This paper reports the results of investigations carried out to determine the microstructure and magnetic properties of some cast and homogenized praseodymium-based alloys and magnets represented by the formula Pr14Fe79.9xCoxB6Nb0.1. Permanent magnets were prepared from cast and annealed alloys using the hydrogenation, disproportionation, desorption and recombination process. The effect of cobalt content on the magnetic properties of these magnets was investigated. Cobalt has a significant effect on the magnetic behaviour of these alloys. Microstructural examinations revealed that the grain size of the matrix phase increased with increasing cobalt content in the Pr-based alloys and niobium had a grain-refining role.
Permanent magnet powders prepared by the hydrogenation, disproportionation, desorption and recombination (HDDR) process are high coercivity materials and these can be obtained by exposing Nd-based alloys to hydrogen at elevated temperatures (examples in Refs.). Cobalt free Pr-based powders have been produced via the HDDR process, but with inferior magnetic properties, compared to Nd-based materials. PrFeCoBGaZr-type HDDR powders with good remanence have also been reported. The intrinsic coercivity in these materials is still quite poor. Recently, it was shown that powders based on the composition Pr13,7Fe63,5Co16,7B6M0.1 (M=Zr or Nb), with good remanence (Br = 1000 mT) and reasonable coercivity (iHc = 790 kAm-1 ) can be produced by the HDDR process (pH2 = 0:1 MPa). This paper reports the results of further work carried out on Pr14Fe79.9xCoxB6Nb0.1-type alloys (where x ¼ 0; 4, 8, 10, 12, 16). This investigation was undertaken to optimise the cobalt content with respect to the magnetic properties of the HDDR magnets. The microstructures of the magnetic alloys were observed with a scanning electron microscope (SEM) and the phase compositions were determined with the aid of an energy dispersive X-ray (EDX) spectrometer system coupled to the SEM.
2. Experimental procedure
Various commercial alloys in the as-cast state and after annealing in vacuum at 1100C for 20 h were studied. The chemical analyses of the as-cast alloys are given in Table 1. All the alloys contain about 0.1 wt% aluminium as an impurity (as per the supplier’s specification). The niobium-free alloy was included in this study for comparison. The following procedure was adopted to produce the Pr-based magnets via the HDDR process. The as-cast and annealed alloys were crushed into coarse lumps and 9 g batches were placed in the HDDR reactor. This reactor was then evacuated to the backing-pump pressure (=10-1 mbar) and hydrogen introduced until the pressure varied between 0.08 and 0.25 MPa (0.8– 2.5 bar). The temperature of the reactor was held at 100C for 30 min to provide sufficient time for the hydrogen decrepitation (HD) reaction to go to completion. The reactor was then heated to 770C at 15C/ min and further up to the desorption temperature (860C) at 5C/ min, with a dwell time of 15 min prior to desorptionx. Subsequent desorption and recombination was carried out under vacuum at the same temperature until a pressure of 101 mbar was achieved (B10 min). Subsequent rapid cooling of the material was carried out by removing the furnace from the HDDR reactor and by coupling a water-cooled copper coil to the reactor tube.
The resultant powder was crushed in air until all the material passed through ao106 mm sieve. The fine powder was subsequently encapsulated in a small cylindrical rubber bag, pulsed in a magnetic field of 6.0 T and pressed isostatically at 200 MPa. The resultant green compacts were consolidated by placing wax in the bag and heating to 100C, to enable the molten wax to penetrate the HDDR powder compact. The mixture was then allowed to cool to room temperature and the excess wax removed to yield a cylindrical magnet.
Magnetic characterisation of the HDDR magnets was carried out using a permeameter. Measurements were performed after saturation in a pulsed field of 6.0 T. Remanence values have been normalized assuming 100% density for the HDDR sample, and by also considering a linear relationship between density and remanence. Microstructural characterization of the alloys was carried out with the aid of a SEM.
3. Results and discussion
Variation in remanence of HDDR magnets, produced from as-cast and annealed Pr-based alloys, as a function of cobalt content is shown in Fig. 1. Good remanence values were achieved in samples prepared from annealed Co-containing alloys. In the presence of 4.0 at% Co, the remanence increased from 790 to 840 mT. Higher Co contents produced a slight decrease in remanence. Overall, and as expected, the remanence of the HDDR magnets produced from the alloy in the annealed condition was higher than that of magnets produced from the as-cast alloy. The large variations in remanence values of HDDR magnets produced from the as-cast alloys can be attributed to the heterogeneous condition of the alloys.
Fig. 2 shows the variation in intrinsic coercivity of HDDR magnets produced from as-cast and annealed Pr-based alloys as a function of cobalt content. Good intrinsic coercivity values were achieved in samples prepared using the annealed alloys. In the presence of 8.0 at% cobalt, a magnet with the best intrinsic coercivity (970 kA/m) was obtained. At higher Co contents this magnetic property decreased. Again, as expected, the coercivity of HDDR magnets produced from annealed alloys was higher than in magnets produced from as-cast alloys. It was also noted that the HDDR magnet produced using the Cofree alloy in the annealed condition presented good intrinsic coercivity (923 kA/m).
