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Characterisation of Sm–Co–Sn alloys
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Characterisation of Sm–Co–Sn alloys Hamid Zaigham⁎, F. Ahmad Khalid Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, KPK, Pakistan
AR TIC LE D ATA
ABSTR ACT
Article history:
Cast SmCo5−xSnx (x = 0.00, 0.01, 0.06 and 0.12) alloys were prepared by arc melting with
Received 30 June 2010
subsequent thermal homogenisation. Microstructural and x-ray diffraction studies revealed
Received in revised form
that the alloys were constituted of three phases i.e. SmCo5, Sm2Co7 and Sm2Co17. The
5 September 2010
addition of Sn caused grain refinement of the alloys. It was found that the Sn addition
Accepted 7 September 2010
promoted nucleation of Sm2Co17, segregation of solute atoms and increase in unit cell volume; consequently, significant augmentation in remanence to maximum magnetization ratios was achieved.
Keywords:
© 2010 Elsevier Inc. All rights reserved.
Sm–Co–Sn alloys Microstructure XRD Grain refinement Solute segregation Remanence
1.
Introduction
The formation of Sm(Co, M)5–7 alloys (where M is Cu, Zr, Si, Ti, Ag, Fe, Hf or Cr) with the structural and magnetic properties have been extensively investigated by many researchers so far. The significance of the alloys includes high temperature applications and improved magnetic characteristics [1–7]. However, very few researchers have reported their work on Sm–Co–Sn alloy system. Sn addition in SmCo5 alloy was first investigated in as cast alloys by Washko et al. [8]. Washko first prepared the alloy by casting, than he crushed it to a certain particle size and finally aligned the powder in magnetic field. The resulting magnetic properties were reported inferior to that of as cast binary SmCo5 alloy. The effect of Sn addition in melt spun ribbons of hyper-stoichiometric Sm–Co alloy was investigated by Kundig et al. [9]. The magnetic properties were reported to be influenced by Sn contents i.e. increasing Sn contents resulted in higher magnetization and lower coercivity. According to Kundig, Sn caused smoothening of the grain boundaries; resulting in reduced reverse domain nucleation. The formation
of separated particles of Sn rich phases at grain boundaries and within grains was reported as pinning sites and to be responsible for coercivity increase at lower Sn concentrations. Recently, Romaka et al. reported their work on phase equilibrium of Sm–Co–Sn ternary system at higher temperatures. They had also proposed ternary phase diagrams of the system for higher Sn concentrations (>50% Sn) [10]. In the present study the effect of low Sn contents on microstructures, phases and magnetic properties has been investigated.
2.
Experimental
High purity elemental (99.999%) Sm, Co and Sn were used for making the alloys. Co and Sm were cut to grit size of 5 mm. In order to remove oxidation layer, the alloying elements were treated in 5% nitric acid solution followed by ultrasonic cleaning in ethanol. The Sm rich [SmCo5−xSnx (x = 0.00, 0.01, 0.06 and 0.12)] ternary alloys were prepared in a DC arc button furnace under
⁎ Corresponding author. Tel.: +92 938 271863; fax: +92 938 271865. E-mail address: [email protected] (H. Zaigham). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.09.002
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Table 1 – Results of elemental analyses with point and bulk stoichiometric ratios. Specimen
Alloy-1
Alloy-2
Alloy-3
Alloy-4
Region of analysis Matrix Grain Grain boundary Matrix Grain Grain boundary Matrix Grain Grain boundary Matrix Grain Grain boundary
Stoichiometric ratio Point Analyses Bulk analyses SmCo5.2 SmCo8.5 SmCo3.5 SmCo5.2Sn0.01 SmCo8.8Sn0.05 SmCo3.6 Sn0.06 SmCo4.7Sn0.06 SmCo8.6Sn0.03 SmCo4.4Sn0.06 SmCo4.8Sn0.12 SmCo8.2Sn0.02 SmCo3.6Sn0.3
SmCo4.8
SmCo4.6Sn0.01
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cutting, the samples were mounted in cold moulding resin and prepared to final polished optical microscopic specimens. Further, the specimens were electrolytically etched in 10% NaOH solution with a stainless steel electrode. After the preparation, all the specimens were subjected to optical microscopy, scanning electron microscopy (SEM), EDS analyses, and x-ray diffraction (XRD) measurements. The magnetic properties of the alloys were measured at room temperature.
