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Cast-in crystals in amorphous alloy ribbons
Materials Science and Engineering, 49 (1981) 47 - 56
47
Cast-in Crystals in Amorphous Alloy Ribbons J. L. WALTER, J. D. LIVINGSTON and A. M. DAVIS Metallurgy Laboratory, General Electric Co., Research and Development Center, Schenectady, NY 12301 (U.S.A.) (Received October 14, 1980; in revised form December 15, 1980)
SUMMARY
Cast-in crystals were observed in amorphous alloy ribbons o f Co-B, Fe-B, N i - B , ( F e , N i ) - B and (Fe, C o ) - B prepared by the chill-blockcasting technique. Their presence was determined by optical microscopy, microradiography, X-ray diffraction, scanning and transmission electron microscopies and Bitter and Kerr domain observation techniques. The cast-in crystals are often very large, indicating growth velocities greater than 1 cm s 1 and growth temperatures greater than 1000 °C. Most o f the crystals are o f the expected stable phase but in the F e - B series the cast-in crystals are o f the metastable bodycentered tetragonal Fe 3B phase. 1. INTRODUCTION Amorphous alloy ribbons are c o m m o n l y prepared by the chill-block-casting technique in which a stream of molten alloy is ejected o n t o a rapidly moving chill surface. Alloy ribbons that are partly or fully crystalline can also be prepared by this technique, either directly from casting or by heating of an initially amorphous ribbon. Such ribbons may exhibit unusual physical properties because the high quenching rates of chill block casting can produce microstructures and, in some cases, metastable phases n o t attainable by ordinary casting techniques. Although the crystallization of amorphous alloys by heating has received considerable attention, there has been little discussion of the cast-in crystals that sometimes occur in amorphous ribbons. Whether they are a desired or an undesired part of the final p r o d u c t it is important to characterize these crystals and to understand their formation. We report here observations on cast-in crystals in a n u m b e r of transition metal-B 0025-5416/81/0000-0000/$02.50
alloys. Since cast-in crystals in small volume fractions often escape detection by conventional X-ray techniques, we emphasize a variety of supplementary experimental techniques that have been found to be useful.
2. EXPERIMENTAL PROCEDURE Ingots of Co-B, Fe-B, Ni-B, (Fe,Ni)-B and (Fe,Co)-B were prepared by melting and chill casting under argon in an induction furnace. The starting materials were vacuummelted cobalt, vacuum-melted electrolytic iron, Ni 270 and crystalline boron of 99.8% purity. The ingots were then crushed and used as starting materials in the preparation of alloy ribbons by chill block casting [1]. A copper drum of diameter 10 or 15 cm was used, with a surface speed of about 30 m s-1. The resultingribbon was about 2 7 p m thick and 1 - 2 mm wide. Ribbons were routinely examined by optical microscopy and X-ray diffraction, including both Debye-Scherrer and pinhole transmission diffraction. The pinhole transmission diffraction technique, while covering a similar area of sample, is more sensitive than the D e b y e Scherrer technique in detecting cast-in crystals. X-ray diffraction analysis of residues after etching also proved to be useful. Selected samples were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), microradiography and Bitter and Kerr domain observation techniques.
3. EXPERIMENTAL RESULTS
3.1. C o - B In some cases, cast-in crystals as large as 100 pm are observed, as in the Co74B26 ribbon shown in Fig. 1. Such crystals generally © Elsevier Sequoia/Printed in The Netherlands
48 TABLE1
Fig. 1. Crystals on the surface of Co74B26 ribbon.
have depressed centers on the air side of the ribbon, suggesting that crystal growth terminated on this surface while the ribbon was still liquid. Such crystals are also usually associated with depressed areas, generally much larger than the crystals themselves, on the drum side of the ribbon. These areas result from entrapped air pockets at the m e l t - d r u m interface. The resulting reduction in local quenching rate apparently contributes to the nucleation and growth of cast-in crystals. Debye-Scherrer analysis of this and other Co-B ribbons failed to reveal the presence of cast-in crystals in amounts less than about 5 vol.%, although they could be revealed both by optical microscopy and by measurements of the temperature coefficient of resistivity [ 2]. The crystals were identified by D e b y e Scherrer analysis of the residue remaining after electrolytically thinning the ribbon in 7% hydrochloric acid-methanol, which preferentially dissolves the amorphous matrix. Such residue analysis for a series of Co-B ribbons identified the cast-in crystals as tetragonal Co2B for CoToB3o and Co72B2s , as orthorhombic Co3B for C075B25 , C077B23 and Cos0B20, and as a mixture of Co3B and f.c.c. cobalt for Cos4Ble. The crystal structures of the crystals examined in all alloys are given in Table 1. Contact microradiography was applied to each of the above alloys after etching to remove some of the amorphous matrix and to expose the crystals. In this technique a photographic emulsion is placed in direct contact with the ribbon surface and an image is obtained by differential absorption of the X-rays.
