THE INFLUENCE OF HEAT TREATMENT REGIME ON THE MICROSTRUCTURE AND MAGNETIC PROPERTIES OF MELT-SPUN Nd-Fe-B WITH Nd LOW CONTENT

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1 THE INFLUENCE OF HEAT TREATMENT REGIME ON THE MICROSTRUCTURE AND MAGNETIC PROPERTIES OF MELT-SPUN Nd-Fe-B WITH Nd LOW CONTENT N. TALIJAN 1, J. STAJIĆ-TROŠIĆ 1, A. GRUJIĆ 1, V. ĆOSOVIĆ 1, D. NEDELJKOVIĆ 1, R. ALEKSIĆ 2 1 Institute of Chemistry, Technology and Metallurgy, Belgrade, Serbia and Montenegro, cmm@tmf.bg.ac.yu 2 Faculty of Technology and Metallurgy, Belgrade, Serbia and Montenegro Received December 21, 2004 In recent time the study topic of many scientific institutions throughout the world is the process of obtaining high-coercive magnetic materials based on Nd-Fe-B, by the method of rapid quenching from the alloy melt of desired chemical composition, so-called melt spinning method. Cooling speed range in which optimal results are achieved is rather narrow so that heat treatment of R/Q Nd-Fe-B alloys is needed in order to achieve the maximal coercivity. Crystallization and thermomagnetic behaviour of melt-spun Nd-Fe-B alloy with Nd low content was observed through phase transformation, for defined cooling rate and different heat treatment regime. The crystallization kinetics and activation energy of investigated alloy were observed by DTA measurements. The phase composition of the samples was investigated by conventional XRD analysis and Mössbauer spectroscopic phase analysis (MS). For determining the critical temperature of phase and thermomagnetic transformations, magnetic measurements were additionally carried out. Comparison of the experimental results obtained by different investigation techniques enabled more complete comprehension of the crystallization process during the heat treatment. Analysis of experimental results also enabled better insight in relationship between microstructure and magnetic properties in function of heat treatment regime. Key words: permanent magnet, rapid quenching, Nd-Fe-B-low content Nd, microstructure, phase composition, heat treatment, crystallization process, phase transformation, magnetic properties. 1. INTRODUCTION In recent time the study topic of many scientific institutions throughout the world is the process of obtaining high-coercive magnetic materials based on Paper presented at the 5th International Balkan Workshop on Applied Physics, 5 7 July 2004, Constanþa, Romania. Rom. Journ. Phys., Vol. 50, Nos. 9 10, P , Bucharest, 2005

2 1034 N. Talijan et al. 2 Nd-Fe-B, by the method of rapid quenching from the alloy melt of desired chemical composition. The advantage of using the rapid quenching (R/Q) technology for obtaining high-coercive Nd-Fe-B magnets reflects in the possibility to influence through the quenching speed directly on the grain size and microstructure aimed to achieving so-called magnetic structure which provides maximal magnetic energy of finished magnetic materials of this type. In previous research, influence of cooling rate to magnetic properties of R/Q Nd-Fe-B was investigated and optimal cooling speed for defined chemical composition was selected [1, 2, 3, 4]. Cooling speed range in which optimal results are achieved is rather narrow so that heat treatment of R/Q Nd-Fe-B alloys is needed in order to achieve the maximal coercivity. For this reason the study is extended to testing of effects and selection of the most convenient regime of heat treatment for the optimal selected cooling rate and for initial defined chemical compositions of R/Q Nd-Fe-B alloy. Magnetic material with the reduced content of Nd under the suitably selected conditions of synthesis and heat treatment has, in spite of reduced coercivity, favourable other magnetic characteristics. It is suitable for interpretation and comparison of coercitive mechanism responsible phase transformations in the function of the chemical composition and cooling rate with magnetic properties. In this work, evolution of microstructure trough phase transformations and magnetic properties of melt-spun Nd-Fe-B alloy with Nd low content during the heat treatment was observed. 2. EXPERIMENTAL In presented research different methods of analysis were used. The thermal and crystallization behavior were studied by differential thermal analysis (DTA) [1, 2, 3]. The thermomagnetic behavior was examined using a thermomagnetic analyzer (TM). The phase identification was carried out by X-ray diffractometric analysis (XRD) [1]. Mössbauer spectroscopy (MS) was used to study the magnetic microstructure [4, 5, 6]. The magnetic properties of annealed Nd-Fe-B alloy were measured using a vibrating sample magnetometer (VSM) with the magnetic field strength of 50 koe. 3. RESULTS AND DISCUSSION Investigated Nd-Fe-B alloy with Nd low content was prepared by rapid quenching (R/Q) under Ar atmosphere with optimal selected cooling rate [1, 3, 4].

3 3 Heat treatment of melt-spun Nd-Fe-B 1035 The as-quenched alloy had Nd-12.0 mass %, Pr-02 mass %, B-4.2 mass %, Fe-balance. SEM micrograph of melt-spun Nd-Fe-B alloy is shown in Fig. 1. Platelets with width between μm and thickness between μm are typical. Fig. 1. SEM micrograph of melt-spun Nd-Fe-B alloy. Thermal behavior of investigated alloy was done by DTA in the temperature range from 20 C to 900 C for the reason of getting optimal heat treatment regime. According to the most of authors, in investigated alloy with low Nd content in temperature interval from 600 C to 700 C Fe 3 B, Nd 2 Fe 14 B, Nd 2 Fe 23 B 3 phases and α-fe phase are present [7 10]. On the DTA curve for investigated sample (Fig. 2) first crystallization peak which corresponds with primary crystallization of Fe 3 B phase (phase of high magnetization) can be observed. Fig. 2. DTA curves of investigated melt-spun Nd-Fe- B alloy as a function of the annealing temperature. Annealing time: 5 min.