SmCo4.6Sn0.06
3. SmCo4.8Sn0.12
flowing argon along with activated calcium placed in the closed vicinity of the melt cavity. Calcium acted as gettering element to accommodate the losses of Sm due to the dissolved oxygen in Sm and argon as an impurity. Several re-melting were carried out to ensure compositional homogeneity. Finally, the alloys were cast into 10 mm diameter cylindrical copper moulds. The samples were sealed under vacuum in quartz tubes accompanying elemental Sm as gettering charge. Afterwards, the samples were homogenized at 1100 °C for 12 h. These samples were subjected to energy dispersive spectroscopic (EDS) analyses to assure the final compositions (Table 1). For optical microscopic studies, the cross sectional samples were cut using slow speed diamond cut-off wheel. After
Results and Discussion
Optical microscopy of various alloys was carried out using an Olympus optical microscope. All the specimens were studied in etched condition. Fig. 1 represents typical dendritic structure for as cast Sm–Co alloy without addition of Sn (Alloy-1) having an inter-dendritic spacing of 50 μm. The specimen was studied using differential interference contrast mode. The microstructure contained discernable features of uniformly distributed grey dendrites in a bright contrast matrix. It was observed that the dendritic grain boundaries were isolated by hyper-stoichiometric compositions containing Sm2Co7 phase. Whereas, the dendrites (grey areas) were hypo-stoichiometric compositions containing Sm2Co17 phase. The matrix represented near-stoichiometric composition of SmCo5. These analyses were persistent with previously quoted observations for unalloyed Sm–Co as cast specimens [11]. The inset in Fig. 1 shows the high resolution image of the microstructure elaborating the distribution of various phases. Radical changes in microstructural features were observed even after a minute addition of Sn (0.01%) in the alloy. Fig. 2
Fig. 1 – Optical micrographs of as cast Sm–Co alloy without addition of Sn (Alloy-1) in etched condition; a) isolating grain boundary Sm2Co7 phase, b) hypo-stoichiometric Sm2Co17 phase dendrites and c) Sm Co5 matrix.
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Fig. 2 – Optical micrographs of as cast Sm–Co alloy with 0.01% Sn addition (Alloy-2); a) nucleating Sm2Co7 phase at grain boundaries, b) isolated growth of hyper-stoichiometric Sm2Co17 phase surrounded by Sm2Co7 grain boundaries and c) Sm Co5 matrix. Sm2Co17 phase is also nucleated in matrix without having Sm2Co7 on grain boundaries (rectangle marked in high resolution inset micrograph).
Fig. 3 – Optical micrographs of as cast Sm–Co alloy with 0.06% Sn addition (Alloy-3); a) Sm2Co7, b) Sm2Co17 phase and c) SmCo5.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
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Fig. 4 – Optical micrographs of as cast Sm–Co alloy with 0.12% Sn addition (Alloy-4); a) Sm2Co7, b) Sm2Co17 phase and c) Sm Co5.
Fig. 5 – SEM images of as cast Sm–Co alloy with and without Sn addition (Alloy-1 to -4); a) Sm2Co7, b) Sm2Co17 and c) SmCo5.
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Fig. 6 – XRD scans of as cast Sm–Co alloys with and without Sn addition (Alloy-1 to -4); a, b and c are the standard JCPDS-ICDD cards of SmCo5, Sm2Co7, and Sm2Co17 phases, respectively. Peaks of each respective card are marked on the individual XRD scans of the alloys.