Alloy composition
Crystal structure
Co84B16 Co80B20 C077B23 Co75B25 Co72B28 Co70B30 Fe76B24 Fe75B25 Fe75B25
Orthorhombic Co3B and f.c.c. Co Orthorhombic Co3B Orthorhombic Co3B Orthorhombic Co3B Tetragonal C02B Tetragonal Co2B B.c.t. a Fe3B B.c.t. a as cast B.c.t. a Fe3B and b.c.t, a Fe2B + a-Fe as annealed F.c.c. (Fe,Ni)23B 6 F.c.c. (Fe,Ni)23B 6 Orthorhombic (Fe,Co)3B Orthorhombic (Fe,Co)3B Orthorhombic Ni4B 2
Fe43Ni43B14 Fe39Ni39B22 Fe38Co38B24 Fe44Co44B12 Ni62B38
aB.c.t., body-centered tetragonal.
Microradiographs of as-cast Co80B20 and Co70B30 ribbons are shown in Fig. 2. The light regions are the crystals.
(a)
(b) Fig. 2. (a) A microradiograph of crystals in CosoB20 ribbon; (b) a microradiograph of crystals in Co70B30 ribbon.
49
The difference in magnetic properties between cast-in crystals and the amorphous matrix can also be useful in characterizing such crystals. Orthorhombic CoaB has a strongly preferred crystallographic axis of magnetization. When this axis has a component normal to the surface, the emerging fields strongly attract the ferrofluid used in the Bitter domain observation technique as seen in Fig. 3(a) which shows crystals in Co76B24 ribbon. On a mechanically polished surface these emerging fields also result in strong domain contrast in polarized light by the polar Kerr effect (Fig. 3(b)). The cast-in Co2B crystals in the upper boron ribbons neither attract ferrofluid nor show Kerr contrast. This results from the easy-plane magnetic s y m m e t r y of Co2B, which allows the magnetization to remain parallel to the specimen surface. Domain patterns such as those in Figs. 3(a) and 3(b) are determined by the orientation of the magnetic easy axis of CoaB. Thus Bitter and Kerr techniques can be helpful n o t only in identifying the phases of cast-in crystals but also in determining their orientation. TEM of as-cast Co75B25 ribbon reveals Co3B crystals with widely varying microstructures. Some contain fault-like features (Figs. 4(a) and 4(b)), some contain cellular features (Figs. 4(c) and 4(d)) and some contain an extensive ultrafine structure (Figs. 4(e) and 4(f)). The microstructure of CoaB crystals in Co78B22 is alSO different both for cast-in crystals (Figs. 5(a) - 5(d)) and for crystals f o r m e d on heating to 709 K (Figs. 5(e) and 5(f)). The diffraction pattern of the crystals f o r m e d on heating (Figs. 5(e) and 5(f)) shows highly faulted crystals, and these crystals are reminiscent o f FeaB crystals f o r m e d by heating Fes0B20 and FesoNiaoB20 ribbons [3]. The heating of amorphous ribbons containing cast-in crystals leads both to the formation of fine crystals from the amorphous phase and to the further growth of the large cast-in crystals (Fig. 6(a)). This layer of new growth can also be seen from the microradiography (Fig. 6(b)) and SEM (Fig. 6(c)) of ribbons in which the matrix had been partly etched away. 3.2. F e - B As reported elsewhere [4], F e - B ribbons with less than 14% B or more than 21% B were found to contain cast-in crystals. The
4o~ ,
(a)
(b) Fig. 3. (a) Magnetic domains in crystals in Co76B24 ribbon using the Bitter technique; (b) magnetic domains revealed by the Kerr technique in crystals in Co76B24 ribbon.
low boron ribbons contained round a-Fe crystals which were quite different in morphology from the dendritic a-Fe crystals produced on heating these hypoeutectic alloys [ 5]. The high boron ribbons contained cast-in crystals of metastable tetragonal FeaB which attracted ferrofluid strongly (Fig. 7(a)). The use of dark field microscopy reveals magnetic domain patterns in the surrounding amorphous matrix (Fig. 7(b)). Interpretation of these patterns [6] indicates that the matrix is under compression in the direction of the c axis of the FeaB crystal and is under tension in directions normal to the c axis. These stresses presumably result from anisotropic differential thermal contraction between the matrix and the crystal during cooling; the crystal contracts more along the a axis, but less along the c axis, than the matrix does.
50
o •
t it
0 O
(a)
(b)
•
(c)
•
(d)
0 ,
e •
• t
(e)
*
•
(f)
Fig. 4. TEM micrographs and selected area diffraction patterns of Co3B crystals in Co75B25 ribbon with (a), (b) fault-like features in a (746) crystal plane, (c), (d) cell-like features in a (201) crystal plane and (e), (f) ultrafine structure in a (114) crystal plane.
