Engineering Materials and Their Applications
Effect of Microstructure on the Coercivity of SmCo5 Intermetallic Compound
2020 Volume 61 Issue 11 Pages 2195-2200
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2020 Volume 61 Issue 11 Pages 2195-2200
SmCo5 is well known for its high coercive properties. This property helped the compound to become stable even at high temperatures. A lot of efforts had been made to improve this important property but so far only a few percentages of the theoretical coercivity values were achieved. Improving processing parameters or doping by other alloying elements are two popular ways to manipulate the properties of SmCo5. In this research work, the cooling temperature of the indigenously developed water-cooled copper mold was manipulated to control the solidifying peritectic structure. The obtained casting was milled to powder and the final sintered product was produced. It was noted that high coercive values i.e. 32.9 kOe was achieved at low water inlet temperature. The results were interpreted by using a scanning electron microscope (SEM), Differential thermal analysis and X-Ray diffraction analysis. SEM results revealed peritectic nano-structure in SmCo5 compounds. These nano-structures seem helped to improve the coercivity of SmCo5.
Fig. 7 M-H curves comparing the magnetic properties of three samples i.e. Fast cooled, medium cooled and slow cooled.
Coercivity is a property of magnets that determine the resistance against demagnetization whether in a magnetic field or high temperature. Thus, an increase in coercivity is the stringent requirement for modern magnets which may have great applications at high temperatures.
Researchers have been worked on the coercivity of the SmCo5 by adopting different techniques i.e. either by altering the processing parameters or by the addition of some alloying element. According to Zhexu Zhang et al.,1) for a single-phase binary alloy of Sm–Co, by far the maximum coercivity is 36.76 kOe, which was achieved in case of Sm5Co19 phase. Though, Lei Fang et al.,2) demonstrated that dry milling of SmCo5 yielded 41 kOe coercivity. Similarly, A.M. Gabay et al.3) obtained 41.5 kOe values for SmCo5 powder. Several other researchers worked on binary Sm–Co system achieving maximum, 30 kOe coercivity by J. Ding et al.4) C. Rong et al.,5) demonstrated that 26 kOe coercivity can be achieved in the SmCo5 system. S. Aich et al.,6) achieved 25 kOe while Hiroaki Nishio7) claimed 24 kOe values in SmCo5. Similarly, S.K. Pal et al.8) achieved 23 kOe coercivity, without any alloying addition. For reference9–21) Table 1 shows the coercivity values achieved by some other researchers without the addition of any dopant.
In this research work single-phase SmCo5 is considered to improve the coercivity without the addition of any alloying element.
Samarium–cobalt alloy was prepared by induction melting technique so that maximum homogeneity can be achieved. In this regard, pure samarium and cobalt were utilized, 5% (by w.t.) samarium losses were considered before the melting operation. The pouring temperature of the molten pool in the crucible was kept at 1450°C, before pouring, while the temperature was monitored by using the Fluke IR-Pyrometer. The molten metal was poured into an indigenously made water-cooled copper mold. The inlet temperature of the water is controlled by using a chiller. Three inlet water temperatures were selected to control the solidification of the molten metal. Therefore, the grain size is expected to vary with the change in these temperatures and hence the final magnetic properties of SmCo5. The nomenclature and the inlet temperatures for the three experiments are summarized in Table 2.
The obtained casting was crushed and ball milled to produce powder in the range of 4–6 micrometers. The chemical composition of each powder was determined by Energy Dispersive Spectrometer (EDS) attached with Scanning Electron Microscope (SEM). The chemical analysis of the three powders revealed that samarium is in range of 34.4–34.8% (by w.t.) while the balance is cobalt. General ball-milling parameters are given in Table 3.
