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Glass forming ability and microstructure of hard magnetic Nd60Al20Fe20 glass forming alloy
Intermetallics 14 (2006) 1098–1101 www.elsevier.com/locate/intermet
Glass forming ability and microstructure of hard magnetic Nd60Al20Fe20 glass forming alloy L. Xia *, S.S. Fang, C.L. Jo, Y.D. Dong Institute of Materials and Center for Microanalysis, Shanghai University, Shanghai 200072, China Available online 13 March 2006
Abstract Glass forming ability (GFA), magnetic properties and microstructure of Nd60Al20Fe20 as-cast rod were investigated and further compared with Nd60Al10Fe30 glass forming alloy. The rod prepared by suction casting with a diameter of 3 mm exhibits the typical amorphous nature in XRD pattern, distinct glass transition in DSC traces and hard magnetic properties. It is found that the diameter of cast Nd60Al20Fe20 glassy rod is much larger than the critical section thickness (Zc) of bulk metallic glass (BMG) predicted from DSC measurements. A few nano-crystalline particles with the structure and composition similar to Ax phase in Nd–Fe alloys were found embedded randomly in amorphous matrix and could be the origin of hard magnetic properties of the as-cast rods. The GFA of the alloy appears to be enhanced by the precipitation of metastable nanoparticles with small positive forming enthalpy and the real Zc of the alloy could be less than 1 mm predicted by parameter g. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.50.Kj; 61.43.Dq; 64.75.Cg; 81.05.Pj Keywords: B. Glasses, metallic; B. Magnetic properties; D. Microstructure
Recent years, Nd–TM (TMZFe, Co) based bulk metallic glasses (BMGs) have attracted increasing interests because they exhibit excellent glass forming ability (GFA), hard magnetic properties at room temperature, and absence of glass transition temperature (Tg) in differential scanning calorimetry (DSC) traces [1–10]. The absence of glass transition in DSC traces makes it difficult to estimate the exact GFA of the alloys [2,11–13]. Inoue et al. investigated the GFA of Nd–Al–Fe ternary alloys and supposed that the glass transition temperature of Nd60Al10Fe30 BMG might be higher than the crystallization temperature (Tx) and thus the reduced glass transition temperature (Trg) was estimated to be higher than 0.9 [2]. Nevertheless, recent experimental results have shown that Nd–Al–(Fe,Co) BMGs were essentially partially crystalline materials [3,5,6] and the absence of glass transition in DSC traces could be due to the chemical inhomogeneity of the amorphous phase [7]. The primary crystallization (at about 460 K) obtained from the DSC trace of Nd60Al10Fe30 ribbon (30 m/s) suggests that the bulk samples are already partially crystallized [8], and as the case in Nd60Al10Fe20Co10 glass forming alloys [9], the exact glass transition temperature might * Corresponding author. E-mail address: [email protected] (L. Xia).
