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Anomalous eutectic formation in the solidification of undercooled Co–Sn alloys
Journal of Crystal Growth 358 (2012) 20–28
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Anomalous eutectic formation in the solidification of undercooled Co–Sn alloys L. Liu a, X.X. Wei a, Q.S. Huang a, J.F. Li a,n, X.H. Cheng b, Y.H. Zhou a a b
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2011 Received in revised form 10 July 2012 Accepted 25 July 2012 Communicated by Silvere Akamatsu Available online 4 August 2012
Three Co–Sn alloys with compositions around the eutectic point were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification structure. It is revealed that the primary phase during rapid solidification changes complexly with the increasing undercooling in the off-eutectic alloys, while coupled eutectic growth takes place at all undercoolings in the eutectic alloy. Two types of anomalous eutectics form in the alloys: one evolving from coupled eutectics and the other from single phase dendrites or seaweeds. The crystallographic orientation of eutectic phases in the anomalous eutectic is dependent on which type their precursors belong to. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Eutectics A1. Solidification B1. Metals
1. Introduction Eutectic structure is composed at least of two phases arranged alternatively. Therefore, eutectic alloys are of good performances and widely applied as structural and functional materials in automobile, aerospace, electronic and other industries. Their solidifications are investigated intensively [1–6]. Depending on the physical features of eutectic phases, various geometrical morphologies can be exhibited in the eutectic alloys under near-equilibrium conditions. With near volume fractions between two eutectic phases, lamellar eutectics form. Otherwise, if a phase presents a much smaller volume fraction, it tends to distribute in another phase with fibrous morphology. When solidification proceeds far from equilibrium conditions, for example, the eutectic alloy melt has been undercooled deeply, a transition from lamellar or rod to anomalous eutectic may occur, as observed in Ni–Sn [7,8], Co–Sn [9–11], and Cu–Ag [12–14] eutectic alloys. In anomalous eutectics, a phase of granular shape is discontinuously embedded in the matrix of another phase. Several theories have been advanced to explain the anomalous eutectic formation. Based on the fact that Cu particles in the anomalous eutectic were discontinuously distributed in the Ag matrix, Powell and Hogan [12] thought that repeated nucleation resulted in anomalous eutectic formation in the solidification of
n
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0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.041
undercooled Ag–Cu eutectic alloy. In studying the solidification of undercooled Ni–Sn eutectic alloy, Kattamis and Flemings [7] proposed that the primary solid was a highly supersaturated aNi solid solution of eutectic composition which grew dendritically and decomposed subsequently to the equilibrium a-Ni and b-Ni3Sn during the post-recalescence, leading to the formation of anomalous eutectic. Jones [15] suggested that when the melt undercooling exceeded a critical value, the coupled eutectic growth interface became instable due to a large difference in interface undercooling between two phase, and thus led to anomalous eutectics. Wei et al. [8,16] ascribed the transition from lamellar to anomalous eutectics to the independent nucleation and cooperatively branching growth of two eutectic phases. A significantly different mechanism was proposed by Goetzinger et al. [17], who thought that anomalous eutectics were resulted from the fragmentation of the primary eutectic lamellae driven by the reduction of the interfacial energy. They further pointed out that the eutectic lamellae formed in rapid solidification was solute-supersaturated due to solute trapping and would be partially remelted when the interdendritic liquid solidified under near-equilibrium conditions at higher temperature. Since the excess solute trapped in the primary formed eutectic phases was very limited, they thought that remelting played a minor role in the formation of anomalous eutectic. Recently, Li et al. [18] advanced a new mechanism for anomalous eutectic formation. They argued that the primary solid formed in rapid solidification was rich in solutes due to the deviation of the solidification temperature from the equilibrium melting point and thus was
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partially remelted during recalescence. As a result, the primary solid broke into pieces. It should be noted that the type of primary solid was not confined by Li et al., i.e. both coupled and decoupled growths of eutectic phases could result in anomalous eutectics. Such a viewpoint was supported by Yang et al. [19] later. Obviously, there is a dispute about the dependence of anomalous eutectic formation on the type of primary solid. Considering that the solid morphology would undergo considerable evolution during solidification of undercooled eutectic alloys, it is hard to accurately judge from the post-solidification structure morphology whether the primary solid is a single phase or coupled eutectic at large undercoolings, which blocks the understanding of the origin of anomalous eutectic. However, it is well known that there is a coupled eutectic growth zone below the eutectic line for eutectic alloy systems [20–24]. The alloy melt undercooled into the coupled eutectic growth zone, whether it is with off-eutectic or eutectic composition, solidifies with regular eutectic as the primary solid. At the other undercoolings, a single phase forms primarily. The change of primary phase must cause significant differences in the cooling curves. Through monitoring the thermal history of a sample, it is feasible to deduce the crystal growth mode in the solidification. In the present paper, the recalescence processes and solidification microstructures of undercooled Co–Sn alloys with eutectic and off-eutectic compositions were analyzed systematically. It is verified that both rapid coupled and decoupled growth can result in anomalous eutectic formation in the solidification structure.
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3. Results 3.1. Cooling curves Fig. 1 shows the cooling curves of Co–20at%Sn, Co–24at%Sn and Co–28at%Sn alloys solidifying at different undercoolings (the temperature interval between the equilibrium liquidus temperature
2. Experiments The experimental materials were Co–20at%Sn hypo-eutectic alloy, Co–24at%Sn eutectic alloy [25] and Co–28at%Sn hypereutectic alloy. The alloy ingots were prepared by arc melting Co and Sn of 99.999 wt% and 99.99 wt% purities, respectively, in a Ti-gettered argon atmosphere. The undercooling experiment was carried out in a vacuum chamber back-filled with ultra-pure argon. Covered by the glass purifier made from 50% B2O3, 30% Na2SiO3 and 20% Na2B4O7 and dehydrated at 1273 K for 6 h in advance, the alloy ingot placed in a quartz glass crucible was induction melted, and cyclically superheated and cooled under the protection of the molten glass flux until the desired undercooling was achieved. The superheating degree of the melt was controlled to be about 300 K by adjusting the input power of the induction coil. An infrared pyrometer with an accuracy of 1 K and a response time of 1 ms was utilized to monitor the thermal history of the sample during the entire experimental cycle. The temperature data were recorded in a computer. The as-cast sample was a short column of about 8 mm in diameter. The sample was sectioned along the longitudinal direction, polished and then etched for structural observation under an optical microscope (OM). The etching agent was a mixed solution of CuSO4, HCl and ethanol. The phases in the solidification structures were determined by X-ray diffraction (XRD) with Cu Ka radiation. Electron backscattered diffraction (EBSD) measurement was performed using a JSM7600F field emission gun scanning electron microscope (SEM) operated in the secondary electron mode to interesting areas. Since EBSD is very sensitive to crystalline perfections, a well prepared sample is a prerequisite to obtain a good diffraction pattern. For this purpose, the section of the sample for measurement was first mechanically polished with diamond polishing agent from 5 mm to 0.5 mm roughness and subsequently vibration polished with alumina polishing agent to 0.05 mm roughness. During EBSD analysis, the sample was tilted at 651 to the horizontal plane to optimize both the contrast in the diffraction pattern and the fraction of electrons scattered from the sample. An acceleration voltage of 18 kV and probe current of 10 nA were used.
Fig. 1. Cooling curves at different undercoolings: (a) Co–24at%Sn eutectic alloy, (b) Co–20at%Sn hypo-eutectic alloy and (c) Co–28at%Sn hyper-eutectic alloy.