represents optical micrographs of as cast Sm–Co alloy with 0.01% Sn addition (Alloy-2). The inter-dendritic spacing was reduced to 20 μm with very fine distribution of hyper- and hypo-stoichiometric phases in near-stoichiometric matrix. Fine growth of dendrites was accompanied by nucleation of hypo-stoichiometric phase (Sm2Co17) within the matrix. It was also observed that hyper-stoichiometric phase (Sm2Co7) was grown to small islands on the dendritic grain boundaries. The nucleation and growth of hyper- and hypo-stoichiometric phases was increased with further increase in Sn contents in the alloy. Hypo-stoichiometric phase (Sm2Co17) nucleated at initial stages of the Sn alloying and as Sn contents increased to
0.12%, it almost replaced 70% of the matrix of near-stoichiometric phase (see Figs. 3 and 4). In the succession of optical microscopic studies, SEM based EDS analyses were performed on the specimens. Selected area and point analyses techniques were used to quantify the constituents of various phases. Fig. 5 shows the four SEM images at the same magnification. It is quite evident in the images that the microstructural details became finer with increasing Sn contents, which is consistent with optical microscopic observations. Detailed EDS analysis results are given in Table 1, which shows that the addition of Sn caused Co and Sm replacement in the matrix and the grain
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Table 2 – Lattice parameters, c/a ratio, unit cell volume (Vo), and maximum magnetization of the alloys as a function of Sn content. Specimen
Snx
a (°A)
c (°A)
c/a
Vo (°A3)
Mmax (T)
Alloy-1 Alloy-2 Alloy-3 Alloy-4
0 0.01 0.06 0.12
4.8684 ± 0.0003 5.0136 ± 0.0006 5.0281 ± 0.0004 4.9413 ± 0.0006
4.0764 ± 0.0001 3.9739 ± 0.0003 4.0984 ± 0.0002 3.9705 ± 0.0002
0.8373 0.7926 0.8151 0.8035
83.67 86.50 89.73 83.95
0.351 0.504 0.513 0.439
boundaries, respectively. While a shifting behaviour of Sm to Co substitution was observed in the grains with increasing Sn concentration. A Siemens diffractometer with CoKα radiation source was used to analyse the specimens. In Fig. 6, four XRD graphs are superimposed on each other with three standard reference XRD spectra of the phases identified during microscopic studies. The standard card numbers are JCPDS-ICDD Card Nos: 35-1400, 71-0387, and 35-1368 for SmCo5, Sm2Co7 and Sm2Co17 with cobalt Kα radiation source, respectively. During XRD studies it was observed that relative quantity of hypostoichiometric phase (Sm2Co17) was increased with the increase of Sn contents in the alloys. Moreover, it was also observed that the addition of Sn in the alloy caused a preferential growth of hypo-stoichiometric and near-stiochiometric phases. This effect of preferential growth was dominated for hypo-stoichiometric phase than near-stoichiometric phase in such a way that it became maximal at 0.12% Sn contents. A similar kind of trend was also reported by Washko et al. [8]. Additionally, lattice parameters, c/a ratio and cell volume were also calculated from the individual XRD spectrum. The lattice parameters “a” and “c” were determined by using Eqs. (1) and (2) [12]: a=
1 λ h2 + k2 + hk =2 3O sinθ
ð1Þ
c=
lλ 2 sin θ
ð2Þ
The results are given in Table 2.
A Riken Denshi electro-magnetic hysteresis graph (EMH) was used to measure initial magnetization of the specimens in a field of 1 T (10 kOe). To determine maximum magnetization (Mmax) and coercivity (Hci) a “Metis pulsed field magnetometer” was used at 5 T (50 kOe) field. All the magnetic measurements were made at room temperature. The initial magnetization curves for the specimens are shown in Fig. 7. All the specimens showed an abrupt increase in magnetization at low applied magnetic fields, a typical characteristic of the nucleation type magnetization mechanism [13]. It indicated that the Sn additions did not affect the typical nucleation behaviour of the alloy. The calculated results of magnetic properties are given in Table 3. There was a sharp increase in Mr/Mmax ratio even after minor addition of Sn. Such type of comportment also indicates grain refinement and reduction in energy loss. Similarly maximum magnetization (Mmax) showed increasing trend with Sn addition until 0.06% Sn. Further increase in Sn contents resulted in a certain decrease in Mmax. A twofold increase in remanence (Mr) was observed after the addition of Sn in the alloys. It also resulted in increase in intrinsic coercivity, see Fig. 8. The effect of lattice parameters, c/a ratio, unit cell volume (Vo) were also compared with achieved maximum magnetization (Mmax) at room temperature (Table 2). The Vo first increased with Sn addition and then reduced slightly. The increase in Vo up to 0.06% Sn indicates the insertion of Sn atom into the Sm–Co lattice and resulted in the enhancement in Mmax. In specimens with 0.12% Sn the insertion of Sn atom in the lattice was smaller than the predecessor alloys. The reduced Sn solubility in Sm–Co was also supported by reduction in Vo and the results of EDS point analyses (see Table 1). The point analyses revealed the fact that in Alloy-4, Sn was precipitated on the grain boundaries. Moreover, the Sn addition resulted in reduction of the grain size in comparison to the binary SmCo5 alloy. The addition of Sn caused the segregation of solute atoms when its concentration was increased from certain limit (i.e. > 0.06%), which consequently, resulted in further grain refinement. Similar kinds of observations were quoted by some other researchers as well [9,14]. The retardation of
Table 3 – Magnetic properties of various alloys.