0966-9795/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2006.01.033
be lower than 460 K. Therefore, the reduced glass transition temperature of Nd60Al10Fe30 glass forming alloy might be much lower than the claimed value. However, up to now, as glass transition is difficult to be observed in the DSC traces of Nd–Al–Fe ternary BMGs, the GFA of the alloys have not been studied systematically. Recently we obtained glass transition using DSC from some as-cast Nd–Al–Fe ternary alloys. In this work, we reported ascast Nd60Al20Fe20 rod showing typical amorphous characters in X-ray diffraction (XRD) pattern and distinct glass transition in DSC traces. The glass transition allowed us to investigate the GFA of the alloy using Trg (TrgZTgjTl, where Tl is the liquidus temperature) [11] and the recently defined parameter g (gZTx/ (TgCTl)) [12,13]. However, it is found that the obtained diameter of the rod is much larger than the critical section thickness the theory would predict. Thus, we decide to investigate the GFA of the alloy as well as the microstructure and the magnetic properties of the as-cast rod in more detail. Pre-alloys with nominal compositions of Nd60Al20Fe20 and Nd60Al10Fe30 were prepared separately by arc-melting of 99.9% (at.%) pure Nd, Al and Fe in titanium–gettered argon atmosphere. Cylinders of the alloys were prepared in the shape of 3 mm in diameter by suction casting under pure argon atmosphere. The structure of the samples was characterized by XRD on a Philips diffractometer using Cu Ka radiation. DSC measurements were carried out under a purified argon
L. Xia et al. / Intermetallics 14 (2006) 1098–1101
atmosphere in a Perkin Elmer DSC-7 at a heating rate of 20 K/min. High-temperature DSC (HTDSC) curve was measured in a TA INSTRUMENT SDT-Q600 DSC under argon atmosphere in order to obtain the melting and liquidus temperature of the alloys. A vibrating sample magnetometer (VSM) was used for the magnetic measurements of the as-cast rods. The field applied was 1432 kA/m. The microstructure of the as-cast rod and the rough composition of nano-particles were observed on a JEOL JEM-2010 F high-resolution electron microscope (HREM) with an Oxford INCA energy dispersive spectrometer (EDS). Specimens for HREM observation were thinned under pure argon atmosphere on a rotating plate cooled by liquid nitrogen. The XRD pattern of the as-cast Nd60Al20Fe20 rod with a 3 mm in diameter is shown in Fig. 1. The typical broad diffraction maxima of amorphous phases for the rod indicate the glassy characters of the alloy, no obvious crystalline peaks were observed within the XRD detection limit. DSC traces obtained from as-cast Nd60Al20Fe20 and Nd60Al10Fe30 rods at a heating rate of 20 K/min are shown in Fig. 2. Whereas there is no glass transition in the DSC trace of Nd60Al10Fe30, Nd60Al20Fe20 exhibits clearly an endothermic glass transition followed by three exothermic crystallization peaks. Primary crystallization has been found in the DSC traces of amorphous Nd60Al10Fe30 (30 m/s) and Nd60Al10Fe20Co10 as-spun ribbons but is invisible in their bulk counterparts [8,9]. The onset temperatures of the glass transition (Tg) and crystallizations (Tx1, Tx2 and Tx3) of Nd60Al20Fe20 as-cast rod are about 468, 509, 620 and 762 K. The melting and liquidus temperatures (Tm and Tl) of Nd60Al20Fe20 alloy obtained from the HTDSC curve (not shown in present paper) are about 865 and 953 K. Thus, the supercooled liquid region (DTZTx1KTg) and the reduced glass transition temperature (TrgZTg/Tl) of the alloy are about 41 and 0.49 K, respectively. Turnbull has calculated that liquids with TrgZ0.5 (larger than that of Nd60Al20Fe20 as-cast rod) could be chilled into glassy state only in droplet with diameter less than 60 mm at the cooling rate of K106 K sK1 [11], this is obviously not in accordance with the apparent GFA of Nd60Al20Fe20 alloy. To investigate the GFA of Nd60Al20Fe20 glass-forming alloy in detail, parameter (gZTx/(TgCTl)) [12,13] is employed for the evaluation of GFA. The parameter has been successfully
Fig. 1. XRD pattern of Nd60Al20Fe20 as-cast rod.
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Fig. 2. DSC curves of Nd60Al20Fe20 and Nd60Al10Fe30 as-cast rods at a heating rate of 20 K/min.