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TL and the onset temperature TN of solidification). Their TL is 1438, 1385 and 1407 K respectively. It can be seen that only one temperature recalescence happens during the solidification of Co–24at%Sn eutectic alloy at all undercoolings studied (Fig. 1). As undercooling increases, crystal growth velocity rises gradually, leading to an increasing recalescence rate (the ratio of the difference between the end and onset temperatures of recalescence to the recalescence time). For Co–20at%Sn hypo-eutectic alloy, the cooling curve changes complexly as undercooling increases. As shown in Fig. 1b, there are two critical undercoolings DTc1 (about 100 K) and DTc2 (about 240 K) across which the temperature recalescence behavior significantly changes. When undercooling is less than DTc1, for example at 40 K and 70 K, the cooling curve shows two recalescence events. The first one becomes obvious at larger undercoolings while the end temperature of the second one is constantly lower than the eutectic temperature. It should be noted that a relative long temperature plateau follows the second recalescence but not the first one. When the alloy melt is undercooled between DTc1 and DTc2, for instance at 110 K and 208 K, only one recalescence happens. When undercooling exceeds DTc2, for example at 270 K, two recalescences occur again. With the end of the first rescalescence, the temperature falls rapidly to the onset temperature of the second recalescence. The typical cooling curves of Co–28at%Sn hyper-eutectic alloy are shown in Fig. 1c. In the undercooling range studied, only one critical undercoolings DTc1 (about 70 K) exist. At undercooling lower than DTc1, for example at 17 K and 52 K, two recalescences happen during solidification. Once again, only one recalescence is seen on the cooling curve when undercooling exceeds DTc1, as occurred at 97 K and 200 K. 3.2. Solidification structures In the solidification of undercooled alloy melt, particularly at low undercooling, multiple nucleation event often happens, leading to several freezing fronts propagating simultaneously in the rapid solidification. The latent heat released from these freezing domains may influence mutually, causing the thermal inhomogeneities across the sample. As a result, the rapid
solidification microstructures in the sample are inhomogeneous. When undercooling is large enough, single nucleation event happens in the rapid solidification. In such a case, the primary solid almost forms at the same interface undercooling during rapid solidification and therefore the resultant microstructures should be uniform if the crystal growth mode is the same. For better comparison and analysis, the microstructures near the nucleation site were used for all the three Co–Sn alloys in this paper. Since the characteristic size of the microstructure may also be influenced by local solidification time, the microstructures selected also located at about half radius of the sample. 3.2.1. Co–24at%Sn eutectic alloy Fig. 2 shows the sectional microstructures of the Co–24at%Sn eutectic samples solidified at different undercoolings. The microstructure is composed completely of regular lamellar eutectics at very low undercooling (Fig. 2a). According to the Co–Sn binary phase diagram [25], the phases in the microstructure are rather complex as solid-state phase transformations may be involved in the post-solidification cooling process, especially at slow cooling rates. So the samples solidified at low and large undercoolings respectively were analyzed through XRD, and the results indicate that four phases, a-Co, b-Co3Sn2, e-Co and a-Co3Sn2 coexist in the microstructure (Fig. 3). Among them the latter two are the transformation products of a-Co and b-Co3Sn2 at lower temperatures. However, only two phases can be identified in the OM observation, i.e. the darker Co phase and the brighter Co3Sn2 phase (Fig. 2). So, in the following description a-Co and e-Co as well as b-Co3Sn2 and a-Co3Sn2 will not be distinguished and only noted by Co and Co3Sn2, respectively, unless otherwise pointed out. Anomalous eutectics begin to form once undercooling increases to 21 K (Fig. 2b), in which the Co phase is of particulate or short rod shape, coexisting with traces of eutectic lamellae. Around the anomalous eutectic are lots of regular lamellar eutectics (not shown here). As undercooling rises, the fraction of anomalous eutectics increases. When undercooling increases up to the maximum value 203 K, most of the sample is occupied by anomalous eutectics except a small amount of lamellar eutectics lying between them (comparing Fig. 2c and d). The average
Fig. 2. Sectional microstructures of the Co–24at%Sn eutectic alloy undercooled by (a) 15 K, (b) 21 K, (c) 110 K and (d) 203 K.
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diameter of Co particles in the anomalous eutectic is exhibited in Fig. 4a. Clearly, the size of Co particles increases first and then decreases as undercooling rises. The volume fraction of anomalous eutectics is evaluated by measuring the area of the anomalous eutectic zone on the sample cross-section and the result indicates that it is nearly in direct proportion to undercooling (Fig. 4b). 3.2.2. Co–20at%Sn hypo-eutectic alloy The cross-sectional microstructures of Co–20at%Sn samples solidified at different undercoolings are shown in Fig. 5. The
Fig. 3. XRD patterns of the Co–24at%Sn eutectic alloy solidified at undercooling of 15 K and 203 K.
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solidification structure evolution with undercooling is very complex. At low undercoolings, the solidification microstructure is composed of coarse primary Co dendrites, surrounded by Co3Sn2 haloes and regular lamellar eutectics (Fig. 5a). When undercooling increases to about 70 K the microstructure changes to a mixture of coarse broken Co dendrites and anomalous eutectics (Fig. 5b). Careful observation indicates that the anomalous eutectics at this time can be divided into two types: one with large Co particles and the other with small Co particles, denoted by A and B respectively in the image. These two types of anomalous eutectics are distributed in the whole sample. As undercooling increases further, the Co particles become rounder and rounder while their sizes are still obviously different between two types of anomalous eutectics. Once undercooling increases up to about 100 K (DTc1), the microstructure significantly changes again. The microstructure of the sample undercooled by 110 K is shown in Fig. 5c, where clear eutectic colonies are found. Different from the one shown in Fig. 2c, the edge of the eutectic colony in Co–20at%Sn consist of coarse Co strips embedded in the Co3Sn2 matrix instead of lamellar eutectics though the central structure belongs to the same type of anomalous eutectic. Such anomalous eutectics exists until undercooling increase up to DTc2. At larger undercoolings, eutectic colony structure disappears and the microstructure consists completely of anomalous eutectics. Different from the anomalous eutectics described above, all the Co particles within such anomalous eutectics are coarse and nearly spherical and of the same size (Fig. 5d). The size of Co particles in the anomalous eutectics of Co–20at%Sn changes complicatedly with undercooling (Fig. 4c).
Fig. 4. Average diameter of Co particles in the anomalous eutectic and volume fraction of anomalous eutectics in the solidification microstructure of Co–24at%Sn eutectic alloy (a,b) and Co–20at%Sn hypo-eutectic alloy (c,d), respectively.
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Fig. 5. Sectional microstructures of the Co–20at%Sn hypo-eutectic alloy undercooled by (a) 25 K, (b) 70 K, (c) 110 K and (d) 270 K.