51
o
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 5. TEM micrographs and electron diffraction patterns of Co3B crystals oriented in the (001), (74i') and (452) planes: (a) - (d) cast Co78B22 ribbon; (e), (f) ribbon heated to 709 K and quenched.
52
(a)
(b)
(c) Fig. 6. (a) Optical micrographs of cast-in crystals in longitudinal sections of Co80B20 ribbon showing growth caused by heating; (b) a microradiograph of a crystal in Co70B30 ribbon heated to 735 K and quenched; (c) an SEM micrograph of a crystal in Co70B30 ribbon heated to 735 K and quenched.
(a)
(b)
Fig. 7. Magnetic domains in Fe76B24 ribbon: (a) in crystals revealed by the Bitter technique; (b) in the adjacent amorphous matrix revealed by dark field microscopy.
53
From TEM the cast-in tetragonal Fe3B crystals are usually nearly featureless [7], although some heavily structured finer FesB crystals are found imbedded within the larger crystals (Fig. 8(a)). These heavily structured finer crystals decompose to the equilibrium Fe2B and ~-Fe phases on heating to 773 K (Fig. 8(b)), whereas the larger featureless crystals do not decompose until much higher temperatures. Fe3B crystals formed from the amorphous phase in Fe75B25 and Fe76B24 by heating also appear to be featureless. Heating Fes4Bi6 produces primary ~-Fe particles and featureless FesB [5]. However, heating Fes0B20 produced only FeaB crystals with an ultrafine but strong columnar microstructure [3].
3.3. (Fe,Ni)-B TEM of as-cast Fe43Ni43B14 and Fe39Ni39B22 ribbons indicate that most cast-in crystals have the f.c.c. (Fe,Ni)zsB6 structure (isostructural with Cr23C6). Some crystals are essentially featureless while others, sometimes even adjacent crystals, appear to be highly faulted [7]. In addition to very large cast-in crystals, as discussed above for Co-B and Fe-B, there were also numerous finer crystals (Fig. 9). An area originally thought to be amorphous (Fig. 10(a)) was found, from its diffraction pattern {Fig. 10(b)) and dark field microscopy (Fig. 10(c)), to be microcrystalline with an average grain size of about 250 £. 3.4. (Fe, Co)-B Ribbons of Fe38Co38B24 contain cast-in crystals of orthorhombic (Fe,Co)3B with no microstructural features. However, ribbons of Fe44Co44Bz2 contain cast-in crystals, identified as (Fe,Co)aB by diffraction analysis, that contain a pronounced substructure. One such crystal appears in Fig. 11, in which the diffraction pattern indicates a spread of about 12 ° about the [311] normal to this crystal. 3.5. N i - B In the alloys discussed so far, most cast-in crystals (both large and small) appear to be relatively unfaceted. In contrast, some of the cast-in crystals in Ni62B3s ribbon are hexagonal in form and contain a dendritic substructure which is very similar to a snowflake (Fig. 12). These crystals were identified from X-ray diffraction as orthorhombic NiaB3.
(a)
O
(b)
F'ezB ÷ aFe
(c) Fig. 8. (a) A TEM mierograph and (b) electron diffraction pattern of Fe3B crystals (featureless and microstructured) in cast Fe75B25; (c) featureless and decomposed (Fe2B and ~-Fe) crystals in Fe75B25 heated to 773 K.
54
(a)
(a)
W
O
(b)
(b)
(c)
(c)
Fig. 9. (a) A TEM micrograph of large (left-hand side) and fine (Fe,Ni)23B 6 crystals in Fe43Ni43B14 ribbon; (b) an electron diffraction pattern of the large (111) crystal plane area; (c) an electron diffraction pattern of the fine crystal area.
Fig. 10. (a) A TEM micrograph of Fe43Ni43B14 ribbon containing crystals of average size 250 •; (b) an electron diffraction pattern of the area in (a); (c) a dark field micrograph of the area in (a).
55 4. DISCUSSION
(a)
(b) Fig. 11. (a) A TEM micrograph of a large crystal with substructure in Fe44Co44B12 ribbon; (b) an electron diffraction pattern of the (311) plane of the crystal in (a).
Fig. 12. An optical micrograph of crystals on the surface of Ni62B3s ribbon.