demonstrated in various glass forming systems. The value of g calculated for Nd60Al20Fe20 BMG is about 0.36, and the critical cooling rate (RcZC1 exp[(K1n C1/g0)g], where C1 and g0 are constants) and section thickness (ZcZ2.80!10K7exp(41.70g) of the BMG can be predicted accordingly [12,13] to be about 160 K sK1 and 1 mm, respectively. The predicted critical thickness is also not in accordance with the obtained diameter of the rod. Therefore, it seems that both Trg and g are not suitable for the evaluation of GFA of Nd60Al20Fe20 BMG. It should be noted that the Nd60Al20Fe20 as-cast rod is hard magnetic. Fig. 3 shows the hysteresis loops of Nd60Al20Fe20 and Nd60Al10Fe30 as cast rods. The lower value of magnetization of Nd60Al20Fe20 as-cast rod under the field of 1432 kA/m is due to the lower concentration of Fe element. The coercivity of Nd60Al20Fe20 bulk sample is about 160 kA/m and much lower than that of Nd60Al10Fe30 as cast rod (about 230 kA/m). Recent experimental results in Nd–Al–Fe and Nd–Al–Fe–Co hard magnetic BMGs have revealed that these hard magnetic BMGs usually have the microstructure of nano-scaled particles dispersed uniformly in the amorphous matrix and the high coercivity of the rods is closely related to the existence of these nano-particles [3,5,6,9]. Therefore, the hard magnetic
Fig. 3. Hysteresis loops of Nd60Al20Fe20 and Nd60Al10Fe30 as-cast rods.
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L. Xia et al. / Intermetallics 14 (2006) 1098–1101
properties of Nd60Al20Fe20 as-cast rod suggest that the rod could be also partially crystalline and the XRD pattern of the rod indicate that the possibly existed crystalline particles could be too small to be reflected in XRD patterns. Furthermore, as the coercivity of Nd–Al–Fe(Co) as-cast rods is dominated by the size and distribution of the particles [9,14], the lower coercivity of Nd60Al20Fe20 rod suggests that the particles could be smaller or fewer in Nd60Al20Fe20 than in Nd60Al10Fe30 ascast rods. The microstructure of Nd60Al20Fe20 as-cast rod has demonstrated the suggestions. Some isolated particles were found distributed randomly in the amorphous matrix in as-cast Nd60Al20Fe20 rod, as shown in Fig. 4. The interplanar distance of the particles is about 0.275 nm. It has been reported that the interplanar distance of the nano-particles in as-cast Nd60Al10Fe20Co10 is about 0.273 nm [5,6,9], and in the as cast Nd60Al10Fe30 it is about 0.32 nm [3]. They are roughly correspondent to (002) dhkl (Z0.275 nm) and (111) dhkl (Z 0.32 nm) of the metastable Ax phase with fcc structure of aZ 0.55 nm in Nd-riched Nd–Fe alloys [15]. On the other hand, the average composition of the particles detected by EDS is about Nd79Fe21. Thus, the nano-crystalline particles in Nd60Al20Fe20 as-cast rod could be near-eutectic or pseudo-eutectic metastable Ax phase [15]. The nano-crystalline region of the particles with the same structure of Ax phase, which is supposed to be the same as the well known highly anisotropic A1 phase, could be the origin of high coercivity in Nd–Al– Fe(Co) nano-glasses [15]. The formation of nano-crystallites in Nd–Al–Fe(Co) alloy system has evoked increasing research interests because classical nucleation from a homogeneous undercooled liquid state should result in a rather coarse microstructure upon crystallization [3,16]. It is argued that the typical morphology of fine particles dispersed within the amorphous matrix in Nd–Al–Fe and Nd–Al–Fe–Co hard magnetic BMGs is induced by the phase separation of the melt or the slightly undercooled
Fig. 4. HREM image of Nd60Al20Fe20 as-cast rod.
