig. 6. Compressive stress–strain curves of bulk Gd65 Fe20 Al15 amorphous alloy with a diameter of 2 mm.
of the fracture surface morphology of the Gd65 Fe20 Al15 bulk amorphous alloy rod. The fracture surface consists of a number of small fracture zones and their zone planes are tilted nearly 90◦ to the direction of applied load, which is nearly the same with the previous results of Fe-based bulk glassy alloys [22,23] and Fe-rich Fe–Nd–Al bulk amorphous alloys. However, the tilting angel of nearly 90◦ is different from the previous results for typical bulk metallic glasses where the fracture frequently occurs along the maximum shear plane at 45◦ to the direction of applied load [24–26]. The simultaneous generations of a number of small fracture zones is presumably due to the easy initiation of fracture at many sites, which may be resulted from the mechanism that the initiation of crack occurs at a high stress level and resulting in a shock wave and induces cracks at different sites. Furthermore, the fine shell pattern caused by the propagation of the cracks (Fig. 7(b)) also supported the fracture mechanism. Gd–Fe–Al amorphous alloy has a promising application prospects as a new kind of magnetic refrigeration material with high magnetocaloric effect (MCE), because Gd has the Curie temperature near room temperature (294 K) and relative large MCE [27,28]. Recently, a series of Gd5 (Six Ge1−x )4 (0≤ x ≤5) alloys were prepared in Ames Laboratory [29,30]. These results show that the isothermal magnetic entropy change (SM ) is at least two times higher than that of Gd near the room temperature, and 2 to 10 times higher than that of the best magnetocaloric material below room temperature. Furthermore, it is reported that the Curie temperature varies from 20 to 286 K by changing the Si to Ge ratio and by introducing small alloying additions of Ga to Gd5 (Six Ge1−x )4 [31]. However, the exorbitant cost of Ge is a fateful disadvantage of this kind of magnetocaloric material. On the contrary, Gd–Fe–Al bulk amorphous alloys have lower cost than that of the Gd5 (Six Ge1−x )4 alloys. Although the investigations of the magnetocaloric effect of Gd–Fe–Al bulk amorphous alloys are not carried out yet, but the research results of magnetocaloric effect in NdFeAl amorphous ribbon by Si et al. [16] and the peak magnetocaloric effect in Al–Gd–Fe alloys by Provenzano et al. [32] have suggested the possibility of the large magnetocaloric effects of Gd–Fe–Al amorphous alloys. The former shows that the NdFeAl amorphous phase has a relatively high magnetocaloric effect around the Curie temperature of 110 K, and the latter shows that the peak in magnetocaloric effect was found in several alloys around the Al28 Gd60 Fe12 with a much larger SM peak found at temperature about 10 K. 4. Conclusion The Gd–Fe–Al bulk amorphous alloys with high amorphousforming ability were developed. The structure, thermal stability and magnetic properties were investigated primary for cast Gd60–70 Fe15–30 Al10–20 (at%) alloys. The conclusions are drawn as follows:
Fig. 7. (a and b) Fracture surface morphology of the Gd65 Fe20 Al15 bulk amorphous alloy cylinder subjected to compressive deformation test.
(1) The X-ray diffraction results show that the maximum diameters of Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amorphous alloys are 2, 1 and 1 mm, respectively. (2) The reduced crystallization temperatures (Tx /Tm ) of the Gd65 Fe20 Al15 , Gd65 Fe15 Al20 and Gd70 Fe15 Al15 bulk amor-
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phous alloys with a diameter of 1 mm are about 0.82–0.86, and the Tm (=Tm − Tx ) is measured to be 135–168 K. The Gd–Fe–Al bulk amorphous alloys exhibit superparamagnetism at the room temperature, while the amorphous ribbons show the paramagnetism at the same temperature. (3) These Gd–Fe–Al bulk amorphous alloys with large amorphous-forming ability are promising for future development as new type of magnetocaloric materials. References [1] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mater. Trans., JIM 32 (1991) 609–616. [2] A. Inoue, T. Nakamura, N. Nishiyama, T. Masumoto, Mater. Trans., JIM 33 (1992) 937–945. [3] A. Inoue, T. Zhang, N. Nishiyama, K. Ohba, T. Masumoto, Mater. Trans., JIM 34 (1993) 1234–1237. [4] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 36 (1995) 391–398. [5] A. Inoue, T. Zhang, N. Nishiyama, K. Ohba, T. Masumoto, Mater. Sci. Eng., A 179–180 (1994) 210–214. [6] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342–2344. [7] A. Inoue, N. Nishiyama, T. Matusda, Mater. Trans., JIM 37 (1996) 181–184. [8] A. Inoue, Y. Shinohara, J.S. Gook, Mater. Trans., JIM 36 (1995) 1427–1433. [9] A. Inoue, T. Zhang, A. Takeuchi, Appl. Phys. Lett. 71 (1997) 464–466. [10] A. Inoue, K. Kita, T. Zhang, T. Masumoto, Mater. Trans., JIM 30 (1989) 722–725. [11] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 31 (1990) 425– 428. [12] A. Inoue, Mater. Sci. Eng., A 226–228 (1997) 357–363. [13] G.J. Fan, W. Loser, S. Roth, J. Eckert, Acta Mater. 48 (2000) 3823–3831.
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