For each alloy and crystallization mode the rate of crystallization will go through a maxim u m with decreasing temperature, producing the familiar C curve [8]. When crystallization occurs on heating an amorphous alloy, it occurs on the lower portion of this C curve. However, most cast-in crystals grew on the upper portion of the C curve, at much higher temperatures and velocities than the crystals formed on heating. If we estimate from known cooling rates [1], large cast-in Fe3B crystals such as those shown in Fig. 7 grew in times of less than 1 0 - 2 s, and hence at growth velocities greater than 1 cm s-1. By extrapolating data from Herold and KSster [9] for Fe3B growth at lower temperatures, such velocities indicate that these crystals grew at temperatures over 1000 °C. The same is probably true for the large (greater than 10 pm) cast-in crystals observed in the other alloys studied. Their association with air pockets on the drum side of the ribbon indicates that their formation was aided by a local lowering of the cooling rate. However, only a small fraction of the air pockets had associated cast-in crystals, indicating that crystal formation was limited by a shortage of nucleation sites. The much smaller cast-in crystals sometimes observed, such as those in Fig. 9 or Fig. 10, probably formed at much lower temperatures on the lower portion of the C curve. The high initial cooling rates characteristic of chill block casting are maintained only while the ribbon maintains contact with the drum. Even when such initial cooling is sufficiently rapid to avoid the nose of the C curve, the much slower cooling that occurs after leaving the drum can allow crystallization to occur at lower temperatures. Herold and KSster [9] have distinguished between three different crystallization modes in alloys such as these: (1) p o l y m o r p h o u s crystallization (single-phase growth with no composition change); (2) primary crystallization (single-phase growth with rejection of solute); (3) eutectic crystallization (simultaneous two-phase growth). Most of the castin crystals are probably examples of polymorphous crystallization, the crystallization m o d e that requires the least a m o u n t of diffusion.
56 In most of the alloys studied, the phase of the castAn crystals is present in the equilibrium phase diagram, although it often formed at a stoichiometry that was far from the equilibrium stoichiometry. The exception is Fe-B where the tetragonal FeaB phase is a metastable phase. This phase forms on crystallization of heated amorphous ribbons [3, 5] but, on further heating to 600 °C or above, decomposes to the equilibrium Fe2B and ~-Fe phases [9]. It is interesting that the cast-in crystals grow with the metastable Fe3B structure at temperatures well in excess of these decomposition temperatures. Apparently the short times involved favored polymorphous crystallization in the same way t h a t lowered diffusion rates favor this mode at lower temperatures. The presence of FesB crystals in the cast ribbon might be expected since it appears that the chemical short-range order of the amorphous Fe75B25 is similar to the short-range order of crystalline FeaB [5, 10]. It has been suggested that this unique ordering may extend to the Fe-B melt [5] which could account for the preference for the cast-in metastable Fe3B crystals over the stable Fe2B crystals which grow from the melt at lower cooling rates [4]. Since cast-in crystals and crystals formed on heating grow under very different conditions, it is n o t surprising that their microstructures, as observed by TEM, are often different. The high growth rates themselves m a y sometimes lead to faulted structures but faults may also be caused by off-stoichiometric compositions. Slight deviations from stoichiometry can be accommodated by faults t h a t do n o t preserve the composition, such as crystallographic shear planes [11]. It may sometimes be difficult to distinguish between such faults and fine second-phase regions, i.e. fine eutectic crystallization. Some of the highly structured crystals often seen, such as those in Figs. 5 and 11 and the FeaB crystals in heated FesoB2o ribbon [3], may in fact be two phase on a very fine scale, even though the minority metallic phase was n o t detected in diffraction patterns. The small structured FeaB crystals seen in Fig. 8(a) may contain such fine iron inclusions
which would provide easy nucleation sites for the decomposition to the equilibrium Fe2B and a-Fe phases (Fig. 8(b)). Finally, we note the utility of a broad variety of experimental techniques in detecting and characterizing cast-in crystals. X-ray diffraction of ribbons or etching residue, optical microscopy, TEM, SEM microradiography and resistivity measurements all may contribute helpful information. Where the phases formed are ferromagnetic, Bitter and Kerr domain observation techniques are also useful.
ACKNOWLEDGMENTS
Optical microscopy was performed by C. R. Rodd and TEM was performed by E. F. Koch, Inorganic Materials and Structures Laboratory.