liquids into two metastable phases [3]. Frankwicz et al. have revealed that the precipitation of intermediate phase (metastable I-phase) balanced the dynamic equilibrium between the reordered and disordered configuration of Zr65Al7.5Ni10Cu17.5 water-quenched cylinders and led to the formation of refined microstructure over thermally stabilized nanocrystallites [17]. Supposed that one of the separated phases has higher forming enthalpy, the formation of this phase will lower the forming enthalpy of the remainders and thus stabilize the others. We have obtained the composition of nano-scaled particles in Nd60Al10Fe20Co10 as-cast rod to be about Nd69Fe21Co10 [6,9]. Based on the Miedema’s model [18–20], we calculated the forming enthalpies of Nd69Fe21Co10 and Nd79Fe21 phases and observed that they are all small positive. The precipitation of the phases with small positive value of forming enthalpy as Nd69Fe21Co10 and Nd79Fe21 could stabilize the amorphous matrix and thus the apparent glass forming ability of the alloys could be enlarged by the phase separation in Nd–Al–Fe(Co) glass forming alloys. In addition, as the results obtained in Nd– Fe rapidly quenched alloys [20], the small positive value of forming enthalpy of the separated phases could reduce the formation of stable intermetallic compounds and therefore, induce the formation of metastable phases as the nano-scaled particles in Nd–Al–Fe(Co) as-cast rods under favorable thermal conditions [6]. On the other hand, the composition of the remaining amorphous matrix is different from the nominal one because of the presence of nano-crystalline particles with different composition. Thus, the critical section thickness predicted by either Trg or parameter g resulting from Tg and Tx of the remaining amorphous matrix could not reflect the real GFA of the alloy. As the glass forming enthalpy of Nd60Al20Fe20 alloy is higher than that of the amorphous matrix, Zc of Nd60Al20Fe20 alloy could be lower than the predicted value, in a nutshell, less than 1 mm. In conclusion, we studied the GFA, magnetic properties, and microstructure of as-cast Nd60Al20Fe20 rod. The material exhibited distinct glass transition in DSC traces and typical amorphous features in XRD patterns. Trg and g of the as-cast Nd60Al20Fe20 rod obtained from the DSC measurements were about 0.49 and 0.36, respectively. The critical section thickness of the BMG predicted by either Trg (less than 60 mm) or g (about 1 mm) is much lower than the diameter of experimentally obtained glassy rod (about 3 mm). Structural observation showed that isolated nanocrystalline particles, which have a structure and composition similar to the near-eutectic or pseudo-eutectic metastable Ax phase, embedded randomly in the amorphous matrix. The nano-crystalline region was expected to be the origin of high coercivity in Nd–Al–Fe(Co) nano-glasses due to the highly anisotropic structure of these nano-particles. We believe the apparent GFA of Nd60Al20Fe20 BMG could be greatly enhanced by the precipitation of metastable nanoparticles resulting from phase separation in Nd–Al–Fe alloys, although the small positive forming enthalpy of the precipitates and the real Zc of the alloy could be less than 1 mm predicted by parameter g.
L. Xia et al. / Intermetallics 14 (2006) 1098–1101
Acknowledgements The authors are grateful for the financial supports of the National Nature Science Foundation of China (Grant No. 50471099) and the Development Foundation of Shanghai Educational Commission (No. 04-58-04AB08). References [1] Inoue A, Zhang T, Zhang W, Takeuchi A. Mater Trans JIM 1996;37:99. [2] Inoue A, Takeuchi A, Zhang T. Metall Mater Trans A 1998;29:1779. [3] Schneider S, Bracchi A, Samwer K, Seibt M, Thiyagarajan P. Appl Phys Lett 2002;80:1749. [4] Fan GJ, Lo¨ser W, Roth S, Eckert J, Schultz L. J Mater Res 2000;15:1556. [5] Xia L, Wei BC, Zhang Z, Pan MX, Wang WH, Dong YD. J Phys D 2003; 36:775. [6] Xia L, Wei BC, Pan MX, Zhao DQ, Wang WH, Dong YD. J Phys Condens Matter 2003;15:3531. [7] Wei BC, Lo¨ser W, Xia L, Roth S, Pan MX, Wang WH, et al. Acta Mater 2002;50:4357.