Generally, the Co particle in type A anomalous eutectic is much larger than that in type B anomalous eutectic in the undercooling range studied. The average diameter of Co particles in type A anomalous eutectic decreases continuously when undercooling below DTc1 and then increases again to a higher value at 270 K. The average diameter of Co particles in type B anomalous eutectic is almost unchanged when undercooling is below DTc1. At larger undercooling (DTc1 DTc1), however, the size of Co particles becomes large and decreases gradually as undercooling increases. As for the volume fraction of anomalous eutectics, it increases with undercooling as occurred in Co–24at%Sn eutectic alloy (Fig. 4d). 3.2.3. Co–28at%Sn hyper-eutectic alloy Typical microstructures of Co–28at%Sn hyper-eutectic alloy are shown in Fig. 6. At low undercoolings, the solidification structure consists of coarse primary Co3Sn2 crystals with haloes of Co and regular lamellar eutectics (Fig. 6a). The primary Co3Sn2 phase is not of dendritic morphology but of seaweed pattern due to the weak anisotropy in the solid–liquid interface [11]. When undercooling increases to about 52 K or more, as shown in Fig. 6b, the primary Co phase becomes coarse spherical particles and fine anomalous eutectics forms between them. As undercooling increases to the range where only one recalescence happens during solidification, i.e. larger than 70 K, the microstructure contains another type of eutectic colony: fine anomalous eutectics at the center and coarse vermiculate Co3Sn2 crystals around them (Fig. 6c). Out of the eutectic colony is the mixture of Co3Sn2 seaweeds and regular lamellar eutectics. As undercooling increases, the volume fraction of anomalous eutectics in the microstructure also increases constantly. 3.3. EBSD measurement As described above, different morphologies of anomalous eutectics form in the three alloys. To uncover the anomalous eutectic formation mechanism, crystal orientations in the anomalous eutectic were analyzed using EBSD. Fig. 7 shows the results of the Co–20at%Sn hypo-eutectic alloy solidified at undercooling
of 70 K. All of the four phases, a-Co, b-Co3Sn2, e-Co and a-Co3Sn2 are indexed respectively. Since e-Co and a-Co3Sn2 resulted from the partial solid-state phase transformation of a-Co or b-Co3Sn2 phases and anomalous eutectic formation has nothing to do with the e-Co and a-Co3Sn2 phases, only a-Co and b-Co3Sn2 phases are further demonstrated in the following. In the examination area shown in Fig. 7, type A and type B anomalous eutectics coexist. Clearly, the b-Co3Sn2 phase in each type of anomalous eutectic show a sole crystallographic orientation according to the EBSD pattern (Fig. 7e) and the [001] inverse pole figure (IPF) (Fig. 7f). As shown in Fig. 7h, the [001] IPF of a-Co phase in the type A anomalous eutectics shows two different diffraction directions. But the [001] IPF of a-Co phase in the type B anomalous eutectics only displays a diffraction direction (Fig. 7i). The EBSD measurement results of the anomalous eutectic at the center of the eutectic colony in the sample undercooled by 120 K are shown in Fig. 8. When the a-Co and b-Co3Sn2 phases are indexed independently, completely random orientation distributions exhibit in their [001] IPFs (Fig. 8b and c), which are very similar to the EBSD results of the anomalous eutectic in Co–24at%Sn eutectic alloy [11]. The microstructure of the sample undercooled by 270 K is also analyzed by the EBSD and the results are given in Fig. 9. The [001] IPF of the a-Co phase exhibits the feature of completely random orientation (Fig. 9b). However, the [001] IPF of b-Co3Sn2 phase only shows two diffraction directions (Fig. 9c).
4. Discussions 4.1. Crystal growth modes in off-eutectic alloys In our previous work on the solidification of undercooled Co–24at%Sn eutectic melt it had already been confirmed that only coupled eutectic growth happens in the undercooling range studied (0–203 K) [11]. As a result, all the cooling curves have the same characteristic: a sole temperature recalescence. In contrast, the cooling curves in the solidification of Co–20at%Sn hypoeutectic alloy and Co–28at%Sn hyper-eutectic alloy are complex as undercooling increases. Considering that any temperature
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Fig. 6. Sectional microstructures of the Co–28at%Sn hyper-eutectic alloy undercooled by (a) 17 K, (b) 50 K, (c) 110 K and (d) 200 K.
recalescence starts at a temperature far higher than 773 K below which a-Co and b-Co3Sn2 transform into e-Co and a-Co3Sn2, respectively [26]. Therefore, the phenomenon of double recalescences results from solidification, meaning that coupled eutectic growth is not the sole solidification mode of the Co–Sn offeutectic alloy melts. In the microstructures of Co–20at%Sn hypo-eutectic alloy, coarse a-Co dendrites are surrounded by eutectic structures at undercoolings below 70 K (Fig. 5a and b). Obviously, a-Co dendrites are primarily formed in the undercooled melt, although the alloy melt may have been undercooled below the eutectic line, i.e. undercooling has exceeded the temperature interval between TL (1438 K) and eutectic temperature TE (1385 K). In this case, as a-Co dendrites grow, the liquid around them become rich in Sn, and the Sn concentration gradually decreases along the direction away from the a-Co solid. Once the temperature is low enough, b-Co3Sn2 nucleates, and coupled eutectic growth occurs in the remaining liquid away from the a-Co solid, while divorced eutectic growth occurs in the remaining liquid in contact with a-Co. As a result, secondary recalescence takes place. As undercooling increases further, only one recalescence event happens in the whole solidification process as occurred in Co–24at%Sn eutectic melt. The resultant anomalous eutectic at the center of the eutectic colony in the Co–20at%Sn alloy exhibits the same feature in the EBSD measurement as those in the Co–24at%Sn alloy. These mean that the melt has been undercooled into the coupled eutectic growth zone and regular eutectics first form as the primary phase instead of a-Co. When undercooling is very large, two recalescences happen again in the solidification (Fig. 1b), indicating that the primary phase returns to a-Co. In this case, the volume fraction of the primary a-Co solid is so large at the start of the second recalescence that the remaining liquid zone among the a-Co crystal is very thin, and only divorced eutectic growth occur in the following solidification. In comparison with Co–20at%Sn hypo-eutectic alloy, Co–28at%Sn hyper-eutectic alloy exhibits similar recalescence behaviors and solidification structure evolutions at low and intermediate undercoolings. But it does not experience the transition from single recalescence to double recalescences at large undercoolings, and the microstructure consisting of full anomalous eutectics as shown in
Fig. 5d does not form in the alloy. It is clear that the Co–28at%Sn hyper-eutectic alloy melt first solidifies into b-Co3Sn2 seaweeds and then to lamellar eutectics when undercooling is smaller than DTc, accompanied with two recalescence events. At larger undercoolings, coupled eutectic growth first takes place, followed by the solidification of single b-Co3Sn2 phase in the remaining liquid and another eutectic growth at the last. In this case, there is only one recalescence since nucleation is not needed for the solidification of single b-Co3Sn2 in the mixture of the primary eutectic and the remaining liquid. 4.2. Formation mechanisms of anomalous eutectics As stated above, the primary phase during rapid solidification can be either coupled eutectic or single phase, depending on the alloy composition and undercooling. Since anomalous eutectic is the evolution product of the rapidly solidified primary solid, its formation closely relates with the crystal growth mode of the primary solid. 4.2.1. Co–24at%Sn eutectic alloy For the Co–24at%Sn eutectic alloy, coupled eutectic growth goes the whole solidification process. Due to the negative temperature gradient ahead of the solid/liquid (S/L) interface, the eutectic growth interface branches during rapid solidification. Because the temperature at the eutectic branch tip is lower than the equilibrium eutectic temperature, the compositions of two eutectic phases at the S/L interface deviate from the solid composition at the equilibrium eutectic temperature, i.e. the rapidly solidified eutectic phases are rich in solutes. As release of latent heat raises the temperature of the system, the primary eutectic is superheated. Once undercooling exceeds a critical value, the superheating degree becomes large enough for the primary lamellar eutectic to be partially remelted and disintegrated during recalescence [18]. For the eutectic branches under rapid growth, the melt undercooling ahead of the S/L interface decreases gradually as the position leaves away from the tip. Correspondingly, the supersaturation degree of solute in the eutectic branches falls as they become thick until their
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Fig. 7. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 70 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co3Sn2 phase, (c) [001] IPF of e-Co phase, (d) EBSD pattern indexed to b-Co3Sn2 phase, (e) [001] IPF of b-Co3Sn2 phase in type A anomalous eutectic, (f) [001] IPF of b-Co3Sn2 phase in type B anomalous eutectic, (g) EBSD pattern indexed to a-Co phase, (h) [001] IPF of a-Co phase in type A anomalous eutectic and (i) [001] IPF of a-Co phase in type B anomalous eutectic.
Fig. 8. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 110 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co phase and (c) [001] IPF of b-Co3Sn2 phase.
compositions close to the equilibrium values at equilibrium eutectic temperature. Therefore, the eutectic formed at the late stage of the rapid solidification as well as the slow solidification
stage cannot be remelted and keeps the morphology of regular lamellae. This leads to the microstructure shown in Fig. 2c that anomalous eutectic lies at the center of the eutectic branch, of
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Fig. 9. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 270 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co phase and (c) [001] IPF of b-Co3Sn2 phase.
which the edge is occupied by regular lamellar eutectic. As undercooling increases, the composition of the primary eutectics deviates more severely from the equilibrium value at eutectic temperature, and thus more of them are remelted during recalescence, leading to a rising volume fraction of anomalous eutectics. When undercooling is not too large, the superheating degree in the primary eutectic lamellae is relative low and only a small portion of the solid is remelted. The ripening and coarsening of the broken lamellae is very limited in the following solidification. As a result, a-Co particles in the anomalous eutectic is very small and their diameter closes to the eutectic lamellar spacing. As undercooling increases, the superheating degree in the primary eutectic increases and the remelting volume fraction rises correspondingly. In this case, ripening and coarsening take place on a large scale in the broken eutectic lamellae, leading to an increase of the a-Co particle size in the anomalous eutectic. As undercooling increases further, solidification time reduces, which restricts the growth of a-Co particles in the anomalous eutectic. So the size of a-Co particles in the anomalous eutectic of Co–24at%Sn eutectic alloy increases first and then decrease as undercooling increases.