REFERENCES
1 J. L. Walter, Proc. 3rd Int. Conf. on Rapidly Quenched Metals, Brighton, 1978, Vol. 1, Metals Society, London, 1978, p. 30. 2 A. Mogro-Campero and J. L. Walter, GE Rep. 80CRD073, 1980 (General Electric); J. Phys. (Paris), 41 (C8) (1980) 497 - 500. 3 J. L. Walter, S. F. Bartram and R. R. Russell, Metall. Trans. A, 9 (1978) 803. 4 J. L. Walter, J. Non-Cryst. Solids, in the press. 5 J. L. Walter, S. F. Bartram and I. Mella, Mater. Sci. Eng., 36 (1978) 193. 6 J. D. Livingston, Phys. Status Solidi A,56 (1979) 637. 7 J. L. Walter, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Proc. 2nd Int. Conf. on Rapid Solidification Processing, VA, March 1980,
Claitor's Publication Division, Baton Rouge, LA, 1980, p. 294. 8 H. A. Davies, Phys. Chem. Glasses, 17 (1976) 159. 9 U. Herold and U. KSster, in B. Cantor (ed.), Proc. 3rd Int. Conf. on Rapidly Quenched Metals, Brighton, 1978, Vol. VI, Metals Society, London,
1978, p. 281. U. KSster and U. Herold, Metallic Glasses, Springer, Berlin, 1980. 10 I. Vincze, D. S. Boudreaux and M. Tegze, Phys. Rev. B, 19 (1979) 4896. 11 J. S. Anderson, J. Chem. Soc., Dalton Trans., 10 (1973) 1107.
47
Cast-in Crystals in Amorphous Alloy Ribbons J. L. WALTER, J. D. LIVINGSTON and A. M. DAVIS Metallurgy Laboratory, General Electric Co., Research and Development Center, Schenectady, NY 12301 (U.S.A.) (Received October 14, 1980; in revised form December 15, 1980)
SUMMARY
Cast-in crystals were observed in amorphous alloy ribbons o f Co-B, Fe-B, N i - B , ( F e , N i ) - B and (Fe, C o ) - B prepared by the chill-blockcasting technique. Their presence was determined by optical microscopy, microradiography, X-ray diffraction, scanning and transmission electron microscopies and Bitter and Kerr domain observation techniques. The cast-in crystals are often very large, indicating growth velocities greater than 1 cm s 1 and growth temperatures greater than 1000 °C. Most o f the crystals are o f the expected stable phase but in the F e - B series the cast-in crystals are o f the metastable bodycentered tetragonal Fe 3B phase. 1. INTRODUCTION Amorphous alloy ribbons are c o m m o n l y prepared by the chill-block-casting technique in which a stream of molten alloy is ejected o n t o a rapidly moving chill surface. Alloy ribbons that are partly or fully crystalline can also be prepared by this technique, either directly from casting or by heating of an initially amorphous ribbon. Such ribbons may exhibit unusual physical properties because the high quenching rates of chill block casting can produce microstructures and, in some cases, metastable phases n o t attainable by ordinary casting techniques. Although the crystallization of amorphous alloys by heating has received considerable attention, there has been little discussion of the cast-in crystals that sometimes occur in amorphous ribbons. Whether they are a desired or an undesired part of the final p r o d u c t it is important to characterize these crystals and to understand their formation. We report here observations on cast-in crystals in a n u m b e r of transition metal-B 0025-5416/81/0000-0000/$02.50
alloys. Since cast-in crystals in small volume fractions often escape detection by conventional X-ray techniques, we emphasize a variety of supplementary experimental techniques that have been found to be useful.
2. EXPERIMENTAL PROCEDURE Ingots of Co-B, Fe-B, Ni-B, (Fe,Ni)-B and (Fe,Co)-B were prepared by melting and chill casting under argon in an induction furnace. The starting materials were vacuummelted cobalt, vacuum-melted electrolytic iron, Ni 270 and crystalline boron of 99.8% purity. The ingots were then crushed and used as starting materials in the preparation of alloy ribbons by chill block casting [1]. A copper drum of diameter 10 or 15 cm was used, with a surface speed of about 30 m s-1. The resultingribbon was about 2 7 p m thick and 1 - 2 mm wide. Ribbons were routinely examined by optical microscopy and X-ray diffraction, including both Debye-Scherrer and pinhole transmission diffraction. The pinhole transmission diffraction technique, while covering a similar area of sample, is more sensitive than the D e b y e Scherrer technique in detecting cast-in crystals. X-ray diffraction analysis of residues after etching also proved to be useful. Selected samples were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), microradiography and Bitter and Kerr domain observation techniques.
3. EXPERIMENTAL RESULTS
3.1. C o - B In some cases, cast-in crystals as large as 100 pm are observed, as in the Co74B26 ribbon shown in Fig. 1. Such crystals generally © Elsevier Sequoia/Printed in The Netherlands
48 TABLE1
Fig. 1. Crystals on the surface of Co74B26 ribbon.