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[8] Wang XZ, Li Y, Ding J, Si L, Kong HZ. J Alloys Compd 1999;290: 209. [9] Xia L, Tang MB, Pan MX, Zhao DQ, Wang WH, Dong YD. J Phys D 2003;36:2954. [10] Xing LQ, Eckert J, Lo¨ser W, Roth S, Schultz L. J Appl Phys 2000;88: 3565. [11] Turnbull D. Contemp Phys 1969;10:473. [12] Lu ZP, Liu CT. Acta Mater 2002;50:3501. [13] Lu ZP, Liu CT. Phys Rev Lett 2003;91:115501–5. [14] Lo¨ffler JF, Braun HB, Wagner W. Phys Rev Lett 2000;85:1990. [15] Menushenkov VP, Andersen SJ, Høier R. Proceedings of the 10th international symposium on magnetic anisotropy and coercivity in rareearth transition metal alloys. Dresden: Werkstoff-Informationsgesellschaft; 1998. p. 97. [16] Busch R, Schneider S, Peker A, Johnson WL. Appl Phys Lett 1995;67: 1544. [17] Frankwicz PS, Ram S, Fecht HJ. Mater Lett 1996;28:77. [18] Miedema AR, Boom R, De Boer FR. J Less Common Met 1975;41: 283. [19] Takeuchi A, Inoue A. Mater Trans JIM 2001;42-7:1435. [20] Delamare J, Lemarchand D, Vigier P. J Alloys Compd 1994;216:273.
Glass forming ability and microstructure of hard magnetic Nd60Al20Fe20 glass forming alloy L. Xia *, S.S. Fang, C.L. Jo, Y.D. Dong Institute of Materials and Center for Microanalysis, Shanghai University, Shanghai 200072, China Available online 13 March 2006
Abstract Glass forming ability (GFA), magnetic properties and microstructure of Nd60Al20Fe20 as-cast rod were investigated and further compared with Nd60Al10Fe30 glass forming alloy. The rod prepared by suction casting with a diameter of 3 mm exhibits the typical amorphous nature in XRD pattern, distinct glass transition in DSC traces and hard magnetic properties. It is found that the diameter of cast Nd60Al20Fe20 glassy rod is much larger than the critical section thickness (Zc) of bulk metallic glass (BMG) predicted from DSC measurements. A few nano-crystalline particles with the structure and composition similar to Ax phase in Nd–Fe alloys were found embedded randomly in amorphous matrix and could be the origin of hard magnetic properties of the as-cast rods. The GFA of the alloy appears to be enhanced by the precipitation of metastable nanoparticles with small positive forming enthalpy and the real Zc of the alloy could be less than 1 mm predicted by parameter g. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.50.Kj; 61.43.Dq; 64.75.Cg; 81.05.Pj Keywords: B. Glasses, metallic; B. Magnetic properties; D. Microstructure
Recent years, Nd–TM (TMZFe, Co) based bulk metallic glasses (BMGs) have attracted increasing interests because they exhibit excellent glass forming ability (GFA), hard magnetic properties at room temperature, and absence of glass transition temperature (Tg) in differential scanning calorimetry (DSC) traces [1–10]. The absence of glass transition in DSC traces makes it difficult to estimate the exact GFA of the alloys [2,11–13]. Inoue et al. investigated the GFA of Nd–Al–Fe ternary alloys and supposed that the glass transition temperature of Nd60Al10Fe30 BMG might be higher than the crystallization temperature (Tx) and thus the reduced glass transition temperature (Trg) was estimated to be higher than 0.9 [2]. Nevertheless, recent experimental results have shown that Nd–Al–(Fe,Co) BMGs were essentially partially crystalline materials [3,5,6] and the absence of glass transition in DSC traces could be due to the chemical inhomogeneity of the amorphous phase [7]. The primary crystallization (at about 460 K) obtained from the DSC trace of Nd60Al10Fe30 ribbon (30 m/s) suggests that the bulk samples are already partially crystallized [8], and as the case in Nd60Al10Fe20Co10 glass forming alloys [9], the exact glass transition temperature might * Corresponding author. E-mail address: [email protected] (L. Xia).
