The effect of niobium on the extent of columnar growth of grains and on the magnetic properties in Alnico alloys has been studied. Alloys containing 1 % Nb show maximum growth of columnar crystals. In columnar alloys, coercivity increases slowly with increasing niobium content up to approximately 1 %. With further increase in niobium, coercivity and remanence decrease. The maximum energy product (55 kJ m-3) has been obtained at 1 % Nb. Niobium addition has also been found to suppress the precipitation of the undesirable gamma phase

Mohanty, O N

Pramanik, S

Rao, Venkatesh

English

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E E 0 Hot top Refractory sleeve High silica sand 0 If 1 AluMina tube Water-cooled copper plate 1,m2m5 100 mm AL OF MATERIALS SCIENCE 28 (1993) 1237-1244 ct of niobium on the directional dification and properties of ico alloys RAMANIK, V. RAO, O. N. MOHANTY nal Metallurgical Laboratory, Jamshedpur 831 007, India ffect of niobium on the extent of columnar growth of grains and on the magnetic prop-in Alnico alloys has been studied. Alloys containing 1% Nb show maximum growth of nar crystals. In columnar alloys, coercivity increases slowly with increasing niobium con-.p to — 1 %. With further increase in niobium, coercivity and remanence decrease. The mum energy product (55 kJ m-3 ) has been obtained at 1°A Nb. Niobium addition has been found to suppress the precipitation of the undesirable y phase. troduction iderable work has been reported in the literature inico alloys [1-6]. Alnico V which contains 1, 14% Ni, 24% Co, 3% Cu and balance Fe, is to give rise to a maximum energy product ),iiax] of 40-44 kJ m -3. Further improvement in pi- lies is possible through directional solidific-ti. 1 .1 Directionally solidified Alnico alloys have :=11:),s4arallel to the <1 0 0> direction resulting in r remanence (1.3-1.4 T), good coercivity 6-kA m -1) and a (BH),„a„ of 50-70 kJ m t studies [7-10] have shown that the properties ectionally solidified Alnico alloys can further be yed by addition of small amounts of niobium. A atic study, however, on the effect of niobium on tent of columnar crystallization and magnetic has not been reported in the literature. e present work pertains to the studies carried out e effect of niobium (0-2 wt %) on columnar llization and magnetic properties in Alnico V s. xperimental procedure alloys were made. Low-carbon iron, cobalt, elec-ic copper and nickel were melted in a 2 kg Lion furnace and then electrolytic aluminium dded. After deoxidizing with small amounts of n, the melt was held in the furnace for 2 min. quently, niobium was added in the form of 'obium. Molten metal was then poured into a ally made refractory mould (Fig. 1). bulds were made from 4 in (10 cm) diameter ttory sleeves and 1 in (2.5 cm) diameter silliman-bes which were fixed at the centre of the sleeves „sand and sodium silicate). The sillimanite tube d in obtaining castings with smooth surfaces, by avoiding columnar growth of crystals from ails of the mould. The moulds were heated to 'C and placed on a water-cooled copper plate. 2461 © 1993 Chapman & Hall Figure 1 Schematic diagram showing the mould arrangements for directional solidification. 1250°C,1/2 h 0, 920°C,1/2 h = R L.. 650°C, 6h 4, es. 600°  C i  560°C, 10 h 5 I-- A B Ti me Figure 2 Schematic diagram of heat-treatment schedule. R = 32°C in a magnetic field. 1237 Alloy No.3 , tempered Alloy No.3 quenched Alloy No.1 quenched 40 418 5 64 72 " 100 100 116 Z e (deg) Figure 3 X-ray diffraction patterns of quenched and tempered Alnico alloys. 1238 Nominal composition of Alnico alloys (wt %) 8 14 24 3 8 14 24 3 0.5 8 14 24 3 1.0 8 14 24 3 1.5 8 14 24 3 2.0 al was then poured at a temperature of :and exothermic mixture was added on the hot edd over the heated mould. teens•10-20 mm thick were cut from the bot-well as the upper portions of each ingot for eatment were magnetic measurements. All the ens re homogenized at a temperature of C,,for 30 min and cooled in a blast of air. The ens were cooled at the rate of 0.5 °C s -1  from 920-600 °C in a magnetic field of 20 kA m', then cooled to room temperature without the magnetic field. The specimens were tempered first at 650 °C for 6 h and finally at 560 °C for 10 h. A schematic repre-sentation of the heat-treatment schedule is given in Fig. 2. To study the extent of columnar growth of crystals, 2 in (5 cm) long samples were cut from the bottom portion of each ingot and sectioned longitudinally into two halves. The inside surface was macro-etched with an etching solution containing 5 g cupric chlor-ide, 1 g stannous chloride and 10 g ferric chloride dissolved in 100 ml HCI and made up to 500 ml with water. For X-ray diffraction studies, 5 mm thick specimens from the bottom portion of each ingot were .used. X-ray diffractograms were taken on Siemens X-ray difTractometer, model D-500 using CrKG, radiation. X-ray and metallographic studies were made in the as-cast, air-quenched and double-tempered conditions. Macrostructures of columnar Alnico alloys in as-cast conditions. Alloy: (a) I. (b) 2, (c) 3. (d) 4, (e) 5. 