have depressed centers on the air side of the ribbon, suggesting that crystal growth terminated on this surface while the ribbon was still liquid. Such crystals are also usually associated with depressed areas, generally much larger than the crystals themselves, on the drum side of the ribbon. These areas result from entrapped air pockets at the m e l t - d r u m interface. The resulting reduction in local quenching rate apparently contributes to the nucleation and growth of cast-in crystals. Debye-Scherrer analysis of this and other Co-B ribbons failed to reveal the presence of cast-in crystals in amounts less than about 5 vol.%, although they could be revealed both by optical microscopy and by measurements of the temperature coefficient of resistivity [ 2]. The crystals were identified by D e b y e Scherrer analysis of the residue remaining after electrolytically thinning the ribbon in 7% hydrochloric acid-methanol, which preferentially dissolves the amorphous matrix. Such residue analysis for a series of Co-B ribbons identified the cast-in crystals as tetragonal Co2B for CoToB3o and Co72B2s , as orthorhombic Co3B for C075B25 , C077B23 and Cos0B20, and as a mixture of Co3B and f.c.c. cobalt for Cos4Ble. The crystal structures of the crystals examined in all alloys are given in Table 1. Contact microradiography was applied to each of the above alloys after etching to remove some of the amorphous matrix and to expose the crystals. In this technique a photographic emulsion is placed in direct contact with the ribbon surface and an image is obtained by differential absorption of the X-rays.
Alloy composition
Crystal structure
Co84B16 Co80B20 C077B23 Co75B25 Co72B28 Co70B30 Fe76B24 Fe75B25 Fe75B25
Orthorhombic Co3B and f.c.c. Co Orthorhombic Co3B Orthorhombic Co3B Orthorhombic Co3B Tetragonal C02B Tetragonal Co2B B.c.t. a Fe3B B.c.t. a as cast B.c.t. a Fe3B and b.c.t, a Fe2B + a-Fe as annealed F.c.c. (Fe,Ni)23B 6 F.c.c. (Fe,Ni)23B 6 Orthorhombic (Fe,Co)3B Orthorhombic (Fe,Co)3B Orthorhombic Ni4B 2
Fe43Ni43B14 Fe39Ni39B22 Fe38Co38B24 Fe44Co44B12 Ni62B38
aB.c.t., body-centered tetragonal.
Microradiographs of as-cast Co80B20 and Co70B30 ribbons are shown in Fig. 2. The light regions are the crystals.
(a)
(b) Fig. 2. (a) A microradiograph of crystals in CosoB20 ribbon; (b) a microradiograph of crystals in Co70B30 ribbon.
49
The difference in magnetic properties between cast-in crystals and the amorphous matrix can also be useful in characterizing such crystals. Orthorhombic CoaB has a strongly preferred crystallographic axis of magnetization. When this axis has a component normal to the surface, the emerging fields strongly attract the ferrofluid used in the Bitter domain observation technique as seen in Fig. 3(a) which shows crystals in Co76B24 ribbon. On a mechanically polished surface these emerging fields also result in strong domain contrast in polarized light by the polar Kerr effect (Fig. 3(b)). The cast-in Co2B crystals in the upper boron ribbons neither attract ferrofluid nor show Kerr contrast. This results from the easy-plane magnetic s y m m e t r y of Co2B, which allows the magnetization to remain parallel to the specimen surface. Domain patterns such as those in Figs. 3(a) and 3(b) are determined by the orientation of the magnetic easy axis of CoaB. Thus Bitter and Kerr techniques can be helpful n o t only in identifying the phases of cast-in crystals but also in determining their orientation. TEM of as-cast Co75B25 ribbon reveals Co3B crystals with widely varying microstructures. Some contain fault-like features (Figs. 4(a) and 4(b)), some contain cellular features (Figs. 4(c) and 4(d)) and some contain an extensive ultrafine structure (Figs. 4(e) and 4(f)). The microstructure of CoaB crystals in Co78B22 is alSO different both for cast-in crystals (Figs. 5(a) - 5(d)) and for crystals f o r m e d on heating to 709 K (Figs. 5(e) and 5(f)). The diffraction pattern of the crystals f o r m e d on heating (Figs. 5(e) and 5(f)) shows highly faulted crystals, and these crystals are reminiscent o f FeaB crystals f o r m e d by heating Fes0B20 and FesoNiaoB20 ribbons [3]. The heating of amorphous ribbons containing cast-in crystals leads both to the formation of fine crystals from the amorphous phase and to the further growth of the large cast-in crystals (Fig. 6(a)). This layer of new growth can also be seen from the microradiography (Fig. 6(b)) and SEM (Fig. 6(c)) of ribbons in which the matrix had been partly etched away. 3.2. F e - B As reported elsewhere [4], F e - B ribbons with less than 14% B or more than 21% B were found to contain cast-in crystals. The
4o~ ,
(a)
(b) Fig. 3. (a) Magnetic domains in crystals in Co76B24 ribbon using the Bitter technique; (b) magnetic domains revealed by the Kerr technique in crystals in Co76B24 ribbon.
low boron ribbons contained round a-Fe crystals which were quite different in morphology from the dendritic a-Fe crystals produced on heating these hypoeutectic alloys [ 5]. The high boron ribbons contained cast-in crystals of metastable tetragonal FeaB which attracted ferrofluid strongly (Fig. 7(a)). The use of dark field microscopy reveals magnetic domain patterns in the surrounding amorphous matrix (Fig. 7(b)). Interpretation of these patterns [6] indicates that the matrix is under compression in the direction of the c axis of the FeaB crystal and is under tension in directions normal to the c axis. These stresses presumably result from anisotropic differential thermal contraction between the matrix and the crystal during cooling; the crystal contracts more along the a axis, but less along the c axis, than the matrix does.
