Garyounis University Press Journal of Science and Its Applications Vol. 1, No. 2, pp 79-101, September 2007 97 Effect of Nb on The HDDR Behaviour and The magnetic Properties of NdFeB Alloys F. M. Alyamani Physics Department, Faculty of Science, Garyounis University Benghazi, Libya. ABSTRACT The hydrogenation, disproportionation, desorption and recombination, (HDDR) behaviour of the Nd11.8Fe81.3Nb1B5.9 (1% Nb alloy) and the Nd11.8Fe80.3Nb2B5.9 (2% Nb alloy), (atomic%) were studied using a high resolution scanning electron microscope (HRSEM) in order to relate the initial, intermediate and final microstructures, to that of NdFeB-type alloys without additions. The disproportionation of the Nd 2 Fe 14 B ( ) matrix phase starts at the grain boundary and at the interface of the Nb 26 Fe 32 B 42 / phase, resulting the formation of a submicron structure consisting of Fe, Fe 2 B and Nd-hydride. The HDDR process proved to be a very rapid and efficient method of homogenizing the as-cast alloys. Promising magnetic properties have been obtained for bonded magnets using the HDDR powder. Key Words: Niobium; Solid HDDR, NdFeB; Permanent magnets.. INTRODUCTION Due to the peritectic nature of the Nd 2 Fe 14 B ( ) phase, alloys of stoichiometric and nearstoichiometric compositions contain large amounts of -Fe dendrites. Normally a very long, high temperature homogenization treatment is required to remove these dendrites and to produce single phase material. Niobium substitution for iron in the stoichiometric alloy was found to reduce the homogenization time considerably and also suppresses the grain growth of the matrix phase (Ahmed, Edgley, & Harris, 1994). Previous work (McGuiness,.Zhang, Yin, & Harris, 1990; Zhang, McGuiness, & Harris, 1991) indicated that homogeneous material can be achieved rapidly if the alloy is subjected to the HDDR process. The HDDR process was discovered by (Takeshita & Nakayama 1994) and reviewed previously (Ahmad, Edgley, & Harris 1994a). In the present work an attempt has been made to incorporate the two approaches in order to produce homogeneous material with improved magnetic properties. EXPERIMENTAL The Nd11.8Fe81.3Nb1B5.9 (1% Nb alloy) and the Nd11.8Fe80.3Nb2B5.9 (2% Nb alloy) used in this work were supplied by Rare Earth Products, UK. The microstructural changes during the HDDR processes were investigated using a High Resolution Scanning Electron Microscope (HRSEM) and the alloy Nd11.8Fe80.3Nb1B5.9 was chosen for this study because it required a relatively low homogenization time and it contained the Nd-rich phase. The soft ferromagnetic Nd 2 Fe 17 phase which was present in alloys containing 3 at % Nb was absent in the 2 at % Nb alloy (Ahmad et al 1994a). The solid HDDR process was carried out at 800 0 C under a hydrogen pressure of 1 bar.
F. M. Alyamani 98 Hydrogen was introduced into the system when the temperature reached 500 0 C, in order to prevent the decrepitation of the material. When the temperature reached 800 0 C the samples were held for 1, 2 or 10 h. In order to study the recombined microstructure, disproportionated samples were heated to 800 0 C in vacuum for 2 or 10 hours and at the end the samples were quenched in order to fix the recombined microstructure. The HDDR powders were optimized by varying the disproportionation and recombination times and temperatures. The magnetic properties of both alloys were measured using a vibrating sample magnetometer (VSM). RESULTS AND DISCUSSION The disproportionation reaction begins at the Nd hydride / interface then progresses into the unreacted phase ( a in fig. 1), and also at the Nd 26 Fe 32 B 42 / interface ( b in fig. 1) proceeding outwards from the hexagonal rods of Nd 26 Fe 32 B 42 into the unreacted phase. It appears that hydrogen diffuses readily along this interface probably because of the incoherent boundary between Nb 26 Fe 32 B 42, and possibly due to the presence of cracks. Fig.2, shows the microstructure of a fully disproportionated sample, in addition to the disproportionation reaction of the matrix ( ) phase, the hydrogen also partially diproportionated the Nd 26 Fe 32 B 42 phase, (A) in Fig.2 but at a much slower rate. As a result of hydrogen removal, the Fe, Fe 2 B and Nd recombined to the original phase, however, with very file stb-micron grains, the average grain size about 0.2 m (as shown in fag. 3). Previous studies (McGuaness et al., 1990) demonstrated that, for mnst NdFeB,type alloys, it was possible to produce a rebombined structure whth a grain size of 0.3 m. 20.0 m Fig. 1. HRSEM image of a partially disproportionated sample of the 2% Nb alloy, A = phase, B = Nb 26 Fe 32 B 42 phase, C = Nd hydride, -Fe or Fe 2 B, E = fine disproportionated mixture, a = the Nd hydride / interface and b = the Nd 26 Fe 32.
Effect of Nb on The HDDR Behaviour and The magnetic Properties of NdFeB Alloys 99 20.0 m Effect of Nb on The HDDR BehaviourFig. 2. HRSEM image of a fully disproportionated sample of the 2% Nb alloy, A 9 Nb 26 Fe 32 B 42 phasa, B = Nd hydride, B = fine disproportionated miptur%. In the present work the r%combhned grain size appeared to be reduced &urther ( 0.2 m ) and this c`n be attributed to the Nb addition which is known to inhibit grain growth (Tokonaga, Harada, & Trout, 1987; Ishizaka, Hamada, & Ohmori, 1988; Ishikawa, Hamada, & Ohmori 1989; Ahmed, et al., 1994; McGuiness, et al., 2004, Wang et al 2005). However, in some cases the grains appeared larger than 0.2 m but this could be because of poor etching, the image was not clearly resolved. In any case, etching of this sort of material is very sensitive and specimens can easily be over etched resulting in poorly defined microstructure.
100 F. M. Alyamani Fig. 3. Recombined structure of the 2% Nb alloy. Fig. 4. VSM traces of the 1 and 2 % Nb alloys after HDDR treatment. Under the particular processing conditions employed in these experiments, the magnetic properties of the HDDR powder containing 1% Nb were found to be much better than the alloy containing 2% Nb. The 1% Nb sample was processed by heating in hydrogen with a 15 min hold at 200 o C followed by 750 o C for 4 h, then switching to vacuum for a further 2 h at 750 o C, before cooling to give i H c = 449 ka/m and B T = 690 mt. The 2% Nb alloy gave i H c = 270 ka/m and B T = 640 mt, by holding in hydrogen at 800 o C for 2h followed by vacuum for 2 h at 800 o C. the demagnetization traces of both alloys are compared in fig. 4. The magnetic properties of the alloy containing 1% Nb look promising for making bonded magnets. The best i H c achieved so far was 449 ka/m (5.64 koe) and it should be possible to improve on this with further optimization of the processing conditions and composition (e.g. increased Ndcontent ). CONCLUSION 1. The solid HDDR process proved to be valuable technique for following the microstructural changes taking place during the HDDR process. 2. The HDDR process resulted in the homogenization of the specimens much faster and at considerably lower temperatures than those used in the conventional homogenisation method. 3. Nb additions appear to refine grain size of recombined material. 4. Disproportionation begins at Nb 26 Fe 32 B 42 / Nd 2 Fe 14 B interface.
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