4.2.2. Co–20at%Sn eutectic alloy It can be deduced from the cooling curves (Fig. 1b) and EBSD measurement results (Fig. 7) that two types of anomalous eutectics form at different stages of solidification in the undercooling range 70–100 K. In this case, the a-Co dendrite firstly forms from the undercooled melt during rapid solidification. Also temperature at the dendrite tip of a-Co is much lower than the equilibrium liquidus temperature of the alloy, which in combination with solute trapping makes the solid rich in solute. With recalescence proceeding, the primary a-Co dendrite is superheated and partially remelted, which makes the dendrite branches broken and form anomalous eutectics. In the resultant anomalous eutectic a-Co particles are of the same order in size as the arms diameter of the primary a-Co dendrite (type A anomalous eutectic in Fig. 5b). As undercooling increases, the dendrite tip radius and the stem diameter decrease due to the rise of growth velocity. Thus, the size of a-Co particles in type A anomalous eutectic decreases (Fig. 4c). Due to solute partitioning, the composition of the remaining liquid approaches to the equilibrium eutectic composition gradually. Once the composition and temperature satisfy the conditions for eutectic to nucleate, regular lamellar eutectics solidify in the remaining melt, causing the second recalescence. In such a case, the undercooling of the remaining liquid is large enough for the formed lamellar eutectics to be partially remelted into anomalous eutectic via the same mechanism as that in the Co–24at%Sn eutectic alloy discussed in Section 4.2.1. Owing to the much thinner eutectic lamella compared with the a-Co dendrite arm diameter, a-Co
particles in type B anomalous eutectic are greatly smaller than those in type A anomalous eutectic. In this undercooling range, the melt undercooling prior to the second recalescence changes little as undercooling increases, as a result the size of a-Co particles in type B anomalous eutectic do not change almost with the original undercooling (Fig. 4c). The fragments of a-Co resulted from partial remelting are surrounded by liquid. They may rotate in the following solidification, which leads to different crystallographic orientations among the Co particles in the anomalous eutectic. To obtain overall useful information about two types of anomalous eutectics, the step length used in the EBSD measurement was very small. Correspondingly the area of the analyzing region was limited, and only several large a-Co particles in type A anomalous eutectic were indexed. Although only two crystallographic orientations were revealed (Fig. 7h), referring to the work on other alloy systems [18,19], it can be predicted that a-Co particles in type A anomalous eutectic should exhibit a feature of random orientation. During the second recalescence, both a-Co and b-Co3Sn2 lamellae are partially remelted. But the broken fragments cannot rotate due to the insufficient surrounding liquid. Therefore crystals of two phases are all orientated. When undercooling exceeds the critical value DTc1, coupled eutectic growth first happens in the undercooled melt and thus the primary solid becomes lamellar eutectic. Due to the severe recalescence-induced superheating in the solid, the primary lamellar eutectics are heavily remelted and evolved into anomalous eutectic. The formation mechanism is the same as that in the solidification of undercooled Co–24at%Sn eutectic alloy. The formed anomalous eutectic belongs to type B. The average size of a-Co particles in the anomalous eutectic decreases with the increasing undercooling. In this case, the remelted fractions of two phases are so large that both a-Co and b-Co3Sn2 crystals in the post-recalescence solidification can rotate freely, leading to completely random orientation as shown in the [001] IPFs (Fig. 8b and c). As the primary eutectic grows, the composition of the remaining liquid becomes rich in Co. Therefore a-Co phase first precipitates surrounding the primary eutectic (Fig. 5c). When undercooling is larger than DTc2, single a-Co solid instead of lamellar eutectic again becomes the primary phase forming in the first recalescence. In such a case, the growth velocity is very high, and the rapidly solidified a-Co dendrite is supersaturated with more solute. Consequently, more of the primary a-Co solid is remelted. In the other hand, the severe solute trapping in the rapidly growing a-Co dendrite greatly reduces the solute enrichment in the liquid ahead of the S/L interface, and thus leads to the increase of the dendrite tip radius and arm diameter, as found in the rapid solidification of undercooled single phase alloy [27]. As a result, the average size of a-Co particles in type B anomalous eutectic increases again (Fig. 4c). In this case, the a-Co fragments have enough time to ripen and
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rotate in the remaining and thus they exhibit spherical morphologies (Fig. 5d) and randomly orientated (Fig. 9b) in the anomalous eutectic. b-Co3Sn2 nucleates and grows in the remaining liquid phase, with which the second recalescence takes place. But the superheating in the b-Co3Sn2 solid is so weak that no obvious remelting occurs in it. In the IPF shown in Fig. 9, there are three crystallographic orientations. The reason is that the b-Co3Sn2 solid in the analyzed region grew from three nucleation sites. As discussed in Section 4.1, there is no lamellar eutectic to form in the solidification. As a result, only type A anomalous eutectic can be found in the microstructure. 4.2.3. Co–28at%Sn eutectic alloy In the undercooling range investigated, the solidification of Co–28at%Sn hyper-eutectic alloy only experiences the change of primary phase from single b-Co3Sn2 to eutectic at the critical undercooling DTc1. The anomalous eutectic formation can be understood according to the description to the solidification of the Co–20at%Sn hypo-eutectic alloy at undercoolings below DTc2. But it should be noted that in the anomalous eutectic associated with the primary b-Co3Sn2 phase, the particulate phase is b-Co3Sn2 rather than a-Co, and correspondingly the matrix phase is a-Co rather than b-Co3Sn2. 5. Conclusions (1) In the whole undercooling range studied (0–203 K), only one recalescence occurs in the solidification of Co–24at%Sn eutectic alloy, and the primary solid is constantly lamellar eutectic. (2) At undercooling below 100 K, the solidification of Co–20at%Sn hypo-eutectic alloy is accompanied with double recalescences. The first one corresponds to the solidification of primary a-Co dendrites, while the second one to the eutectic solidification. In the undercooling range 100–240 K, there is only one recalescence resulting from the rapid growth of the primary lamellar eutectic. At higher undercooling, double recalescences take place again. (3) The solidification of Co–28at%Sn hyper-eutectic alloy at undercoolings below 70 K also results in double recalescences, but the first one is caused by the rapid growth of the primary b-Co3Sn2 phase. In the undercooling range 70–200 K, only one recalescence event associated with primary lamellar eutectic formation occurs. The return to double recalescences at large undercooling in the hypo-eutectic alloy is not revealed in this alloy. (4) Two types of anomalous eutectics form in the Co–Sn off-eutectic alloys. One results from the remelting-induced disintegration of the primary a-Co or b-Co3Sn2 phase. In this case, the granular crystals in the anomalous eutectic are randomly orientated, while the matrix phase is well orientated. Another type of anomalous eutectic results from the remelting and ripening of the lamellar eutectics solidified in the first or second recalescence. In this case, crystals of two phases in the anomalous eutectic are somewhat orientated at low undercoolings, but become randomly orientated at high undercoolings.
Acknowledgments The financial supports from the National Natural Science Foundation of China under Grant no. 50874073, Post-doctoral
Science Foundation of China under Grant no. 2011M500074 and the National Basic Research Program of China under Grant no. 2011CB610405 are greatly appreciated.