Fig. 7 – Initial magnetization curves of various Sm–Co–Sn alloys.
Sample
Snx
Mmax (T)
Mr (T)
Mr/Mmax
Hci (kOe)
Alloy-1 Alloy-2 Alloy-3 Alloy-4
0 0.01 0.06 0.12
0.351 0.504 0.513 0.439
0.123 0.267 0.248 0.244
0.35 0.53 0.48 0.55
0.601 0.946 0.787 0.911
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
REFERENCES
Fig. 8 – The demagnetization curves of the alloys indicating the effect of Sn contents on the Mr and Hci.
grain growth in the crystalline structures may be attributed to the reduced mobility of grain boundaries due to solute drag and eventually resulting in stagnation and/or stabilization of the grain growth process [15–20].
4.
Conclusions
During investigations (i.e. optical microscopy, SEM and XRD) it was found that Alloy-1 was dominatingly two phase (nearand hypo-stoichiometric) material with slight presence of hyper-stoichiometric phase. As Sn was added in Sm–Co, discernable changes in the microstructure and phase distribution were observed; • Increase in Sn concentration caused nucleation of hypostoichiometric phase • The presence of hyper-stoichiometric phase persisted in the alloys despite the nucleation of hypo-stoichiometric phase • Addition of Sn resulted in the refinement of the microstructure • The addition of Sn (> 0.06%) caused its segregation on the grain boundaries. Moreover, addition of Sn caused augmentation in magnetic properties of the Sm–Co alloys until 0.06% Sn, and any further increase in Sn caused decrease in magnetic characteristics.
Acknowledgement Authors are thankful to Asst. Prof. Dr. A. Hakeem of GIK Institute for his valuable suggestions.
[1] Tellez-Blanco JC, Grossinger R, Sato R, Turtelli R. Structure and magnetic properties of SmCo5−xCux alloys. J Alloys Compd 1998;281:1–5. [2] Kerschl P, Handstein A, Khlopkov K, Gutfleisch O, Eckert D, Nenkov K, et al. High magnetic field magnetization of SmCo5 −xCux (x = 2.5) determined in pulse field up to 48 T. J Magn Magn Mater 2005;290–291:420–3. [3] Weng-yong Zhang, Zhang Xiao-dong, Yang Ying-chang, Shen Bao-gen. Effect of Cu substitution on structure and magnetic properties of anisotropic SmCo ribbons. J Alloys Compd 2003;353:274–7. [4] Gjoka M, Panagiotopoulos I, Niarchos D. Structure and magnetic properties of Sm (Co1−x Mx)5 (M = Cu, Ag) alloys. J Mater Process Technol 2005;161:173–5. [5] Liu T, Li W, li XM, Feng WC, Guo YQ. Crystal structure and magnetic properties of SmCo7−xAgx. J Magn Magn Mater 2007;310:e632–4. [6] Yao Z, Jiang CB. Structure and magnetic properties of SmCoxTi0.4-1:7 ribbons. J Magn Magn Mater 2008;320:1073–7. [7] Guo YQ, Li W, Luo J, Feng WC, Liang JK. Structure and magnetic characteristics of novel SmCo-based hard magnetic alloys. J Magn Magn Mater 2006;303:e367–70. [8] Washko S, Gerboc J, Orehotsky J. Magnetic and crystallographic properties of SmCo5 ternary alloys. IEEE Trans Magn 1976(6):974–6 Vol. MAG-12. [9] Kundig AA, Goplan R, Ohkubo T, Hono K. Coercivity enhancement in melt-spun SmCo5 by Sn addition. Scr Mater 2006;54:2047–51. [10] Romaka L, Romaka VV, Konyk M, Melnychenko-Koblyuk N. Phase equilibria in Sm–Co–Sn ternary system at 870 K and 770 K. J Chem Met Alloys 2008;1:198–203. [11] ASM Metals Handbook, metallography and microstructures, Vol-9, Metals Park, Ohio-44073, p-548. [12] Suryanerayana C, Norton MG. X-ray diffraction: a practical approach. New York: Plenum Publishing Corporation; 1998. [13] Skomski R, Coey JMD. Permanent magnetism. Bristol: IOP Publishing; 1999. p. 174. [14] Li Liya, Yi Jianhong, Peng Yundong, Huang Baiyun. The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeB