The growth of pure and modified bismuth-manganese alloys

880

Journal of Crystal Growth 76 (1986) 880-884 North-Holland, Amsterdam

THE GROWTH OF PURE AND MODIFIED BISMUTH-MANGANESE ALLOYS M.A. SAVAS and R.W. SMITH Department of Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6

The Bi-MnBi eutectic is recognised for its remarkable magnetic properties and recently considerable attention has been devoted to the effects of constitutional undercooling, magnetic field and low gravity conditions on its growth behaviour. Its growth mechanism and the details of the resultant microstructure are, however, still unclear, yet an understanding of these is essential in order to produce the desired optimum properties. The work reported here is part of a more general investigation of eutectics with a high entropy of solution (faceting) phase as the major constituent. The effects of temperature gradient GL, growth rate R, and small additions of sodium on the morphology of the MnBi phase have been examined. The growth form of the MnBi primary near the eutectic composition was also studied in order to gain a better insight into the eutectic growth behaviour. In all alloys examined, the morphology of the MnBi phase was influenced by the growth rate and the addition of sodium.

1. Introduction Since Brady [1], various attempts have been made to classify the almost endless variety of eutectic microstructures. One of the more fundamental attempts was presented in 1973 by Croker, Fidler and Smith [2]. Following the earlier solid/liquid interface structure theory of Jackson [3], their classification scheme places the binary metallic eutectics into six morphological groups as a function of entropy of solution (AS,O and the volume fraction of the minor phase (l f), fig. 1. A AS~ value of 5.5 c a l / K , mol (23 J / K - m o l ) separates the non-faceted/non-faceted (normal) eutectics from the faceted/non-faceted (anomalous) eutectics. Whether the normal eutectic morphology is lamellar or rod-like is determined by the volume fraction of the minor phase, i.e., groups 1 and 2 in fig. 1. The anomalous eutectics exhibit more microstructural variety with increasing volume fraction of the faceting phase; they may be broken-lamellar, irregular, complex-regular or quasi-regular (groups 3, 4, 5, and 6 respectively). Very little has been reported about the quasi-regular eutectics. The outstanding characteristic of these systems is that although they belong to the anomalous group, they display nearly regular microstructures similar to those of normal eutectics. Quasi-regularity is believed to be due to

the fact that, being the eutectic matrix, the normally faceting phase does not facet during coupled eutectic growth. It has been proposed that, since it completely surrounds the minor phase, the matrix must grow with a concave solid/liquid interface which provides many re-entrant corners where easy atomic addition can be made from the melt [4,5]. Those sections of the present investigation published earlier [6,7] were concerned with eutectics containing relatively large volume fractions of the minor phase in a bismuth matrix. In the Bi-MnBi eutectic the volume fraction of the MnBi (minor) phase is only about 3 or 4% [8,9]. The purpose of the present study was to examine the changes in eutectic morphology resulting from impurity additions and variations in the growth rate.

2. Experimental procedure The bismuth and manganese metals used in alloy preparation were respectively of 99.99% purity. Table 1 lists the compositions considered in this investigation. The eutectic and hypereutectic (manganese-rich) alloys were prepared by placing weighed amounts of each component into 4 mm Vycor tubes which were then sealed under a partial pressure of argon gas. The charge was melted

0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

M.A. Savas, R. 14/. Smith / Growth of bismuth - manganese alloys

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at 850°C in a tube furnace and, to ensure homogeneity, the molten alloys were left in the furnace for 12 h and shaken periodically. The eutectic alloys were then transferred to slightly larger diameter Pyrex tubes and grown unidirectionally using the vertical Bridgman technique as described previously [7]. The hypereutectic alloys were examined in the as-cast condition. Chemical analysis indicated that the specimens were homo-

Table 1 The compositions of the bismuth-manganese alloys examined in the present study (phase diagram information according to Moffatt [10]) Alloy

Manganese (wt%)

Eutectic Hypereutectic Hypereutectic

0.6 1.5 3.0

geneous and had t h e desired compositions. In order to achieve chemical modification, weighed amounts of sodium were added to both eutectic and hypereutectic alloys. Because of its high reactivity the sodium was not included during initial alloy preparation but instead was added afterwards at a temperature of 450 ° C. The metallographic samples were polished mechanically and examined in the unetched condition.