The magnetic properties of all the magnets produced with alloys in the as-cast and annealed conditions are shown in Table 2. The best energy product (121 kJ/m3 ) was observed in the magnet containing 8 at% cobalt, which was prepared from an aligned HDDR powder and an annealed precursor material. It is worth noting that the Pr14Fe79.9B6Nb0.1 magnet also showed a good energy product value (114 kJ/m3 ). Niobium addition increased the remanence from 680 to 790 mT in HDDR magnets prepared from annealed Co-free alloys but had no effect on iHc (a decrease in iHc was observed for that in the as-cast state) The highest inductive coercivity was achieved in the Pr14Fe67.9Co12B6Nb0.1 HDDR magnet (541 kA/m). The best squareness factor (SF=0.50) was achieved in the magnet containing 8 at% cobalt, which was also prepared from an annealed alloy. The Pr14 Fe69.9 Co10 B6 Nb0.1 HDDR magnet also showed a reasonable squareness factor (0.46).
Back-scattered electron images of the as-cast and annealed Pr-based alloys are shown in Figs. 3–9. It can be seen clearly that annealing at 1100C for 20 h was quite effective in homogenizing all the alloys. Free iron (dark phase) was completely eliminated from the interior of the matrix phase grains (dark grey phase). It can also be clearly seen that niobium addition had a refining effect on the as-cast alloy. The refining effect is not evident in alloys with Co more than 4 at%. Nevertheless, cobalt addition up to 8 at% has a beneficial effect on the intrinsic coercivity. The Curie temperature of Pr2 Fe14B increases with the Co content, at about 11C per at% and this could be a favourable factor for additions beyond 8 at% of Co to these magnetic alloys. Increase in corrosion resistance could be another factor in favour of addition of higher amounts of Co. Further studies are being carried out to determine the effect of cobalt on the corrosion behaviour of these Pr-based alloys.
The composition of the various phases in the Pr14 Fe69.9 Co10 B6 Nb0.1 alloy, both in the as-cast condition and annealed, are presented in terms of the ratio Pr:Fe:Co in Table 3. Cobalt was observed to substitute Fe in the matrix phase. The Pr:Fe:Co ratio is approximately 2:11:2.6 and very close to that reported in a previous study (2:10:2.3) for a Zr-containing alloy (Pr13.7 Fe63.5 Co16.7 B6 Zr0.1). Zirconium and niobium seem to have distinct influence in these alloys. The former appears to inhibit grain growth during heat treatment whereas the latter appears to control grain growth more effectively during the disproportionation and recombination stages of the HDDR process. A small change in composition of the matrix phase was observed, as a result of the heat treatment and this was not observed in the previous investigation. As shown recently, small differences in chemical composition can be observed with a SEM fitted with an EDX system. In all alloys, in the as-cast condition, iron and cobalt were observed in the dark dendritic phase (D), with variable Fe/Co ratio. The white (W) and grey (G) phases have been already identified as Nd3Co and Nd(Fe,Co)2 (Laves phase), respectively. Recently, in an investigation where the effect of disproportionation of powders based on the Pr13.7 Fe63.5 Co16.7 B6 Zr0.1 alloy were studied using X-ray diffraction and Mossbauer spectro- . scopy, it was reported that a boride phase Pr(Fe,Co)12 B6 is present when the alloys are disproportionated at or above 860C. Also according to this study, after recombination, this phase is still present in the early stages and it could play a role in inducing textures in the material upon further treatment. This phase disappears with increase in duration of the recombination treatment. Very recently, significant amounts of intermediate borides t-Fe3B and Pr(Fe,Co)12 B6 have been detected after solid hydrogen disproportionation treatment in Pr13.7 Fe80.3 B6 and Pr13.7 Fe63.5 Co16.7 B6 Zr0.1alloys . Further microstructural investigation is underway to determine the effect of cobalt on the morphology of the HDDR powder.
Pr14 Fe75.9 Co4 B6 Nb0.1 HDDR powders produced from annealed alloys yielded magnets with good remanence, energy product and squareness factor whereas a Pr14 Fe71.9 Co8 B6 Nb0.1 alloy yielded HDDR magnets with better intrinsic coercivity. Good intrinsic coercivity was also obtained in Co-free (Pr14 Fe80 B6 and Pr14 Fe79.9 B6 Nb0.1) HDDR magnets prepared with annealed alloys, although it had slightly reduced remanence. The best inductive coercivity was achieved when homogenized Pr14 Fe67.9- Co12 B6 Nballoy was used. These results show clearly that cobalt is essential to develop high anisotropy and SF, convenient to improve the Curie temperature and inductive coercivity but not necessary for enhancing intrinsic coercivity in Prbased HDDR magnets.