magnets. J Magn Magn Mater 2007;308: 80–4. [15] Li Junjie, Wang Jincheng, Yang Gencang. On the stagnation of grain growth in nanocrystalline materials. Scr Mater 2009;60: 945–8. [16] Millett Paul C, Panneer Selvam R, Saxena Ashok. Stabilizing nanocrystalline materials with dopants. Acta Mater 2007;55: 2329–36. [17] Liu Feng, Kirchheim Reiner. Grain boundary saturation and grain growth. Scr Mater 2004;51:521–5. [18] Kirchheim Reiner. Grain coarsening inhibited by solute segregation. Acta Mater 2002;50:413–9. [19] Natter H, Loffler MS, Krill CE, Hempelmann R. Crystallite growth of nanocrystalline transition metals studied in situ by high temperature synchrotron x-ray diffraction. Scr Mater 2001;44:2321–5. [20] Darling Kris A, Chan Ryan N, Wong Patrick Z, Semones Jonathan E, Scattergood Ronald O, Koch Carl C. Grain size stabilization in nanocrystalline FeZr alloys. Scr Mater 2008;59: 530–3.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Characterisation of Sm–Co–Sn alloys Hamid Zaigham⁎, F. Ahmad Khalid Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, KPK, Pakistan
AR TIC LE D ATA
ABSTR ACT
Article history:
Cast SmCo5−xSnx (x = 0.00, 0.01, 0.06 and 0.12) alloys were prepared by arc melting with
Received 30 June 2010
subsequent thermal homogenisation. Microstructural and x-ray diffraction studies revealed
Received in revised form
that the alloys were constituted of three phases i.e. SmCo5, Sm2Co7 and Sm2Co17. The
5 September 2010
addition of Sn caused grain refinement of the alloys. It was found that the Sn addition
Accepted 7 September 2010
promoted nucleation of Sm2Co17, segregation of solute atoms and increase in unit cell volume; consequently, significant augmentation in remanence to maximum magnetization ratios was achieved.
Keywords:
© 2010 Elsevier Inc. All rights reserved.
Sm–Co–Sn alloys Microstructure XRD Grain refinement Solute segregation Remanence
1.
Introduction
The formation of Sm(Co, M)5–7 alloys (where M is Cu, Zr, Si, Ti, Ag, Fe, Hf or Cr) with the structural and magnetic properties have been extensively investigated by many researchers so far. The significance of the alloys includes high temperature applications and improved magnetic characteristics [1–7]. However, very few researchers have reported their work on Sm–Co–Sn alloy system. Sn addition in SmCo5 alloy was first investigated in as cast alloys by Washko et al. [8]. Washko first prepared the alloy by casting, than he crushed it to a certain particle size and finally aligned the powder in magnetic field. The resulting magnetic properties were reported inferior to that of as cast binary SmCo5 alloy. The effect of Sn addition in melt spun ribbons of hyper-stoichiometric Sm–Co alloy was investigated by Kundig et al. [9]. The magnetic properties were reported to be influenced by Sn contents i.e. increasing Sn contents resulted in higher magnetization and lower coercivity. According to Kundig, Sn caused smoothening of the grain boundaries; resulting in reduced reverse domain nucleation. The formation
of separated particles of Sn rich phases at grain boundaries and within grains was reported as pinning sites and to be responsible for coercivity increase at lower Sn concentrations. Recently, Romaka et al. reported their work on phase equilibrium of Sm–Co–Sn ternary system at higher temperatures. They had also proposed ternary phase diagrams of the system for higher Sn concentrations (>50% Sn) [10]. In the present study the effect of low Sn contents on microstructures, phases and magnetic properties has been investigated.