1 239 :f> • -,x;';- Cb Figure 6 Microstructures of quenched niobium-cOntainin (Alloy 3), (a) equiaxed, (b) columnar. For TEM studies, a standard carbon extraction repli-ca technique was used for samples in the heat-treated condition. The magnetic measurements were made using Walker's Hysteresisgraph, model MH-3020. The sam-ples for measurement were taken from the central portion of the columnar regions where the grains were nearly parallel. 3. Results and discussion Table I gives the nominal composition of the alloys studied. Alloy 1 corresponds to the conventional Alnico V, and the others contain varying amounts of niobium. X-ray diffraction patterns of a few quenched and tempered alloys are shown in Fig. 3. The macrostructures of the cast alloys (Fig. 4) reveal that (a) columnar crystals are quite thick and (b) the length of the columnar crystals formed is maximum when the niobium content is 1%. Fig. 5a shows the microstructure of Alloy 1 containing no niobium, cooled in blast of air from 1250 'C. The structure shows predominantly single phase (a) with some pre-cipitation of a second phase (probably y phase). X-ray diffraction studies have also shown the presence of y Figure 5 Microstructures of columnar alloy containing no niobium (Alloy 1), (a) quenched in blast of air and (b) tempered. 1240 phase. Fig 5b shows the microstructure of the s alloy in the fully tempered condition. The struc shows the presence of a, and al  phases with so phase as indicated by X-ray diffraction data. Fi and b show the microstructures of Alloy3 (conta 1 % Nb) in the air-quenched condition, the (equiaxed) and bottom (columnar) portions of ingot, respectively. As can be seen, in both the c there is no precipitation of 7 phase, as evinced by studies. Scanning electron micrographs of Alloy 3 shown in Fig. 7. The aspect ratio of a t  particles i spinodally aged columnar alloy appears to be larger than that of the equiaxed alloy (Fig. 7c an TEM features of Alloy 1 are shown in Fig. 8. Wh the SAD pattern of the alloy containing no nio (Alloy 1) (Fig. 8b) shows the presence of y phase alloy containing niobium (Alloy 3) has • no y p [11]. Table I I gives the magnetic properties o columnar samples from different alloys in the dally aged condition. This table shows that the d able magnetic properties are not yet developed at stage (Stage B). Alloy 1, though, has exhibited coercivity, has shown a lower remanence compared to Alloy 3. Table III and Figs 9 an depict the magnetic properties of the alloys after 114 .Scanning electron micrographs of Alloy 3: (a, c, e) columnar and (b, d, f) equiaxed. (a, b) Homogenized, (c, d) cooled under a magnetic I tempered. ,eatment (Stage C of Fig. 3). These figures also e the influence of niobium on the magnetic ties in the equiaxed (Fig. 9) as well as columnar S (Fig. 10). It is evident, when one compares ith the magnetic properties obtained at Stage B, here is an appreciable improvement after going gh Stage C. From Fig. 9 it can be seen that the remanence, B„ and energy product, (BH)„,,„, values of equiaxed alloys decrease continuously with the addi-tion of niobium. Coercivity, He , remains constant up to 1.5% Nb and then decreases. The decrease in re-manence and hence (BH)max  is attributed to the dilu-tion effect of non-magnetic niobium addition which decreases the saturation induction and hence Br  [12]. 1241 (a) 028 • .116wrwmil Figure 8 (a) Transmission electron micrograph (replica) of Alloy 1, air quenched. (b) The corresponding SAD pattern and indexing. TABLE II Magnetic properties of columnar Alnico alloys after spinodal ageing (Stage B, Fig. 2) Alloy He(kAm -1 ) Br ( T) (BH),,,ar (kJ m" 3  ) 1 33.0 1.0 12.48 2 20.4 1.13 8.96 3 12.3 1.04 4.88 4 26.6 1.0 8.88 5 8.8 0.76 2.48 Beyond 1.5% Nb, the particles are probably of multi-domain size, thus reducing the coercive force. In the columnar specimens (Fig. 10) it may be seen that remanence continuously, but only marginally, decreases from 1.3 T to 1.27 T with the addition of niobium up to 2%, as expected, due to the dilution effect of niobium [10]. The remanence values of col-umnar samples have been found to be much higher than those of the equiaxed