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Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Anomalous eutectic formation in the solidification of undercooled Co–Sn alloys L. Liu a, X.X. Wei a, Q.S. Huang a, J.F. Li a,n, X.H. Cheng b, Y.H. Zhou a a b
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2011 Received in revised form 10 July 2012 Accepted 25 July 2012 Communicated by Silvere Akamatsu Available online 4 August 2012
Three Co–Sn alloys with compositions around the eutectic point were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification structure. It is revealed that the primary phase during rapid solidification changes complexly with the increasing undercooling in the off-eutectic alloys, while coupled eutectic growth takes place at all undercoolings in the eutectic alloy. Two types of anomalous eutectics form in the alloys: one evolving from coupled eutectics and the other from single phase dendrites or seaweeds. The crystallographic orientation of eutectic phases in the anomalous eutectic is dependent on which type their precursors belong to. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Eutectics A1. Solidification B1. Metals
1. Introduction Eutectic structure is composed at least of two phases arranged alternatively. Therefore, eutectic alloys are of good performances and widely applied as structural and functional materials in automobile, aerospace, electronic and other industries. Their solidifications are investigated intensively [1–6]. Depending on the physical features of eutectic phases, various geometrical morphologies can be exhibited in the eutectic alloys under near-equilibrium conditions. With near volume fractions between two eutectic phases, lamellar eutectics form. Otherwise, if a phase presents a much smaller volume fraction, it tends to distribute in another phase with fibrous morphology. When solidification proceeds far from equilibrium conditions, for example, the eutectic alloy melt has been undercooled deeply, a transition from lamellar or rod to anomalous eutectic may occur, as observed in Ni–Sn [7,8], Co–Sn [9–11], and Cu–Ag [12–14] eutectic alloys. In anomalous eutectics, a phase of granular shape is discontinuously embedded in the matrix of another phase. Several theories have been advanced to explain the anomalous eutectic formation. Based on the fact that Cu particles in the anomalous eutectic were discontinuously distributed in the Ag matrix, Powell and Hogan [12] thought that repeated nucleation resulted in anomalous eutectic formation in the solidification of
n
Corresponding author. Tel./fax: þ 86 21 54748530. E-mail address: jfl[email protected] (J.F. Li).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.041
undercooled Ag–Cu eutectic alloy. In studying the solidification of undercooled Ni–Sn eutectic alloy, Kattamis and Flemings [7] proposed that the primary solid was a highly supersaturated aNi solid solution of eutectic composition which grew dendritically and decomposed subsequently to the equilibrium a-Ni and b-Ni3Sn during the post-recalescence, leading to the formation of anomalous eutectic. Jones [15] suggested that when the melt undercooling exceeded a critical value, the coupled eutectic growth interface became instable due to a large difference in interface undercooling between two phase, and thus led to anomalous eutectics. Wei et al. [8,16] ascribed the transition from lamellar to anomalous eutectics to the independent nucleation and cooperatively branching growth of two eutectic phases. A significantly different mechanism was proposed by Goetzinger et al. [17], who thought that anomalous eutectics were resulted from the fragmentation of the primary eutectic lamellae driven by the reduction of the interfacial energy. They further pointed out that the eutectic lamellae formed in rapid solidification was solute-supersaturated due to solute trapping and would be partially remelted when the interdendritic liquid solidified under near-equilibrium conditions at higher temperature. Since the excess solute trapped in the primary formed eutectic phases was very limited, they thought that remelting played a minor role in the formation of anomalous eutectic. Recently, Li et al. [18] advanced a new mechanism for anomalous eutectic formation. They argued that the primary solid formed in rapid solidification was rich in solutes due to the deviation of the solidification temperature from the equilibrium melting point and thus was
L. Liu et al. / Journal of Crystal Growth 358 (2012) 20–28
partially remelted during recalescence. As a result, the primary solid broke into pieces. It should be noted that the type of primary solid was not confined by Li et al., i.e. both coupled and decoupled growths of eutectic phases could result in anomalous eutectics. Such a viewpoint was supported by Yang et al. [19] later. Obviously, there is a dispute about the dependence of anomalous eutectic formation on the type of primary solid. Considering that the solid morphology would undergo considerable evolution during solidification of undercooled eutectic alloys, it is hard to accurately judge from the post-solidification structure morphology whether the primary solid is a single phase or coupled eutectic at large undercoolings, which blocks the understanding of the origin of anomalous eutectic. However, it is well known that there is a coupled eutectic growth zone below the eutectic line for eutectic alloy systems [20–24]. The alloy melt undercooled into the coupled eutectic growth zone, whether it is with off-eutectic or eutectic composition, solidifies with regular eutectic as the primary solid. At the other undercoolings, a single phase forms primarily. The change of primary phase must cause significant differences in the cooling curves. Through monitoring the thermal history of a sample, it is feasible to deduce the crystal growth mode in the solidification. In the present paper, the recalescence processes and solidification microstructures of undercooled Co–Sn alloys with eutectic and off-eutectic compositions were analyzed systematically. It is verified that both rapid coupled and decoupled growth can result in anomalous eutectic formation in the solidification structure.
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3. Results 3.1. Cooling curves Fig. 1 shows the cooling curves of Co–20at%Sn, Co–24at%Sn and Co–28at%Sn alloys solidifying at different undercoolings (the temperature interval between the equilibrium liquidus temperature
2. Experiments The experimental materials were Co–20at%Sn hypo-eutectic alloy, Co–24at%Sn eutectic alloy [25] and Co–28at%Sn hypereutectic alloy. The alloy ingots were prepared by arc melting Co and Sn of 99.999 wt% and 99.99 wt% purities, respectively, in a Ti-gettered argon atmosphere. The undercooling experiment was carried out in a vacuum chamber back-filled with ultra-pure argon. Covered by the glass purifier made from 50% B2O3, 30% Na2SiO3 and 20% Na2B4O7 and dehydrated at 1273 K for 6 h in advance, the alloy ingot placed in a quartz glass crucible was induction melted, and cyclically superheated and cooled under the protection of the molten glass flux until the desired undercooling was achieved. The superheating degree of the melt was controlled to be about 300 K by adjusting the input power of the induction coil. An infrared pyrometer with an accuracy of 1 K and a response time of 1 ms was utilized to monitor the thermal history of the sample during the entire experimental cycle. The temperature data were recorded in a computer. The as-cast sample was a short column of about 8 mm in diameter. The sample was sectioned along the longitudinal direction, polished and then etched for structural observation under an optical microscope (OM). The etching agent was a mixed solution of CuSO4, HCl and ethanol. The phases in the solidification structures were determined by X-ray diffraction (XRD) with Cu Ka radiation. Electron backscattered diffraction (EBSD) measurement was performed using a JSM7600F field emission gun scanning electron microscope (SEM) operated in the secondary electron mode to interesting areas. Since EBSD is very sensitive to crystalline perfections, a well prepared sample is a prerequisite to obtain a good diffraction pattern. For this purpose, the section of the sample for measurement was first mechanically polished with diamond polishing agent from 5 mm to 0.5 mm roughness and subsequently vibration polished with alumina polishing agent to 0.05 mm roughness. During EBSD analysis, the sample was tilted at 651 to the horizontal plane to optimize both the contrast in the diffraction pattern and the fraction of electrons scattered from the sample. An acceleration voltage of 18 kV and probe current of 10 nA were used.
Fig. 1. Cooling curves at different undercoolings: (a) Co–24at%Sn eutectic alloy, (b) Co–20at%Sn hypo-eutectic alloy and (c) Co–28at%Sn hyper-eutectic alloy.