50
o •
t it
0 O
(a)
(b)
•
(c)
•
(d)
0 ,
e •
• t
(e)
*
•
(f)
Fig. 4. TEM micrographs and selected area diffraction patterns of Co3B crystals in Co75B25 ribbon with (a), (b) fault-like features in a (746) crystal plane, (c), (d) cell-like features in a (201) crystal plane and (e), (f) ultrafine structure in a (114) crystal plane.
51
o
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 5. TEM micrographs and electron diffraction patterns of Co3B crystals oriented in the (001), (74i') and (452) planes: (a) - (d) cast Co78B22 ribbon; (e), (f) ribbon heated to 709 K and quenched.
52
(a)
(b)
(c) Fig. 6. (a) Optical micrographs of cast-in crystals in longitudinal sections of Co80B20 ribbon showing growth caused by heating; (b) a microradiograph of a crystal in Co70B30 ribbon heated to 735 K and quenched; (c) an SEM micrograph of a crystal in Co70B30 ribbon heated to 735 K and quenched.
(a)
(b)
Fig. 7. Magnetic domains in Fe76B24 ribbon: (a) in crystals revealed by the Bitter technique; (b) in the adjacent amorphous matrix revealed by dark field microscopy.
53
From TEM the cast-in tetragonal Fe3B crystals are usually nearly featureless [7], although some heavily structured finer FesB crystals are found imbedded within the larger crystals (Fig. 8(a)). These heavily structured finer crystals decompose to the equilibrium Fe2B and ~-Fe phases on heating to 773 K (Fig. 8(b)), whereas the larger featureless crystals do not decompose until much higher temperatures. Fe3B crystals formed from the amorphous phase in Fe75B25 and Fe76B24 by heating also appear to be featureless. Heating Fes4Bi6 produces primary ~-Fe particles and featureless FesB [5]. However, heating Fes0B20 produced only FeaB crystals with an ultrafine but strong columnar microstructure [3].
3.3. (Fe,Ni)-B TEM of as-cast Fe43Ni43B14 and Fe39Ni39B22 ribbons indicate that most cast-in crystals have the f.c.c. (Fe,Ni)zsB6 structure (isostructural with Cr23C6). Some crystals are essentially featureless while others, sometimes even adjacent crystals, appear to be highly faulted [7]. In addition to very large cast-in crystals, as discussed above for Co-B and Fe-B, there were also numerous finer crystals (Fig. 9). An area originally thought to be amorphous (Fig. 10(a)) was found, from its diffraction pattern {Fig. 10(b)) and dark field microscopy (Fig. 10(c)), to be microcrystalline with an average grain size of about 250 £. 3.4. (Fe, Co)-B Ribbons of Fe38Co38B24 contain cast-in crystals of orthorhombic (Fe,Co)3B with no microstructural features. However, ribbons of Fe44Co44Bz2 contain cast-in crystals, identified as (Fe,Co)aB by diffraction analysis, that contain a pronounced substructure. One such crystal appears in Fig. 11, in which the diffraction pattern indicates a spread of about 12 ° about the [311] normal to this crystal. 3.5. N i - B In the alloys discussed so far, most cast-in crystals (both large and small) appear to be relatively unfaceted. In contrast, some of the cast-in crystals in Ni62B3s ribbon are hexagonal in form and contain a dendritic substructure which is very similar to a snowflake (Fig. 12). These crystals were identified from X-ray diffraction as orthorhombic NiaB3.
(a)
O
(b)
F'ezB ÷ aFe
(c) Fig. 8. (a) A TEM mierograph and (b) electron diffraction pattern of Fe3B crystals (featureless and microstructured) in cast Fe75B25; (c) featureless and decomposed (Fe2B and ~-Fe) crystals in Fe75B25 heated to 773 K.
54
(a)
(a)
W
O
(b)
(b)
(c)
(c)
Fig. 9. (a) A TEM micrograph of large (left-hand side) and fine (Fe,Ni)23B 6 crystals in Fe43Ni43B14 ribbon; (b) an electron diffraction pattern of the large (111) crystal plane area; (c) an electron diffraction pattern of the fine crystal area.
Fig. 10. (a) A TEM micrograph of Fe43Ni43B14 ribbon containing crystals of average size 250 •; (b) an electron diffraction pattern of the area in (a); (c) a dark field micrograph of the area in (a).