0966-9795/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2006.01.033
be lower than 460 K. Therefore, the reduced glass transition temperature of Nd60Al10Fe30 glass forming alloy might be much lower than the claimed value. However, up to now, as glass transition is difficult to be observed in the DSC traces of Nd–Al–Fe ternary BMGs, the GFA of the alloys have not been studied systematically. Recently we obtained glass transition using DSC from some as-cast Nd–Al–Fe ternary alloys. In this work, we reported ascast Nd60Al20Fe20 rod showing typical amorphous characters in X-ray diffraction (XRD) pattern and distinct glass transition in DSC traces. The glass transition allowed us to investigate the GFA of the alloy using Trg (TrgZTgjTl, where Tl is the liquidus temperature) [11] and the recently defined parameter g (gZTx/ (TgCTl)) [12,13]. However, it is found that the obtained diameter of the rod is much larger than the critical section thickness the theory would predict. Thus, we decide to investigate the GFA of the alloy as well as the microstructure and the magnetic properties of the as-cast rod in more detail. Pre-alloys with nominal compositions of Nd60Al20Fe20 and Nd60Al10Fe30 were prepared separately by arc-melting of 99.9% (at.%) pure Nd, Al and Fe in titanium–gettered argon atmosphere. Cylinders of the alloys were prepared in the shape of 3 mm in diameter by suction casting under pure argon atmosphere. The structure of the samples was characterized by XRD on a Philips diffractometer using Cu Ka radiation. DSC measurements were carried out under a purified argon
L. Xia et al. / Intermetallics 14 (2006) 1098–1101
atmosphere in a Perkin Elmer DSC-7 at a heating rate of 20 K/min. High-temperature DSC (HTDSC) curve was measured in a TA INSTRUMENT SDT-Q600 DSC under argon atmosphere in order to obtain the melting and liquidus temperature of the alloys. A vibrating sample magnetometer (VSM) was used for the magnetic measurements of the as-cast rods. The field applied was 1432 kA/m. The microstructure of the as-cast rod and the rough composition of nano-particles were observed on a JEOL JEM-2010 F high-resolution electron microscope (HREM) with an Oxford INCA energy dispersive spectrometer (EDS). Specimens for HREM observation were thinned under pure argon atmosphere on a rotating plate cooled by liquid nitrogen. The XRD pattern of the as-cast Nd60Al20Fe20 rod with a 3 mm in diameter is shown in Fig. 1. The typical broad diffraction maxima of amorphous phases for the rod indicate the glassy characters of the alloy, no obvious crystalline peaks were observed within the XRD detection limit. DSC traces obtained from as-cast Nd60Al20Fe20 and Nd60Al10Fe30 rods at a heating rate of 20 K/min are shown in Fig. 2. Whereas there is no glass transition in the DSC trace of Nd60Al10Fe30, Nd60Al20Fe20 exhibits clearly an endothermic glass transition followed by three exothermic crystallization peaks. Primary crystallization has been found in the DSC traces of amorphous Nd60Al10Fe30 (30 m/s) and Nd60Al10Fe20Co10 as-spun ribbons but is invisible in their bulk counterparts [8,9]. The onset temperatures of the glass transition (Tg) and crystallizations (Tx1, Tx2 and Tx3) of Nd60Al20Fe20 as-cast rod are about 468, 509, 620 and 762 K. The melting and liquidus temperatures (Tm and Tl) of Nd60Al20Fe20 alloy obtained from the HTDSC curve (not shown in present paper) are about 865 and 953 K. Thus, the supercooled liquid region (DTZTx1KTg) and the reduced glass transition temperature (TrgZTg/Tl) of the alloy are about 41 and 0.49 K, respectively. Turnbull has calculated that liquids with TrgZ0.5 (larger than that of Nd60Al20Fe20 as-cast rod) could be chilled into glassy state only in droplet with diameter less than 60 mm at the cooling rate of K106 K sK1 [11], this is obviously not in accordance with the apparent GFA of Nd60Al20Fe20 alloy. To investigate the GFA of Nd60Al20Fe20 glass-forming alloy in detail, parameter (gZTx/(TgCTl)) [12,13] is employed for the evaluation of GFA. The parameter has been successfully
Fig. 1. XRD pattern of Nd60Al20Fe20 as-cast rod.