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TL and the onset temperature TN of solidification). Their TL is 1438, 1385 and 1407 K respectively. It can be seen that only one temperature recalescence happens during the solidification of Co–24at%Sn eutectic alloy at all undercoolings studied (Fig. 1). As undercooling increases, crystal growth velocity rises gradually, leading to an increasing recalescence rate (the ratio of the difference between the end and onset temperatures of recalescence to the recalescence time). For Co–20at%Sn hypo-eutectic alloy, the cooling curve changes complexly as undercooling increases. As shown in Fig. 1b, there are two critical undercoolings DTc1 (about 100 K) and DTc2 (about 240 K) across which the temperature recalescence behavior significantly changes. When undercooling is less than DTc1, for example at 40 K and 70 K, the cooling curve shows two recalescence events. The first one becomes obvious at larger undercoolings while the end temperature of the second one is constantly lower than the eutectic temperature. It should be noted that a relative long temperature plateau follows the second recalescence but not the first one. When the alloy melt is undercooled between DTc1 and DTc2, for instance at 110 K and 208 K, only one recalescence happens. When undercooling exceeds DTc2, for example at 270 K, two recalescences occur again. With the end of the first rescalescence, the temperature falls rapidly to the onset temperature of the second recalescence. The typical cooling curves of Co–28at%Sn hyper-eutectic alloy are shown in Fig. 1c. In the undercooling range studied, only one critical undercoolings DTc1 (about 70 K) exist. At undercooling lower than DTc1, for example at 17 K and 52 K, two recalescences happen during solidification. Once again, only one recalescence is seen on the cooling curve when undercooling exceeds DTc1, as occurred at 97 K and 200 K. 3.2. Solidification structures In the solidification of undercooled alloy melt, particularly at low undercooling, multiple nucleation event often happens, leading to several freezing fronts propagating simultaneously in the rapid solidification. The latent heat released from these freezing domains may influence mutually, causing the thermal inhomogeneities across the sample. As a result, the rapid
solidification microstructures in the sample are inhomogeneous. When undercooling is large enough, single nucleation event happens in the rapid solidification. In such a case, the primary solid almost forms at the same interface undercooling during rapid solidification and therefore the resultant microstructures should be uniform if the crystal growth mode is the same. For better comparison and analysis, the microstructures near the nucleation site were used for all the three Co–Sn alloys in this paper. Since the characteristic size of the microstructure may also be influenced by local solidification time, the microstructures selected also located at about half radius of the sample. 3.2.1. Co–24at%Sn eutectic alloy Fig. 2 shows the sectional microstructures of the Co–24at%Sn eutectic samples solidified at different undercoolings. The microstructure is composed completely of regular lamellar eutectics at very low undercooling (Fig. 2a). According to the Co–Sn binary phase diagram [25], the phases in the microstructure are rather complex as solid-state phase transformations may be involved in the post-solidification cooling process, especially at slow cooling rates. So the samples solidified at low and large undercoolings respectively were analyzed through XRD, and the results indicate that four phases, a-Co, b-Co3Sn2, e-Co and a-Co3Sn2 coexist in the microstructure (Fig. 3). Among them the latter two are the transformation products of a-Co and b-Co3Sn2 at lower temperatures. However, only two phases can be identified in the OM observation, i.e. the darker Co phase and the brighter Co3Sn2 phase (Fig. 2). So, in the following description a-Co and e-Co as well as b-Co3Sn2 and a-Co3Sn2 will not be distinguished and only noted by Co and Co3Sn2, respectively, unless otherwise pointed out. Anomalous eutectics begin to form once undercooling increases to 21 K (Fig. 2b), in which the Co phase is of particulate or short rod shape, coexisting with traces of eutectic lamellae. Around the anomalous eutectic are lots of regular lamellar eutectics (not shown here). As undercooling rises, the fraction of anomalous eutectics increases. When undercooling increases up to the maximum value 203 K, most of the sample is occupied by anomalous eutectics except a small amount of lamellar eutectics lying between them (comparing Fig. 2c and d). The average
Fig. 2. Sectional microstructures of the Co–24at%Sn eutectic alloy undercooled by (a) 15 K, (b) 21 K, (c) 110 K and (d) 203 K.
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diameter of Co particles in the anomalous eutectic is exhibited in Fig. 4a. Clearly, the size of Co particles increases first and then decreases as undercooling rises. The volume fraction of anomalous eutectics is evaluated by measuring the area of the anomalous eutectic zone on the sample cross-section and the result indicates that it is nearly in direct proportion to undercooling (Fig. 4b). 3.2.2. Co–20at%Sn hypo-eutectic alloy The cross-sectional microstructures of Co–20at%Sn samples solidified at different undercoolings are shown in Fig. 5. The
Fig. 3. XRD patterns of the Co–24at%Sn eutectic alloy solidified at undercooling of 15 K and 203 K.
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solidification structure evolution with undercooling is very complex. At low undercoolings, the solidification microstructure is composed of coarse primary Co dendrites, surrounded by Co3Sn2 haloes and regular lamellar eutectics (Fig. 5a). When undercooling increases to about 70 K the microstructure changes to a mixture of coarse broken Co dendrites and anomalous eutectics (Fig. 5b). Careful observation indicates that the anomalous eutectics at this time can be divided into two types: one with large Co particles and the other with small Co particles, denoted by A and B respectively in the image. These two types of anomalous eutectics are distributed in the whole sample. As undercooling increases further, the Co particles become rounder and rounder while their sizes are still obviously different between two types of anomalous eutectics. Once undercooling increases up to about 100 K (DTc1), the microstructure significantly changes again. The microstructure of the sample undercooled by 110 K is shown in Fig. 5c, where clear eutectic colonies are found. Different from the one shown in Fig. 2c, the edge of the eutectic colony in Co–20at%Sn consist of coarse Co strips embedded in the Co3Sn2 matrix instead of lamellar eutectics though the central structure belongs to the same type of anomalous eutectic. Such anomalous eutectics exists until undercooling increase up to DTc2. At larger undercoolings, eutectic colony structure disappears and the microstructure consists completely of anomalous eutectics. Different from the anomalous eutectics described above, all the Co particles within such anomalous eutectics are coarse and nearly spherical and of the same size (Fig. 5d). The size of Co particles in the anomalous eutectics of Co–20at%Sn changes complicatedly with undercooling (Fig. 4c).
Fig. 4. Average diameter of Co particles in the anomalous eutectic and volume fraction of anomalous eutectics in the solidification microstructure of Co–24at%Sn eutectic alloy (a,b) and Co–20at%Sn hypo-eutectic alloy (c,d), respectively.
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Fig. 5. Sectional microstructures of the Co–20at%Sn hypo-eutectic alloy undercooled by (a) 25 K, (b) 70 K, (c) 110 K and (d) 270 K.