2.
Experimental
High purity elemental (99.999%) Sm, Co and Sn were used for making the alloys. Co and Sm were cut to grit size of 5 mm. In order to remove oxidation layer, the alloying elements were treated in 5% nitric acid solution followed by ultrasonic cleaning in ethanol. The Sm rich [SmCo5−xSnx (x = 0.00, 0.01, 0.06 and 0.12)] ternary alloys were prepared in a DC arc button furnace under
⁎ Corresponding author. Tel.: +92 938 271863; fax: +92 938 271865. E-mail address: [email protected] (H. Zaigham). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.09.002
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Table 1 – Results of elemental analyses with point and bulk stoichiometric ratios. Specimen
Alloy-1
Alloy-2
Alloy-3
Alloy-4
Region of analysis Matrix Grain Grain boundary Matrix Grain Grain boundary Matrix Grain Grain boundary Matrix Grain Grain boundary
Stoichiometric ratio Point Analyses Bulk analyses SmCo5.2 SmCo8.5 SmCo3.5 SmCo5.2Sn0.01 SmCo8.8Sn0.05 SmCo3.6 Sn0.06 SmCo4.7Sn0.06 SmCo8.6Sn0.03 SmCo4.4Sn0.06 SmCo4.8Sn0.12 SmCo8.2Sn0.02 SmCo3.6Sn0.3
SmCo4.8
SmCo4.6Sn0.01
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cutting, the samples were mounted in cold moulding resin and prepared to final polished optical microscopic specimens. Further, the specimens were electrolytically etched in 10% NaOH solution with a stainless steel electrode. After the preparation, all the specimens were subjected to optical microscopy, scanning electron microscopy (SEM), EDS analyses, and x-ray diffraction (XRD) measurements. The magnetic properties of the alloys were measured at room temperature.
SmCo4.6Sn0.06
3. SmCo4.8Sn0.12
flowing argon along with activated calcium placed in the closed vicinity of the melt cavity. Calcium acted as gettering element to accommodate the losses of Sm due to the dissolved oxygen in Sm and argon as an impurity. Several re-melting were carried out to ensure compositional homogeneity. Finally, the alloys were cast into 10 mm diameter cylindrical copper moulds. The samples were sealed under vacuum in quartz tubes accompanying elemental Sm as gettering charge. Afterwards, the samples were homogenized at 1100 °C for 12 h. These samples were subjected to energy dispersive spectroscopic (EDS) analyses to assure the final compositions (Table 1). For optical microscopic studies, the cross sectional samples were cut using slow speed diamond cut-off wheel. After
Results and Discussion
Optical microscopy of various alloys was carried out using an Olympus optical microscope. All the specimens were studied in etched condition. Fig. 1 represents typical dendritic structure for as cast Sm–Co alloy without addition of Sn (Alloy-1) having an inter-dendritic spacing of 50 μm. The specimen was studied using differential interference contrast mode. The microstructure contained discernable features of uniformly distributed grey dendrites in a bright contrast matrix. It was observed that the dendritic grain boundaries were isolated by hyper-stoichiometric compositions containing Sm2Co7 phase. Whereas, the dendrites (grey areas) were hypo-stoichiometric compositions containing Sm2Co17 phase. The matrix represented near-stoichiometric composition of SmCo5. These analyses were persistent with previously quoted observations for unalloyed Sm–Co as cast specimens [11]. The inset in Fig. 1 shows the high resolution image of the microstructure elaborating the distribution of various phases. Radical changes in microstructural features were observed even after a minute addition of Sn (0.01%) in the alloy. Fig. 2
Fig. 1 – Optical micrographs of as cast Sm–Co alloy without addition of Sn (Alloy-1) in etched condition; a) isolating grain boundary Sm2Co7 phase, b) hypo-stoichiometric Sm2Co17 phase dendrites and c) Sm Co5 matrix.