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Fig. 2. DSC curves of Nd60Al20Fe20 and Nd60Al10Fe30 as-cast rods at a heating rate of 20 K/min.
demonstrated in various glass forming systems. The value of g calculated for Nd60Al20Fe20 BMG is about 0.36, and the critical cooling rate (RcZC1 exp[(K1n C1/g0)g], where C1 and g0 are constants) and section thickness (ZcZ2.80!10K7exp(41.70g) of the BMG can be predicted accordingly [12,13] to be about 160 K sK1 and 1 mm, respectively. The predicted critical thickness is also not in accordance with the obtained diameter of the rod. Therefore, it seems that both Trg and g are not suitable for the evaluation of GFA of Nd60Al20Fe20 BMG. It should be noted that the Nd60Al20Fe20 as-cast rod is hard magnetic. Fig. 3 shows the hysteresis loops of Nd60Al20Fe20 and Nd60Al10Fe30 as cast rods. The lower value of magnetization of Nd60Al20Fe20 as-cast rod under the field of 1432 kA/m is due to the lower concentration of Fe element. The coercivity of Nd60Al20Fe20 bulk sample is about 160 kA/m and much lower than that of Nd60Al10Fe30 as cast rod (about 230 kA/m). Recent experimental results in Nd–Al–Fe and Nd–Al–Fe–Co hard magnetic BMGs have revealed that these hard magnetic BMGs usually have the microstructure of nano-scaled particles dispersed uniformly in the amorphous matrix and the high coercivity of the rods is closely related to the existence of these nano-particles [3,5,6,9]. Therefore, the hard magnetic
Fig. 3. Hysteresis loops of Nd60Al20Fe20 and Nd60Al10Fe30 as-cast rods.
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L. Xia et al. / Intermetallics 14 (2006) 1098–1101
properties of Nd60Al20Fe20 as-cast rod suggest that the rod could be also partially crystalline and the XRD pattern of the rod indicate that the possibly existed crystalline particles could be too small to be reflected in XRD patterns. Furthermore, as the coercivity of Nd–Al–Fe(Co) as-cast rods is dominated by the size and distribution of the particles [9,14], the lower coercivity of Nd60Al20Fe20 rod suggests that the particles could be smaller or fewer in Nd60Al20Fe20 than in Nd60Al10Fe30 ascast rods. The microstructure of Nd60Al20Fe20 as-cast rod has demonstrated the suggestions. Some isolated particles were found distributed randomly in the amorphous matrix in as-cast Nd60Al20Fe20 rod, as shown in Fig. 4. The interplanar distance of the particles is about 0.275 nm. It has been reported that the interplanar distance of the nano-particles in as-cast Nd60Al10Fe20Co10 is about 0.273 nm [5,6,9], and in the as cast Nd60Al10Fe30 it is about 0.32 nm [3]. They are roughly correspondent to (002) dhkl (Z0.275 nm) and (111) dhkl (Z 0.32 nm) of the metastable Ax phase with fcc structure of aZ 0.55 nm in Nd-riched Nd–Fe alloys [15]. On the other hand, the average composition of the particles detected by EDS is about Nd79Fe21. Thus, the nano-crystalline particles in Nd60Al20Fe20 as-cast rod could be near-eutectic or pseudo-eutectic metastable Ax phase [15]. The nano-crystalline region of the particles with the same structure of Ax phase, which is supposed to be the same as the well known highly anisotropic A1 phase, could be the origin of high coercivity in Nd–Al– Fe(Co) nano-glasses [15]. The formation of nano-crystallites in Nd–Al–Fe(Co) alloy system has evoked increasing research interests because classical nucleation from a homogeneous undercooled liquid state should result in a rather coarse microstructure upon crystallization [3,16]. It is argued that the typical morphology of fine particles dispersed within the amorphous matrix in Nd–Al–Fe and Nd–Al–Fe–Co hard magnetic BMGs is induced by the phase separation of the melt or the slightly undercooled
Fig. 4. HREM image of Nd60Al20Fe20 as-cast rod.