Generally, the Co particle in type A anomalous eutectic is much larger than that in type B anomalous eutectic in the undercooling range studied. The average diameter of Co particles in type A anomalous eutectic decreases continuously when undercooling below DTc1 and then increases again to a higher value at 270 K. The average diameter of Co particles in type B anomalous eutectic is almost unchanged when undercooling is below DTc1. At larger undercooling (DTc1 DTc1), however, the size of Co particles becomes large and decreases gradually as undercooling increases. As for the volume fraction of anomalous eutectics, it increases with undercooling as occurred in Co–24at%Sn eutectic alloy (Fig. 4d). 3.2.3. Co–28at%Sn hyper-eutectic alloy Typical microstructures of Co–28at%Sn hyper-eutectic alloy are shown in Fig. 6. At low undercoolings, the solidification structure consists of coarse primary Co3Sn2 crystals with haloes of Co and regular lamellar eutectics (Fig. 6a). The primary Co3Sn2 phase is not of dendritic morphology but of seaweed pattern due to the weak anisotropy in the solid–liquid interface [11]. When undercooling increases to about 52 K or more, as shown in Fig. 6b, the primary Co phase becomes coarse spherical particles and fine anomalous eutectics forms between them. As undercooling increases to the range where only one recalescence happens during solidification, i.e. larger than 70 K, the microstructure contains another type of eutectic colony: fine anomalous eutectics at the center and coarse vermiculate Co3Sn2 crystals around them (Fig. 6c). Out of the eutectic colony is the mixture of Co3Sn2 seaweeds and regular lamellar eutectics. As undercooling increases, the volume fraction of anomalous eutectics in the microstructure also increases constantly. 3.3. EBSD measurement As described above, different morphologies of anomalous eutectics form in the three alloys. To uncover the anomalous eutectic formation mechanism, crystal orientations in the anomalous eutectic were analyzed using EBSD. Fig. 7 shows the results of the Co–20at%Sn hypo-eutectic alloy solidified at undercooling
of 70 K. All of the four phases, a-Co, b-Co3Sn2, e-Co and a-Co3Sn2 are indexed respectively. Since e-Co and a-Co3Sn2 resulted from the partial solid-state phase transformation of a-Co or b-Co3Sn2 phases and anomalous eutectic formation has nothing to do with the e-Co and a-Co3Sn2 phases, only a-Co and b-Co3Sn2 phases are further demonstrated in the following. In the examination area shown in Fig. 7, type A and type B anomalous eutectics coexist. Clearly, the b-Co3Sn2 phase in each type of anomalous eutectic show a sole crystallographic orientation according to the EBSD pattern (Fig. 7e) and the [001] inverse pole figure (IPF) (Fig. 7f). As shown in Fig. 7h, the [001] IPF of a-Co phase in the type A anomalous eutectics shows two different diffraction directions. But the [001] IPF of a-Co phase in the type B anomalous eutectics only displays a diffraction direction (Fig. 7i). The EBSD measurement results of the anomalous eutectic at the center of the eutectic colony in the sample undercooled by 120 K are shown in Fig. 8. When the a-Co and b-Co3Sn2 phases are indexed independently, completely random orientation distributions exhibit in their [001] IPFs (Fig. 8b and c), which are very similar to the EBSD results of the anomalous eutectic in Co–24at%Sn eutectic alloy [11]. The microstructure of the sample undercooled by 270 K is also analyzed by the EBSD and the results are given in Fig. 9. The [001] IPF of the a-Co phase exhibits the feature of completely random orientation (Fig. 9b). However, the [001] IPF of b-Co3Sn2 phase only shows two diffraction directions (Fig. 9c).
4. Discussions 4.1. Crystal growth modes in off-eutectic alloys In our previous work on the solidification of undercooled Co–24at%Sn eutectic melt it had already been confirmed that only coupled eutectic growth happens in the undercooling range studied (0–203 K) [11]. As a result, all the cooling curves have the same characteristic: a sole temperature recalescence. In contrast, the cooling curves in the solidification of Co–20at%Sn hypoeutectic alloy and Co–28at%Sn hyper-eutectic alloy are complex as undercooling increases. Considering that any temperature
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Fig. 6. Sectional microstructures of the Co–28at%Sn hyper-eutectic alloy undercooled by (a) 17 K, (b) 50 K, (c) 110 K and (d) 200 K.
recalescence starts at a temperature far higher than 773 K below which a-Co and b-Co3Sn2 transform into e-Co and a-Co3Sn2, respectively [26]. Therefore, the phenomenon of double recalescences results from solidification, meaning that coupled eutectic growth is not the sole solidification mode of the Co–Sn offeutectic alloy melts. In the microstructures of Co–20at%Sn hypo-eutectic alloy, coarse a-Co dendrites are surrounded by eutectic structures at undercoolings below 70 K (Fig. 5a and b). Obviously, a-Co dendrites are primarily formed in the undercooled melt, although the alloy melt may have been undercooled below the eutectic line, i.e. undercooling has exceeded the temperature interval between TL (1438 K) and eutectic temperature TE (1385 K). In this case, as a-Co dendrites grow, the liquid around them become rich in Sn, and the Sn concentration gradually decreases along the direction away from the a-Co solid. Once the temperature is low enough, b-Co3Sn2 nucleates, and coupled eutectic growth occurs in the remaining liquid away from the a-Co solid, while divorced eutectic growth occurs in the remaining liquid in contact with a-Co. As a result, secondary recalescence takes place. As undercooling increases further, only one recalescence event happens in the whole solidification process as occurred in Co–24at%Sn eutectic melt. The resultant anomalous eutectic at the center of the eutectic colony in the Co–20at%Sn alloy exhibits the same feature in the EBSD measurement as those in the Co–24at%Sn alloy. These mean that the melt has been undercooled into the coupled eutectic growth zone and regular eutectics first form as the primary phase instead of a-Co. When undercooling is very large, two recalescences happen again in the solidification (Fig. 1b), indicating that the primary phase returns to a-Co. In this case, the volume fraction of the primary a-Co solid is so large at the start of the second recalescence that the remaining liquid zone among the a-Co crystal is very thin, and only divorced eutectic growth occur in the following solidification. In comparison with Co–20at%Sn hypo-eutectic alloy, Co–28at%Sn hyper-eutectic alloy exhibits similar recalescence behaviors and solidification structure evolutions at low and intermediate undercoolings. But it does not experience the transition from single recalescence to double recalescences at large undercoolings, and the microstructure consisting of full anomalous eutectics as shown in
Fig. 5d does not form in the alloy. It is clear that the Co–28at%Sn hyper-eutectic alloy melt first solidifies into b-Co3Sn2 seaweeds and then to lamellar eutectics when undercooling is smaller than DTc, accompanied with two recalescence events. At larger undercoolings, coupled eutectic growth first takes place, followed by the solidification of single b-Co3Sn2 phase in the remaining liquid and another eutectic growth at the last. In this case, there is only one recalescence since nucleation is not needed for the solidification of single b-Co3Sn2 in the mixture of the primary eutectic and the remaining liquid. 4.2. Formation mechanisms of anomalous eutectics As stated above, the primary phase during rapid solidification can be either coupled eutectic or single phase, depending on the alloy composition and undercooling. Since anomalous eutectic is the evolution product of the rapidly solidified primary solid, its formation closely relates with the crystal growth mode of the primary solid. 4.2.1. Co–24at%Sn eutectic alloy For the Co–24at%Sn eutectic alloy, coupled eutectic growth goes the whole solidification process. Due to the negative temperature gradient ahead of the solid/liquid (S/L) interface, the eutectic growth interface branches during rapid solidification. Because the temperature at the eutectic branch tip is lower than the equilibrium eutectic temperature, the compositions of two eutectic phases at the S/L interface deviate from the solid composition at the equilibrium eutectic temperature, i.e. the rapidly solidified eutectic phases are rich in solutes. As release of latent heat raises the temperature of the system, the primary eutectic is superheated. Once undercooling exceeds a critical value, the superheating degree becomes large enough for the primary lamellar eutectic to be partially remelted and disintegrated during recalescence [18]. For the eutectic branches under rapid growth, the melt undercooling ahead of the S/L interface decreases gradually as the position leaves away from the tip. Correspondingly, the supersaturation degree of solute in the eutectic branches falls as they become thick until their
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Fig. 7. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 70 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co3Sn2 phase, (c) [001] IPF of e-Co phase, (d) EBSD pattern indexed to b-Co3Sn2 phase, (e) [001] IPF of b-Co3Sn2 phase in type A anomalous eutectic, (f) [001] IPF of b-Co3Sn2 phase in type B anomalous eutectic, (g) EBSD pattern indexed to a-Co phase, (h) [001] IPF of a-Co phase in type A anomalous eutectic and (i) [001] IPF of a-Co phase in type B anomalous eutectic.
Fig. 8. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 110 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co phase and (c) [001] IPF of b-Co3Sn2 phase.
compositions close to the equilibrium values at equilibrium eutectic temperature. Therefore, the eutectic formed at the late stage of the rapid solidification as well as the slow solidification
stage cannot be remelted and keeps the morphology of regular lamellae. This leads to the microstructure shown in Fig. 2c that anomalous eutectic lies at the center of the eutectic branch, of
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Fig. 9. EBSD analysis of the anomalous eutectic structure of Co–20at%Sn hypo-eutectic alloy undercooled by 270 K. (a) EBSD pattern indexed to all the phases, (b) [001] IPF of a-Co phase and (c) [001] IPF of b-Co3Sn2 phase.
which the edge is occupied by regular lamellar eutectic. As undercooling increases, the composition of the primary eutectics deviates more severely from the equilibrium value at eutectic temperature, and thus more of them are remelted during recalescence, leading to a rising volume fraction of anomalous eutectics. When undercooling is not too large, the superheating degree in the primary eutectic lamellae is relative low and only a small portion of the solid is remelted. The ripening and coarsening of the broken lamellae is very limited in the following solidification. As a result, a-Co particles in the anomalous eutectic is very small and their diameter closes to the eutectic lamellar spacing. As undercooling increases, the superheating degree in the primary eutectic increases and the remelting volume fraction rises correspondingly. In this case, ripening and coarsening take place on a large scale in the broken eutectic lamellae, leading to an increase of the a-Co particle size in the anomalous eutectic. As undercooling increases further, solidification time reduces, which restricts the growth of a-Co particles in the anomalous eutectic. So the size of a-Co particles in the anomalous eutectic of Co–24at%Sn eutectic alloy increases first and then decrease as undercooling increases.