3. Results and discussion

3.1. The morphology of the MnBi primary phase As described generally in the introduction, faceting of the bismuth matrix in the Bi-MnBi eutectic is believed not to be possible due to a

882

M.A. Savas, R. W. Smith / Growth of bismuth-manganese alloys

growth interface curvature effect, despite its relatively high entropy of solution value. In theory it is possible to determine the faceting tendency of the MnBi phase through entropy of solution calculations, yet this is made difficult because of the complicated crystallography of the intermetallic phase. Information on the faceting characteristics can, however, be deduced from an examination of the shapes of the MnBi primaries in alloys slightly removed from the eutectic composition [2]. When the hypereutectic castings were solidified at relatively high cooling rates (4°C/s), a large proportion of the MnBi primaries displayed growth forms similar to distorted and fragmented idiomorphic crystals. Fig. 2a illustrates this for the 1.5 wt% Mn alloy. The MnBi phase has a CPH lattice structure [11,12], with the hexagonal symmetry reflected in the morphology of some of the primaries seen in fig. 2a. An increase in the manganese content to 3.0 wt% leads to further supersaturation with respect to bismuth which results in morphological instability of the MnBi primaries. This instability is responsible for the transition to a dendritic growth form, fig. 2b. These dendrites display typical faceted growth characteristics, i.e., the dendrite arms are enclosed by flat faces which join at sharp corners, and they also contain "holes" which are not uncommon in faceted crystals [13,14]. It is known that the alkali metals are chemisorbed strongly on bismuth, with sodium modification of the complex-regular eutectics Bi-Pb2Bi, Bi-T12Bi and Bi-Sn being reported by Baragar et al. [15]. Although no evidence could be found in the literature for the chemisorption of alkali metals on MnBi, the effect of sodium interaction with this phase may be inferred from microstructural changes following sodium additions. Fig. 2c is a photomicrograph of the 3.0 wt% Mn alloy containing 0.05 wt% Na. It is generally accepted that chemisorption may result in a change in the relative growth rates of various crystal planes, often leading to additional facets at the solid/liquid

Fig. 2. The growth forms of the MnBi phase in hypereutectic as-cast alloys (MnBi appears as the dark phase in all photomicrographs): (a) Bi-l.5 wt% Mn; idiomorphicMnBi primaries;

(b) Bi-3.0 wt%Mn; faceted MnBi dendrites; (c) Bi-3.0 wt% Mn containing0.05 wt% Na; modifiedMnBi dendrites.

883

M.A. Savas, R. W. Smith / Growth of bismuth-manganese alloys

interface [13,14]. In fig. 2c it can be seen that the influence of sodium is effectively to introduce another facet at the dendrite arm tip, resulting in a more equiaxed phase morphology. Similar results were reported by Fredriksson et al. [13] in their study of impurity chemisorption onto silicon primaries in hypereutectic A1-Si melts.

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3.2. The growth of the B i - M n B i eutectic

F r o m the growth form of the primary crystals it is apparent that the MnBi phase does facet close to the eutectic composition. This is not unexpected considering that the MnBi phase grows from a very dilute (about 0.6 wt% Mn) melt. This dilution effect also promotes faceting in a number of m e t a l - m e t a l eutectics, e.g., the P b - A g [5] and S n - Z n [14] systems. Work by Ravishankar et al. [16] ascribes a faceted rod structure to the eutectic MnBi phase in alloys solidified in the range R = 10 -2 to 10 -3 c m / s . According to Croker et al. [2], however, the eutectic morphology should be broken-lamellar (since the MnBi phase is faceting and has a volume fraction of about 3-4%) corresponding to Region 3 of fig. 1. In order to examine this apparent discrepancy, alloys of eutectic composition were solidified over a wide range of growth rates, from 5.0 X 10 - 4 to 5.0 × 10 .3 c m / s . Over this range of growth rates (at a temperature gradient of 4 5 ° C / c m ) two different types of structure and produced, viz.: (i) When grown at R = 5.0 × 10 -3 c m / s e c an extremely fine faceted rod-like MnBi phase is produced, fig. 3. This is similar to that observed by Ravishankar et al. [16]. (ii) Smaller growth rates of R = 5.0 × 10 - 4 c m / s result in a broken-lamellar morphology, fig. 4. These results indicate that the Bi-MnBi eutectic m a y indeed display a broken-lamellar growth morphology as predicted by the analysis of Croker et al. [2]. The rod-like morphology obtained at relatively high solidification rates (fig. 3) is believed to be a result of growth modification; similar modification behaviour has been observed in m a n y faceted/non-faceted eutectic systems, perhaps the most notable being the flake-to-fiber transition in A1-Si [13,14,17].

Fig. 3. The rod-like morphology of the Bi-MnBi eutectic, GL = 45oC/cm and R = 5.0 × 10- 3cm/s, transverse section.