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Fig. 2 – Optical micrographs of as cast Sm–Co alloy with 0.01% Sn addition (Alloy-2); a) nucleating Sm2Co7 phase at grain boundaries, b) isolated growth of hyper-stoichiometric Sm2Co17 phase surrounded by Sm2Co7 grain boundaries and c) Sm Co5 matrix. Sm2Co17 phase is also nucleated in matrix without having Sm2Co7 on grain boundaries (rectangle marked in high resolution inset micrograph).
Fig. 3 – Optical micrographs of as cast Sm–Co alloy with 0.06% Sn addition (Alloy-3); a) Sm2Co7, b) Sm2Co17 phase and c) SmCo5.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
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Fig. 4 – Optical micrographs of as cast Sm–Co alloy with 0.12% Sn addition (Alloy-4); a) Sm2Co7, b) Sm2Co17 phase and c) Sm Co5.
Fig. 5 – SEM images of as cast Sm–Co alloy with and without Sn addition (Alloy-1 to -4); a) Sm2Co7, b) Sm2Co17 and c) SmCo5.
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Fig. 6 – XRD scans of as cast Sm–Co alloys with and without Sn addition (Alloy-1 to -4); a, b and c are the standard JCPDS-ICDD cards of SmCo5, Sm2Co7, and Sm2Co17 phases, respectively. Peaks of each respective card are marked on the individual XRD scans of the alloys.
represents optical micrographs of as cast Sm–Co alloy with 0.01% Sn addition (Alloy-2). The inter-dendritic spacing was reduced to 20 μm with very fine distribution of hyper- and hypo-stoichiometric phases in near-stoichiometric matrix. Fine growth of dendrites was accompanied by nucleation of hypo-stoichiometric phase (Sm2Co17) within the matrix. It was also observed that hyper-stoichiometric phase (Sm2Co7) was grown to small islands on the dendritic grain boundaries. The nucleation and growth of hyper- and hypo-stoichiometric phases was increased with further increase in Sn contents in the alloy. Hypo-stoichiometric phase (Sm2Co17) nucleated at initial stages of the Sn alloying and as Sn contents increased to
0.12%, it almost replaced 70% of the matrix of near-stoichiometric phase (see Figs. 3 and 4). In the succession of optical microscopic studies, SEM based EDS analyses were performed on the specimens. Selected area and point analyses techniques were used to quantify the constituents of various phases. Fig. 5 shows the four SEM images at the same magnification. It is quite evident in the images that the microstructural details became finer with increasing Sn contents, which is consistent with optical microscopic observations. Detailed EDS analysis results are given in Table 1, which shows that the addition of Sn caused Co and Sm replacement in the matrix and the grain
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
Table 2 – Lattice parameters, c/a ratio, unit cell volume (Vo), and maximum magnetization of the alloys as a function of Sn content. Specimen
Snx
a (°A)
c (°A)
c/a
Vo (°A3)
Mmax (T)
Alloy-1 Alloy-2 Alloy-3 Alloy-4
0 0.01 0.06 0.12
4.8684 ± 0.0003 5.0136 ± 0.0006 5.0281 ± 0.0004 4.9413 ± 0.0006
4.0764 ± 0.0001 3.9739 ± 0.0003 4.0984 ± 0.0002 3.9705 ± 0.0002
0.8373 0.7926 0.8151 0.8035
83.67 86.50 89.73 83.95
0.351 0.504 0.513 0.439
boundaries, respectively. While a shifting behaviour of Sm to Co substitution was observed in the grains with increasing Sn concentration. A Siemens diffractometer with CoKα radiation source was used to analyse the specimens. In Fig. 6, four XRD graphs are superimposed on each other with three standard reference XRD spectra of the phases identified during microscopic studies. The standard card numbers are JCPDS-ICDD Card Nos: 35-1400, 71-0387, and 35-1368 for SmCo5, Sm2Co7 and Sm2Co17 with cobalt Kα radiation source, respectively. During XRD studies it was observed that relative quantity of hypostoichiometric phase (Sm2Co17) was increased with the increase of Sn contents in the alloys. Moreover, it was also observed that the addition of Sn in the alloy caused a preferential growth of hypo-stoichiometric and near-stiochiometric phases. This effect of preferential growth was dominated for hypo-stoichiometric phase than near-stoichiometric phase in such a way that it became maximal at 0.12% Sn contents. A similar kind of trend was also reported by Washko et al. [8]. Additionally, lattice parameters, c/a ratio and cell volume were also calculated from the individual XRD spectrum. The lattice parameters “a” and “c” were determined by using Eqs. (1) and (2) [12]: a=
1 λ h2 + k2 + hk =2 3O sinθ
ð1Þ
c=
lλ 2 sin θ
ð2Þ
The results are given in Table 2.