55 4. DISCUSSION
(a)
(b) Fig. 11. (a) A TEM micrograph of a large crystal with substructure in Fe44Co44B12 ribbon; (b) an electron diffraction pattern of the (311) plane of the crystal in (a).
Fig. 12. An optical micrograph of crystals on the surface of Ni62B3s ribbon.
For each alloy and crystallization mode the rate of crystallization will go through a maxim u m with decreasing temperature, producing the familiar C curve [8]. When crystallization occurs on heating an amorphous alloy, it occurs on the lower portion of this C curve. However, most cast-in crystals grew on the upper portion of the C curve, at much higher temperatures and velocities than the crystals formed on heating. If we estimate from known cooling rates [1], large cast-in Fe3B crystals such as those shown in Fig. 7 grew in times of less than 1 0 - 2 s, and hence at growth velocities greater than 1 cm s-1. By extrapolating data from Herold and KSster [9] for Fe3B growth at lower temperatures, such velocities indicate that these crystals grew at temperatures over 1000 °C. The same is probably true for the large (greater than 10 pm) cast-in crystals observed in the other alloys studied. Their association with air pockets on the drum side of the ribbon indicates that their formation was aided by a local lowering of the cooling rate. However, only a small fraction of the air pockets had associated cast-in crystals, indicating that crystal formation was limited by a shortage of nucleation sites. The much smaller cast-in crystals sometimes observed, such as those in Fig. 9 or Fig. 10, probably formed at much lower temperatures on the lower portion of the C curve. The high initial cooling rates characteristic of chill block casting are maintained only while the ribbon maintains contact with the drum. Even when such initial cooling is sufficiently rapid to avoid the nose of the C curve, the much slower cooling that occurs after leaving the drum can allow crystallization to occur at lower temperatures. Herold and KSster [9] have distinguished between three different crystallization modes in alloys such as these: (1) p o l y m o r p h o u s crystallization (single-phase growth with no composition change); (2) primary crystallization (single-phase growth with rejection of solute); (3) eutectic crystallization (simultaneous two-phase growth). Most of the castin crystals are probably examples of polymorphous crystallization, the crystallization m o d e that requires the least a m o u n t of diffusion.
56 In most of the alloys studied, the phase of the castAn crystals is present in the equilibrium phase diagram, although it often formed at a stoichiometry that was far from the equilibrium stoichiometry. The exception is Fe-B where the tetragonal FeaB phase is a metastable phase. This phase forms on crystallization of heated amorphous ribbons [3, 5] but, on further heating to 600 °C or above, decomposes to the equilibrium Fe2B and ~-Fe phases [9]. It is interesting that the cast-in crystals grow with the metastable Fe3B structure at temperatures well in excess of these decomposition temperatures. Apparently the short times involved favored polymorphous crystallization in the same way t h a t lowered diffusion rates favor this mode at lower temperatures. The presence of FesB crystals in the cast ribbon might be expected since it appears that the chemical short-range order of the amorphous Fe75B25 is similar to the short-range order of crystalline FeaB [5, 10]. It has been suggested that this unique ordering may extend to the Fe-B melt [5] which could account for the preference for the cast-in metastable Fe3B crystals over the stable Fe2B crystals which grow from the melt at lower cooling rates [4]. Since cast-in crystals and crystals formed on heating grow under very different conditions, it is n o t surprising that their microstructures, as observed by TEM, are often different. The high growth rates themselves m a y sometimes lead to faulted structures but faults may also be caused by off-stoichiometric compositions. Slight deviations from stoichiometry can be accommodated by faults t h a t do n o t preserve the composition, such as crystallographic shear planes [11]. It may sometimes be difficult to distinguish between such faults and fine second-phase regions, i.e. fine eutectic crystallization. Some of the highly structured crystals often seen, such as those in Figs. 5 and 11 and the FeaB crystals in heated FesoB2o ribbon [3], may in fact be two phase on a very fine scale, even though the minority metallic phase was n o t detected in diffraction patterns. The small structured FeaB crystals seen in Fig. 8(a) may contain such fine iron inclusions
which would provide easy nucleation sites for the decomposition to the equilibrium Fe2B and a-Fe phases (Fig. 8(b)). Finally, we note the utility of a broad variety of experimental techniques in detecting and characterizing cast-in crystals. X-ray diffraction of ribbons or etching residue, optical microscopy, TEM, SEM microradiography and resistivity measurements all may contribute helpful information. Where the phases formed are ferromagnetic, Bitter and Kerr domain observation techniques are also useful.
ACKNOWLEDGMENTS
Optical microscopy was performed by C. R. Rodd and TEM was performed by E. F. Koch, Inorganic Materials and Structures Laboratory.
REFERENCES
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