4.2.2. Co–20at%Sn eutectic alloy It can be deduced from the cooling curves (Fig. 1b) and EBSD measurement results (Fig. 7) that two types of anomalous eutectics form at different stages of solidification in the undercooling range 70–100 K. In this case, the a-Co dendrite firstly forms from the undercooled melt during rapid solidification. Also temperature at the dendrite tip of a-Co is much lower than the equilibrium liquidus temperature of the alloy, which in combination with solute trapping makes the solid rich in solute. With recalescence proceeding, the primary a-Co dendrite is superheated and partially remelted, which makes the dendrite branches broken and form anomalous eutectics. In the resultant anomalous eutectic a-Co particles are of the same order in size as the arms diameter of the primary a-Co dendrite (type A anomalous eutectic in Fig. 5b). As undercooling increases, the dendrite tip radius and the stem diameter decrease due to the rise of growth velocity. Thus, the size of a-Co particles in type A anomalous eutectic decreases (Fig. 4c). Due to solute partitioning, the composition of the remaining liquid approaches to the equilibrium eutectic composition gradually. Once the composition and temperature satisfy the conditions for eutectic to nucleate, regular lamellar eutectics solidify in the remaining melt, causing the second recalescence. In such a case, the undercooling of the remaining liquid is large enough for the formed lamellar eutectics to be partially remelted into anomalous eutectic via the same mechanism as that in the Co–24at%Sn eutectic alloy discussed in Section 4.2.1. Owing to the much thinner eutectic lamella compared with the a-Co dendrite arm diameter, a-Co
particles in type B anomalous eutectic are greatly smaller than those in type A anomalous eutectic. In this undercooling range, the melt undercooling prior to the second recalescence changes little as undercooling increases, as a result the size of a-Co particles in type B anomalous eutectic do not change almost with the original undercooling (Fig. 4c). The fragments of a-Co resulted from partial remelting are surrounded by liquid. They may rotate in the following solidification, which leads to different crystallographic orientations among the Co particles in the anomalous eutectic. To obtain overall useful information about two types of anomalous eutectics, the step length used in the EBSD measurement was very small. Correspondingly the area of the analyzing region was limited, and only several large a-Co particles in type A anomalous eutectic were indexed. Although only two crystallographic orientations were revealed (Fig. 7h), referring to the work on other alloy systems [18,19], it can be predicted that a-Co particles in type A anomalous eutectic should exhibit a feature of random orientation. During the second recalescence, both a-Co and b-Co3Sn2 lamellae are partially remelted. But the broken fragments cannot rotate due to the insufficient surrounding liquid. Therefore crystals of two phases are all orientated. When undercooling exceeds the critical value DTc1, coupled eutectic growth first happens in the undercooled melt and thus the primary solid becomes lamellar eutectic. Due to the severe recalescence-induced superheating in the solid, the primary lamellar eutectics are heavily remelted and evolved into anomalous eutectic. The formation mechanism is the same as that in the solidification of undercooled Co–24at%Sn eutectic alloy. The formed anomalous eutectic belongs to type B. The average size of a-Co particles in the anomalous eutectic decreases with the increasing undercooling. In this case, the remelted fractions of two phases are so large that both a-Co and b-Co3Sn2 crystals in the post-recalescence solidification can rotate freely, leading to completely random orientation as shown in the [001] IPFs (Fig. 8b and c). As the primary eutectic grows, the composition of the remaining liquid becomes rich in Co. Therefore a-Co phase first precipitates surrounding the primary eutectic (Fig. 5c). When undercooling is larger than DTc2, single a-Co solid instead of lamellar eutectic again becomes the primary phase forming in the first recalescence. In such a case, the growth velocity is very high, and the rapidly solidified a-Co dendrite is supersaturated with more solute. Consequently, more of the primary a-Co solid is remelted. In the other hand, the severe solute trapping in the rapidly growing a-Co dendrite greatly reduces the solute enrichment in the liquid ahead of the S/L interface, and thus leads to the increase of the dendrite tip radius and arm diameter, as found in the rapid solidification of undercooled single phase alloy [27]. As a result, the average size of a-Co particles in type B anomalous eutectic increases again (Fig. 4c). In this case, the a-Co fragments have enough time to ripen and
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rotate in the remaining and thus they exhibit spherical morphologies (Fig. 5d) and randomly orientated (Fig. 9b) in the anomalous eutectic. b-Co3Sn2 nucleates and grows in the remaining liquid phase, with which the second recalescence takes place. But the superheating in the b-Co3Sn2 solid is so weak that no obvious remelting occurs in it. In the IPF shown in Fig. 9, there are three crystallographic orientations. The reason is that the b-Co3Sn2 solid in the analyzed region grew from three nucleation sites. As discussed in Section 4.1, there is no lamellar eutectic to form in the solidification. As a result, only type A anomalous eutectic can be found in the microstructure. 4.2.3. Co–28at%Sn eutectic alloy In the undercooling range investigated, the solidification of Co–28at%Sn hyper-eutectic alloy only experiences the change of primary phase from single b-Co3Sn2 to eutectic at the critical undercooling DTc1. The anomalous eutectic formation can be understood according to the description to the solidification of the Co–20at%Sn hypo-eutectic alloy at undercoolings below DTc2. But it should be noted that in the anomalous eutectic associated with the primary b-Co3Sn2 phase, the particulate phase is b-Co3Sn2 rather than a-Co, and correspondingly the matrix phase is a-Co rather than b-Co3Sn2. 5. Conclusions (1) In the whole undercooling range studied (0–203 K), only one recalescence occurs in the solidification of Co–24at%Sn eutectic alloy, and the primary solid is constantly lamellar eutectic. (2) At undercooling below 100 K, the solidification of Co–20at%Sn hypo-eutectic alloy is accompanied with double recalescences. The first one corresponds to the solidification of primary a-Co dendrites, while the second one to the eutectic solidification. In the undercooling range 100–240 K, there is only one recalescence resulting from the rapid growth of the primary lamellar eutectic. At higher undercooling, double recalescences take place again. (3) The solidification of Co–28at%Sn hyper-eutectic alloy at undercoolings below 70 K also results in double recalescences, but the first one is caused by the rapid growth of the primary b-Co3Sn2 phase. In the undercooling range 70–200 K, only one recalescence event associated with primary lamellar eutectic formation occurs. The return to double recalescences at large undercooling in the hypo-eutectic alloy is not revealed in this alloy. (4) Two types of anomalous eutectics form in the Co–Sn off-eutectic alloys. One results from the remelting-induced disintegration of the primary a-Co or b-Co3Sn2 phase. In this case, the granular crystals in the anomalous eutectic are randomly orientated, while the matrix phase is well orientated. Another type of anomalous eutectic results from the remelting and ripening of the lamellar eutectics solidified in the first or second recalescence. In this case, crystals of two phases in the anomalous eutectic are somewhat orientated at low undercoolings, but become randomly orientated at high undercoolings.
Acknowledgments The financial supports from the National Natural Science Foundation of China under Grant no. 50874073, Post-doctoral
Science Foundation of China under Grant no. 2011M500074 and the National Basic Research Program of China under Grant no. 2011CB610405 are greatly appreciated.
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