liquids into two metastable phases [3]. Frankwicz et al. have revealed that the precipitation of intermediate phase (metastable I-phase) balanced the dynamic equilibrium between the reordered and disordered configuration of Zr65Al7.5Ni10Cu17.5 water-quenched cylinders and led to the formation of refined microstructure over thermally stabilized nanocrystallites [17]. Supposed that one of the separated phases has higher forming enthalpy, the formation of this phase will lower the forming enthalpy of the remainders and thus stabilize the others. We have obtained the composition of nano-scaled particles in Nd60Al10Fe20Co10 as-cast rod to be about Nd69Fe21Co10 [6,9]. Based on the Miedema’s model [18–20], we calculated the forming enthalpies of Nd69Fe21Co10 and Nd79Fe21 phases and observed that they are all small positive. The precipitation of the phases with small positive value of forming enthalpy as Nd69Fe21Co10 and Nd79Fe21 could stabilize the amorphous matrix and thus the apparent glass forming ability of the alloys could be enlarged by the phase separation in Nd–Al–Fe(Co) glass forming alloys. In addition, as the results obtained in Nd– Fe rapidly quenched alloys [20], the small positive value of forming enthalpy of the separated phases could reduce the formation of stable intermetallic compounds and therefore, induce the formation of metastable phases as the nano-scaled particles in Nd–Al–Fe(Co) as-cast rods under favorable thermal conditions [6]. On the other hand, the composition of the remaining amorphous matrix is different from the nominal one because of the presence of nano-crystalline particles with different composition. Thus, the critical section thickness predicted by either Trg or parameter g resulting from Tg and Tx of the remaining amorphous matrix could not reflect the real GFA of the alloy. As the glass forming enthalpy of Nd60Al20Fe20 alloy is higher than that of the amorphous matrix, Zc of Nd60Al20Fe20 alloy could be lower than the predicted value, in a nutshell, less than 1 mm. In conclusion, we studied the GFA, magnetic properties, and microstructure of as-cast Nd60Al20Fe20 rod. The material exhibited distinct glass transition in DSC traces and typical amorphous features in XRD patterns. Trg and g of the as-cast Nd60Al20Fe20 rod obtained from the DSC measurements were about 0.49 and 0.36, respectively. The critical section thickness of the BMG predicted by either Trg (less than 60 mm) or g (about 1 mm) is much lower than the diameter of experimentally obtained glassy rod (about 3 mm). Structural observation showed that isolated nanocrystalline particles, which have a structure and composition similar to the near-eutectic or pseudo-eutectic metastable Ax phase, embedded randomly in the amorphous matrix. The nano-crystalline region was expected to be the origin of high coercivity in Nd–Al–Fe(Co) nano-glasses due to the highly anisotropic structure of these nano-particles. We believe the apparent GFA of Nd60Al20Fe20 BMG could be greatly enhanced by the precipitation of metastable nanoparticles resulting from phase separation in Nd–Al–Fe alloys, although the small positive forming enthalpy of the precipitates and the real Zc of the alloy could be less than 1 mm predicted by parameter g.
L. Xia et al. / Intermetallics 14 (2006) 1098–1101
Acknowledgements The authors are grateful for the financial supports of the National Nature Science Foundation of China (Grant No. 50471099) and the Development Foundation of Shanghai Educational Commission (No. 04-58-04AB08). References [1] Inoue A, Zhang T, Zhang W, Takeuchi A. Mater Trans JIM 1996;37:99. [2] Inoue A, Takeuchi A, Zhang T. Metall Mater Trans A 1998;29:1779. [3] Schneider S, Bracchi A, Samwer K, Seibt M, Thiyagarajan P. Appl Phys Lett 2002;80:1749. [4] Fan GJ, Lo¨ser W, Roth S, Eckert J, Schultz L. J Mater Res 2000;15:1556. [5] Xia L, Wei BC, Zhang Z, Pan MX, Wang WH, Dong YD. J Phys D 2003; 36:775. [6] Xia L, Wei BC, Pan MX, Zhao DQ, Wang WH, Dong YD. J Phys Condens Matter 2003;15:3531. [7] Wei BC, Lo¨ser W, Xia L, Roth S, Pan MX, Wang WH, et al. Acta Mater 2002;50:4357.
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