After powder making brick-like samples were prepared, using 2-tesla magnetic field and 7 MPa pressure, in the magnetic press. The green bricks were then charged into the cold iso-static press where 200 MPa pressure was applied to these samples. These green bricks were then sintered at 1125°C for 100 minutes in a vacuum furnace. After high-temperature sintering, the samples were slowly cooled down i.e. at the rate of 0.5°C/min to 900°C. At 900°C, all the samples were aged for 120 minutes. The samples were then argon quenched with inert gas to room temperature. Heat-treated samples were cut for further material characterization. Optical and scanning electron microscopes (SEM) were used for microstructure evaluation. Energy Dispersive Spectrometer (EDS) attached to SEM was used for chemical analysis. X-ray diffraction (XRD) analysis was done by using Jeol JDX-9C, whereas, Cu-kα radiation was utilized. The scanning step during analysis was kept at 0.05°. Differential thermal analysis (DTA) was done on samples while keeping heating and cooling rate at 10 K/min. Magnetic properties were measured on the Pulsed-field magnetometer. The size of the sample for magnetic properties was 20 × 20 × 20 mm3.
After polishing and etching, the samples were observed both on optical and scanning electron microscopes. Optical microscopy revealed the presence of different phases in the samples, whereas, EDS attached with SEM confirmed the chemical composition of these phases. It was observed that the fastest cooling samples mostly consist of the SmCo5 phase while the small size particles are of the SmCo3 phase, homogeneously distributed in the matrix of SmCo5, Fig. 1. Relatively medium and slow cooled samples showed the presence of the Sm2Co7 phase along with the SmCo5 phase, Fig. 2, and Fig. 3. It was observed that a very small quantity of SmCo3 phase was also present in the later samples i.e. medium and slow cooled. However, it was noted that a relatively smaller quantity of the Sm2Co7 phase is present in the sample cooled at intermediate or medium mode. The tables attached to Fig. 1–3, shows the atomic percentage of different phases, referred to the possible metallurgical phases.
Optical micrograph of fast cooled samples, showing the presence of SmCo3 particles. Attached table shows the atomic percentage of marked sites with possible metallurgical phase.
Optical micrograph of medium cooled sample, showing the presence of Sm2Co7 and SmCo3. Attached table shows the atomic percentage of marked sites with possible metallurgical phase.
Optical micrograph of slow cooled sample, showing the presence of Sm2Co7 and SmCo3. Attached table shows the atomic percentage of marked sites with possible metallurgical phases.
The information that can be destroyed in the polished samples can be witnessed in fracture surfaces. In this regard, fresh fracture surfaces of sintered samples were made and observed under the SEM. The general fracture surfaces showed the cleavage of particles. However, the porosity regions, in the sintered samples, able to preserve some important nano features, Fig. 4. These features are probably similar to the observations made by W. Zhang et al.10) and L. Long et al.,22) called packed crystallites of SmCo5. Accordingly, the packed grains were formed during the solidification process. These features might also be correlated with peritectic structure formed during the fast cooling of molten metal. As per phase diagram of Sm–Co23,24) when the temperature drops from 1300°C, the peritectic reaction occurs i.e. a liquid and a solid phase reacts to form a new solid phase. Hideyaki Yasuda et al.25) discussed some microstructures showing the formation of rod-like structures, formed after peritectic transformation. The diameter of these grains was measured for all the samples and is mentioned in Table 4, where the results are also compared with XRD obtained data. It can be seen that the samples which are fast and medium cooled represent almost the same size i.e. 10–80 nm, whereas, in the case of slower cooled samples, the size is much coarser i.e. in the range of 30–250 nm.
Fracture surface of sintered samples revealing the nano-structure in three cooling conditions of casting: (a) fast cooling, (b) medium cooling, (c) slow cooling.
To confirm the presence of different metallurgical phases, XRD analysis done on sintered samples, Fig. 5. The surface of the XRD samples was perpendicular to the magnetizing direction of the sample. All the main peaks of SmCo5 were observed in all the three samples. Minor peaks of Sm2Co7 were also detected in medium and slow cooled samples.