4 1036 N. Talijan et al. 4 That is proved by XRD analysis and MS analysis [1, 4]. Second peak on DTA curve of weaker intensity corresponds with crystallization of hard magnetic phase Nd 2 Fe 14 B. By increasing of temperature of heat treatment in investigated interval, it was shown that density of fine Fe 3 B particles is increased, and that they are uniformly distributed in the amorphous matrix for the short time of heat treatment [7 10]. By analyzing of XRD diffractogram after heat treatment at 600 C during 5 minutes, following phases are identified: Nd 2 Fe 23 B 3, Fe 3 B, α-fe and Nd 2 Fe 14 B [11]. This corresponds with experimental results of DTA and with theoretical analysis of crystallization flow [9 11]. For the heat treatment at 660 C during 5 minutes, same phases were identified but in different volume rations [4, 11]. Metastable phase Nd 2 Fe 23 B 3 crystallizes as intermediary phase, and in further process of crystallization during heating decompositions to other metastable phases: phase of high magnetization Fe 3 B, main magnetic phase Nd 2 Fe 14 B, and small amount of soft magnetic α-fe phase. Amount of Nd 2 Fe 14 B phase comparing to Fe 3 B increases with increasing of temperature of heat treatment, and that leads to increasing of coercitive force [7 10]. By XRD analysis of investigated alloy heat treated at 700 C during 5 minutes phases Nd 2 Fe 14 B, Nd 1.1 Fe 4 B 4, α-fe and Fe 3 B were identified. Metastable Nd 2 Fe 23 B 3 phase decays completely at 700 C which proves that crystallization of identified phase has been conducted completely [9 11]. According to the results XRD and MS analysis, because of relatively high boron content, Nd 1.1 Fe 4 B 4 phase is also present. Obtained values of Br, Hci and (BH) max of heat treated samples are presented in Table 1. Table 1 Magnetic properties in the function of the heat treatment regime HEAT TREATMENT REGIME Br, kg Hc, koe (BH) max, MGOe 600 C / 5 min C / 5 min C / 5 min This difference in optimum values may arise from the fact that those two different mechanisms: the exchange coupling and the phase content govern Hci and Br. While exchange coupling is important for both Hci and Br, the amount of the soft phase competes with the exchange interaction in its effect on Br [12]. On the basis of DTA (Fig. 2) and XRD results optimal heat treatment regime was selected (660 C for 5 min). Corresponding Kissinger plot is shown in Fig. 3.

5 5 Heat treatment of melt-spun Nd-Fe-B 1037 Fig. 3. Kissinger plot for investigated Nd-Fe-B. According to DTA results, Kissinger plot was used for determination of the activation energy for optimal heat treated Nd-Fe-B alloy with Nd low content. Calculated value of activation energy ( kj/mol) for this Nd-Fe-B system is very close to the value that can be expected according to data found in literature [7, 8]. Mössbauer spectra were taken in the standard transmission geometry with the Co57 (Rh) source at room temperature [5]. The absorbers were prepared spreading such an amount of the material particles that the iron atoms surface density of about 10 mg/cm 2 was reached. For the spectra fitting and decomposition the CONFIT package was used [6]. On the basis of previous thermomagnetic measurements in temperature range 20 C 800 C after second cycle of measurements magnetic hardness of annealed specimens decreases [2, 3, 4]. The Mössbauer spectra of this material after both of thermomagnetic measurement cycles are very complex and not all components could be identified Fig. 4. Mössbauer spectra taken before and after thermomagnetic measurements [A Nd 2 Fe 23 B 3, B α-fe, C Fe 3 B, D Nd 1.1 Fe 4 B 4, E Fe 2 B, F FeB; X belongs probably to the Nd 2 Fe 14 B phase].

6 1038 N. Talijan et al. 6 exactly. MS spectra illustrate the substantial difference between the state with optimised magnetic properties and the state after the decomposition (Fig. 4) induced by the thermomagnetic measurement. MS spectra (Fig. 4) exhibit the decay of the Nd 2 Fe 23 B 3 phase and dramatic increase content of the α-fe phase, above all. The α-fe and the whole set of Fe-B phases prevail. This thermal decomposition will be the main reason for the quality loss of this hard magnetic material. This behavior corresponds to the shape of hysteresis loops shown on Fig. 5. Fig. 5. Hysteresis loop before and after heat treatment. 4. CONCLUSION The comparison of the experimental results obtained by different investigation techniques enabled more complete comprehension of the crystallization process during the heat treatment. Analysis of experimental results also enabled better insight in relationship between microstructure and magnetic properties in function of heat treatment regime. Magnetic microstructure of Nd-Fe-B alloy with reduced Nd content is more sensitive to thermal treatment, which has direct influence on the coercivity. REFERENCES 1. J. Stajić-Trošić, MSc Thesis: Investigation of effect of processing parameter on properties of Nd-Fe-B alloys obtained by the rapid quenching method, Faculty of Technology and Metallurgy, Belgrade, Serbia and Montenegro, N. Talijan, T. Žák, O. Schneeweiss, Ž. Jovanović, Science of Sintering: Current Problems and new Trends, SANU, Belgrade, Serbia and Montenegro, pp , N. Talijan, J. Stajić-Trošić, T. Žák, V. Menushenkov, A. Milutinović-Nikolić, Nd-Fe-B Alloys Synthesis-Structure-Properties, Proc. European Congress & Exhibition on Powder Metallurgy PM 2001, Nice, France, 2: , 2001.

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