A Riken Denshi electro-magnetic hysteresis graph (EMH) was used to measure initial magnetization of the specimens in a field of 1 T (10 kOe). To determine maximum magnetization (Mmax) and coercivity (Hci) a “Metis pulsed field magnetometer” was used at 5 T (50 kOe) field. All the magnetic measurements were made at room temperature. The initial magnetization curves for the specimens are shown in Fig. 7. All the specimens showed an abrupt increase in magnetization at low applied magnetic fields, a typical characteristic of the nucleation type magnetization mechanism [13]. It indicated that the Sn additions did not affect the typical nucleation behaviour of the alloy. The calculated results of magnetic properties are given in Table 3. There was a sharp increase in Mr/Mmax ratio even after minor addition of Sn. Such type of comportment also indicates grain refinement and reduction in energy loss. Similarly maximum magnetization (Mmax) showed increasing trend with Sn addition until 0.06% Sn. Further increase in Sn contents resulted in a certain decrease in Mmax. A twofold increase in remanence (Mr) was observed after the addition of Sn in the alloys. It also resulted in increase in intrinsic coercivity, see Fig. 8. The effect of lattice parameters, c/a ratio, unit cell volume (Vo) were also compared with achieved maximum magnetization (Mmax) at room temperature (Table 2). The Vo first increased with Sn addition and then reduced slightly. The increase in Vo up to 0.06% Sn indicates the insertion of Sn atom into the Sm–Co lattice and resulted in the enhancement in Mmax. In specimens with 0.12% Sn the insertion of Sn atom in the lattice was smaller than the predecessor alloys. The reduced Sn solubility in Sm–Co was also supported by reduction in Vo and the results of EDS point analyses (see Table 1). The point analyses revealed the fact that in Alloy-4, Sn was precipitated on the grain boundaries. Moreover, the Sn addition resulted in reduction of the grain size in comparison to the binary SmCo5 alloy. The addition of Sn caused the segregation of solute atoms when its concentration was increased from certain limit (i.e. > 0.06%), which consequently, resulted in further grain refinement. Similar kinds of observations were quoted by some other researchers as well [9,14]. The retardation of
Table 3 – Magnetic properties of various alloys.
Fig. 7 – Initial magnetization curves of various Sm–Co–Sn alloys.
Sample
Snx
Mmax (T)
Mr (T)
Mr/Mmax
Hci (kOe)
Alloy-1 Alloy-2 Alloy-3 Alloy-4
0 0.01 0.06 0.12
0.351 0.504 0.513 0.439
0.123 0.267 0.248 0.244
0.35 0.53 0.48 0.55
0.601 0.946 0.787 0.911
1280
M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 7 4–1 2 8 0
REFERENCES
Fig. 8 – The demagnetization curves of the alloys indicating the effect of Sn contents on the Mr and Hci.
grain growth in the crystalline structures may be attributed to the reduced mobility of grain boundaries due to solute drag and eventually resulting in stagnation and/or stabilization of the grain growth process [15–20].
4.
Conclusions
During investigations (i.e. optical microscopy, SEM and XRD) it was found that Alloy-1 was dominatingly two phase (nearand hypo-stoichiometric) material with slight presence of hyper-stoichiometric phase. As Sn was added in Sm–Co, discernable changes in the microstructure and phase distribution were observed; • Increase in Sn concentration caused nucleation of hypostoichiometric phase • The presence of hyper-stoichiometric phase persisted in the alloys despite the nucleation of hypo-stoichiometric phase • Addition of Sn resulted in the refinement of the microstructure • The addition of Sn (> 0.06%) caused its segregation on the grain boundaries. Moreover, addition of Sn caused augmentation in magnetic properties of the Sm–Co alloys until 0.06% Sn, and any further increase in Sn caused decrease in magnetic characteristics.
Acknowledgement Authors are thankful to Asst. Prof. Dr. A. Hakeem of GIK Institute for his valuable suggestions.
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