Comparison of XRD graphs for the three samples i.e. fast cooled, medium cooled and slow cooled.
Further, the conventional Sherrer equation was applied to the width of XRD peaks, using shape factor 0.9. Two main crystallographic peaks i.e. (110) and (111) are considered in this regard. It seems that the obtained values represent the diameter of the peritectic rods and not the grain size of the compound. Since the grain size is expected to increase after sintering at high temperature. The results of XRD revealed that the fast and medium cooled samples almost have the same rod diameter while slower cooled samples demonstrated comparatively larger rod diameters. Noise in XRD results probably refers to the amorphization of the sample as discussed by S.K. Pal et al.8) On comparing the XRD and SEM results, it seems XRD represents the average size which is very much comparable with SEM results.
According to M.F. de Campos et al.23) DTA method is well suited for the conformation of different metallurgical phases in the sample. In this regard, a cooling curve is presented in Fig. 6, where the exothermic peaks were demonstrated. The exothermic peak observed at 1200°C, for the fast cooled sample during solidification, represents the formation of SmCo3 and confirmed the observations of SEM, demonstrated in Fig. 1. SmCo3 can produce lattice strain26) in the neighboring regions, this may be due to comparatively larger lattice associated with SmCo3. The lattice strain may act like a pinning site27,28) for domain walls and therefore, contributes to increase in coercivity of the magnetic material. This lattice strain contributes to an increase in the coercivity of the magnetic material. For the samples solidified at a relatively medium and slower pace an exothermic peak was observed at ∼1150°C. According to a phase diagram published by the ASM handbook.24) An isotherm is present at 1150°C. This line demonstrates that upon cooling two phases will be formed i.e. SmCo5 and Sm2Co7. It can be inferred that the exothermic peak formed at ∼1150°C represents the formation of Sm2Co7.
DTA cooling curves obtained for fast cooled, medium cooled and slow cooled samples.
The M-H curve for the three samples is presented in Fig. 7, while the data extracted from the curves are presented in Table 5. It can be seen that the fast cooling samples demonstrated the maximum coercivity i.e. 32.9 kOe which is far higher than the values quoted in literature. This may be attributed to the fine peritectic structure. C.H. Chen et al.,29) Y. Shen et al.30) explained that fine lamellar grains improve the coercivity of the final product. Similarly, N.M. Taliyan et al.31) and D.L.L. Pelecky et al.32) noted that as the grain size decreases the coercivity of the magnetic material increases. The other reason for the increase in coercivity is the presence of the SmCo3 phase, confirmed by the DTA method. As explained earlier, the lattice strain induced by the formation of SmCo3 also contributed to increasing in the coercivity of the sample. Further, fast cooled samples do not show the formation of Sm2Co7. The formation of Sm2Co7 is also noted by Long Li et al.32) on the grain boundaries of SmCo5 and observed a negative effect on the magnetic properties of the sample. W. Zhang et al.10) explained further that the negative effect is associated with low magneto-crystalline anisotropy of the Sm2Co7 phase. Medium cooled samples, which have also the fine peritectic structure, demonstrated high values of coercivity i.e. 24.5 kOe. The relative decrease in coercivity than the former is probably due to the formation of the Sm2Co7 phase.
M-H curves comparing the magnetic properties of three samples i.e. Fast cooled, medium cooled and slow cooled.
Three cooling conditions were adopted to produce the castings of SmCo5. It was observed that the fastest cooled samples yield the highest coercivity i.e. 32.9 kOe. High coercivity is attributed to the formation of a fine peritectic structure formed during the fast cooling along with the formation of SmCo3 and the absence of the Sm2Co7 phase. Relatively medium cooled samples also demonstrated good coercivity i.e. 24 kOe, probably the absence of SmCo3 phase not brought these samples to the range attained by the fast cooled samples. Further, the formation of Sm2Co7 also contributed to the loss of some magnetic properties in medium and slow cooled samples.