ones. The higher values of remanence in columnar alloys may be ascribed to better alignment and distribution of the ferromagnetic a t  phase in a feebly magnetic matrix of a2  phase. The coercivity of columnar alloys is, in general, higher than that of corresponding equiaxed ones. This is attributed to the shape anisotropy of the columnar . (n1) • (311) (331) (000) alloys. The coercivity in columnar alloys was fou increase with increasing niobium content and is imum at 1% Nb (Fig. 10) [10]. Beyond this poin decreases. The maximum Plc  at 1% Nb may be d the reduction in number of domains in the a, p Dispersion of the ferromagnetic a t  phase in the in of a2  phase reduces the demagnetizing field and h the increase in the coercive force. The adverse effe niobium beyond 1% is probably due to the form of multidomain a, particles. As the He  is maxi and the remanence more or less constant at 1°/ one would expect the highest value of (BH) 1 % Nb:  The maximum value in (BH)max, in fact been observed at 1% Nb (Fig. 10). Some det characteristics of the alloy containing 1% Nb (All will now be discussed. Fig. 11 shows the demagn ation and energy product behaviour for Alloy 3. shape of the demagnetization curve shows tha anisotropy has been achieved to the desirable. e taking into account the fact that the coercive for the remanence obtained in the alloy are both se -ably high. From the XRD one finds mostly .th sence of a phase, with some intermetallics su (020) • ( b) TABLE III Magnetic properties of Alnico alloys after final tempering (Stage C of Fig. 2) Alloy Equiaxed  Columnar     H, Br (BH)max H Br (BH)max (kA rn -  ) (T) (kJm -3 ) (kA m -1 ) (T) (kAm- 3  ) 56.5 1.05 26.96 55.7 1.3 40.0 2 56.5 0.95 24.0 61.3 1.29 46.0 3 55.3 0.90 20.72 64.5 1.28 55.0 4 57.3 0.88 19.36 60.5 1.28 48.0 5 42.2 0.85 13.52 56.5 1.27 42.0 1 242 - 1.0 6 80 co"  0.8 a^  64 0.6 .1) 48 0 0.4 32 - •  \- 2 0 1 % Nb ( a) 0/o Nb     56 48 x O — 40 cO  24 16        32     • % Nb 2 0 (b) 1 Nb 2 Effect of niobium on the magnetic properties of equiaxed Figure 10 Effect of niobium on the magnetic properties of columnar alloys. - 1.4 •  - 1.2 - 1.0 - 0.8 - 0.6 ►Is  co - 0.4 - 0.2 • t . 64 48 32 16 0 8 16 24 32 40 48 56 -H (kA rn-1 ) BH (kJ m-3) it Demagnetization and energy product curves of alloy 3 (columnar) after final tempering. 1243 NiFe in the as-cast as well as Stage A (Fig. 3) condi-tions. However, after Stage C, one observes primarily oc1  and a, phases. Further improvement in the mag-netic properties, i.e. (BH)max , can only be expected through either improvement in He  (by addition of elements like titanium) or improvement in remanence (through better alignment of the columnar grains) or both. 4. Conclusions 1. Niobium addition prevents the precipitation of undesirable 7 phase. 2. The length of the columnar crystals is maximum at 1% Nb. More than 1% Nb addition decreases the length of the crystals. 3. Remanence decreases with addition of niobium. 4. Coercive force increases with increase in niobium content and at 1`)/0 Nb it reaches a maximum. Further addition of niobium decreases the coercive force. 5. The energy product is maximum at 1% Nb. Acknowledgements The authors thank Professor S. Banerjee, Director, National Metallurgical Laboratory, Jamshedpur, for his permission to publish this paper, and Mr C. R. Tewari, Scientist, N.M.L., for fruitful discussions. The services rendered by our colleagues of the Mel Services and Electron Miscroscopy Sections are gratefully acknowledged. References I. M. McCAIG, Proc. Phys. Soc. B62, (1949) 652. 2. K. J. De VOS "Alnico Permanent Magnet Alloys", in "M tism and Metallurgy", Vol. 1 (Academic Press, New London, 1969) pp. 473-512. 3. C. D. GRAHAM Jr, "Textured Magnetic Materials", in" netism and Metallurgy", Vol. 2 (Academic Press, New London, 1969) pp. 723-48. 4. J. E. GOULD Proc. Inst. Elec. Eng. 106A (1959) 493. 5. !dem, Cobalt 23 (1964) 82. 6. ANON, ibid. 18 (1963) 26. 7. A. 1. LUTEIJN and K. J. de VOS. Phil. Res. Rep. 11 489. 8. W. S. MESAKIN and B. E. SOMIN Archiv Eisen. 8 ( 315. 9. A. J. BRADLEY and A. TAYLOR, Proc. Roy. Soc. (1938) 353. 10. S. SZYMURA and S. GOLBA J. Magn. & Magn. Mate (1981) 285. 11. SHI MING HAO, K. ISHIDA and T. NISHIZAWA, Trans. Al6 (1985) 179. 12. C. BRONNER, J. P. HABERER, E. PLANCHARD an SAUZE, Cobalt 46 (1970) 15. Received 15 October 1991 and accepted 25 June 1992 

Effect of niobium on the directional solidification and properties of alnico alloys

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Effect of niobium on the directional solidification and properties of alnico alloys

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