Fig. 5 shows the result of a 0.05 wt% N a addition on the eutectic grown at 5.0 × 10 - 4 c m / s . Comparison of this photomicrograph with that of the pure binary eutectic shown in fig. 4 indicates that the addition of sodium results both in a broken-lamellar to rod-like transition as well as a refinement of the overall structure. For magnetic applications it is likely that a rod-like morphology will give the optimum properties [18]. The present work indicates that the rod-like structure can be

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M.A. Savas, R. W. Smith / Growth of bismuth- manganese alloys

Acknowledgements This s t u d y is p a r t of a general investigation of p h a s e t r a n s f o r m a t i o n s which has b e e n in progress in the l a b o r a t o r y of R W S for m a n y years. F i n a n cial s u p p o r t has b e e n received f r o m m a n y source b u t p a r t i c u l a r l y f r o m the N a t i o n a l Science a n d E n g i n e e r i n g R e s e a r c h C o u n c i l of C a n a d a a n d f r o m Q u e e n ' s University. M.A.S. has b e e n in receipt of an A l c a n fellowship.

References

Fig. 5. The rod-like morphology of the Bi-MnBi eutectic containing 0.05 wt% Na, G L = 45°C/cm and R = 5.0×10 4 cm/s, transverse section. o b t a i n e d over a very wide range of solidification conditions, n a m e l y t h r o u g h g r o w t h m o d i f i c a t i o n at R > 1 0 - 3 c m / s . a n d b y s o d i u m m o d i f i c a t i o n at s m a l l e r g r o w t h rates.

4. Conclusions T h e i m p o r t a n c e of the B i - M n B i eutectic has b e e n recognised for some time, yet the i n f o r m a tion available on the factors which d e t e r m i n e its m i c r o s t r u c t u r e is incomplete. T h e s e m u s t b e und e r s t o o d in o r d e r to m a n i p u l a t e the microstructure at will a n d so p r o d u c e d e s i r e d properties. This s t u d y has shown that: (1) T h e eutectic m o r p h o l o g y is g r o w t h rate d e p e n dent, v a r y i n g f r o m b r o k e n - l a m e l l a r to rod-like. (2) T h e r o d - l i k e m o r p h o l o g y m a y be p r o d u c e d at large g r o w t h rates, a n d a similar structure can also b e o b t a i n e d at small solidification rates b y the a d d i t i o n of s o d i u m (i.e., chemical or i m p u r i t y m o d i f i c a t i o n ) . T h e m o d i f i c a t i o n b e h a v i o u r of this eutectic is similar to that of o t h e r f a c e t e d / n o n f a c e t e d eutectics (e.g., A I - S i ) .

[1] F.L. Brady, J. Inst. Metals 28 (1922) 369. [2] M.N. Croker, R.S. Fidler and R.W. Smith, Proc. Roy. Soc. (London) A335 (1973) 15. [3] K.A. Jackson, in: Liquid Metals and Solidification (ASM, Cleveland, OH, 1958) p. 174. [4] J.D. Hunt and D.T.J. Hurle, Trans. AIME 242 (1968) 1043. [5] M.A. Savas, PhD Thesis, Queen's University, Kingston, Ontario (1985). [6] M.A. Savas and R.W. Smith, J. Mater. Sci. 20 (1985) 881. [7] M.A. Savas and R.W. Smith, J. Crystal Growth 71 (1985) 66. [8] W.M. Yim and E.J. Stofka, J. Appl. Phys. 38 (1967) 5211. [9] R.G. PiNch and D.J. Larson, J. Appl. Phys. 50 (1979) 2425. [10] W.G. Moffatt, The Handbook of Binary Phase Diagrams, Vol. 1 (Genium, New York, 1984). [11] B. Ciszewski, J. Kozubowski, T. Patej and J. Sadowski, M6m. Sci. Rev. M6t. 69 (1972) 159. [12] E.M. Savitsky, R.S. Torchinova and S.A. Turanov, J. Crystal Growth 52 (1981) 519. [13] H. Fredriksson, M. Hillert and N. Lange, J. Inst. Metals 101 (1973) 285. [14] R. Elliot, Euteetic Solidification Processing: Crystalline and Glassy Alloys (Butterworths, London, 1983). [15] D. Baragar, M. Sahoo and R.W. Smith, J. Crystal Growth 14 (1977) 278. [16] P.S. Ravishankar, W.R. Wilcox and D.J. Larson, Acta Met. 28 (1980) 1583. [17] M.G. Day and A. Hellawell, Proc. Roy. Soc. (London) A305 (1968) 473. [18] R.G. Pirich and D.J. Larson, in: Materials Processing in the Reduced Gravity Environment of Space, Ed. G.E. Rindone (North-Holland, New York, 1982) p. 523.