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Phase Selection in Solidification of Undercooled Co–B Alloys
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Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Phase Selection in Solidification of Undercooled Co–B Alloys X.X. Wei 1, W. Xu 2, J.L. Kang 1, M. Ferry 2, J.F. Li 1,* 1
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Australian Research Council Centre of Excellence for Design in Light Metals, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia 2
A R T I C L E
I N F O
Article history: Received 13 December 2015 Received in revised form 25 January 2016 Accepted 27 January 2016 Available online Key words: Undercooling Rapid solidification Metastable phase diagram Structure evolution
A series Co-(18.5–20.7) at.% B melts encompassing the eutectic composition (Co81.5B18.5) were solidified at different degrees of undercooling. It is found that the metastable Co23B6 phase solidifies as a substitute for the stable Co3B phase in the alloy melts undercooled above a critical undercooling value of ~60 K. The Co23B6 and α-Co phases make up a metastable eutectic. The corresponding eutectic composition and temperature are Co80.4B19.6 and 1343 K, respectively. On exposure of the metastable Co23B6 phase at a given temperature above 1208 K, it does not decompose even after several hours. But it transforms by a eutectoid reaction to α-Co + Co3B at lower temperature. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Solidification structure changes with the melt undercooling prior to solidification. A high undercooling can be achieved by either rapidly quenching the melt or excluding heterogeneous nucleation substrates from it. In the first case, one is difficult to trace the thermo-physical properties associated with crystal nucleation and growth. In the latter, however, it becomes possible to monitor the temperature recalescence behavior. With the obtained information, a number of high-undercooling induced phenomena, such as the grain refinement in single phase alloys[1–5], the transition from lamellar to anomalous eutectic in eutectic alloys[6–9] and metastable phase formation[10–13] have been well interpreted. More importantly, bulk non-equilibrium materials can be produced by the latter method. M-B (M = Fe, Ni, Co) alloys are widely used as soft magnetic materials. A comprehensive understanding of their structure evolution with undercooling is very important for improving the soft magnetic properties. So far explorations were mainly performed on the solidification behavior of Ni–B[14–16] and Fe–B[17–19] alloys, which resulted in the discovery of metastable Ni23B6 and Fe3B phases at high undercooling. Unfortunately, the Ni23B6, phase is so unstable that it decomposes and transforms into stable Ni and Ni3B phases during the subsequent cooling, which hinders the structural investigation
* Corresponding author. Fax: +86 21 54748530. E-mail address: jfl[email protected] (J.F. Li).
of metastable Ni23B6 phase. Recently, Ohodnicki et al.[20] simulated the crystal structure and energy of possible phases in M-B alloys, and predicted that metastable M23B6 phase is relatively stable in Co–B binary alloys than that in Fe–B and Ni–B binary alloys. Thus, a special solidification structure evolution associated with metastable Co23B6 phase formation may be present in Co–B alloys. In this paper, we report the experimental results of the solidification behavior of undercooled Co–B alloys.
2. Experimental Procedures The Co-rich side of the Co–B phase diagram contains both a eutectic reaction L → α-Co + Co3B at 18.5 at.% B, and a peritectic reaction L + Co2B → Co3B at 25 at.% B. A series of alloys around the eutectic composition (Co81.5B18.5) were selected for investigation. The alloys were synthesized from 99.999 wt% Co and B blocks. To minimize errors in alloy composition caused by the volatilization of B during melting, a Co75B25 master alloy was initially produced. Ingots of various compositions around the eutectic composition were prepared by arc melting a mixture of high purity Co and the master alloy under a high purity argon atmosphere on a water-cooled copper hearth. Each ingot was remelted several times and turned over after each solidification stage to ensure chemical homogeneity. For the undercooling experiments, about 3 g of a given alloy and a small quantity of B2O3 glass flux were put together into a fused silica crucible and inserted in the induction-heating coil. The flux was dehydrated for 6 h at 1273 K before use. After evacuating the
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Please cite this article in press as: X.X. Wei, W. Xu, J.L. Kang, M. Ferry, J.F. Li, Phase Selection in Solidification of Undercooled Co–B Alloys, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.09.012
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vacuum chamber to a pressure of 2 × 10−3 Pa, ultra-high purity argon was back-filled into the chamber, followed by induction melting of each alloy. Covered by the molten flux, each alloy was cyclically superheated and cooled until a desired undercooling was obtained. Each alloy was then cooled to room temperature. An infrared pyrometer with an accuracy of 1 K and a response time of 1 ms was utilized to monitor the thermal history of each alloy, with the subsequent temperature data recorded in a computer. To avoid interfering with nucleation, solidification was designed to take place spontaneously rather than triggered manually. Each solidified alloy was sectioned, polished, and etched with a mixture of glacial acetic acid, hydrochloride acid, nitric acid and water. The solidification microstructures were analyzed using a JSM7600F field emission gun scanning electron microscope (SEM), and the composition of the phases determined by energy dispersive spectroscopy (EDS). The phase constitution of the alloys was determined using a Thermo ARL X-ray diffractometer (XRD). Thermal analysis was performed in a Netzsch (404F3) differential scanning calorimeter (DSC) under a flow of high-purity argon. The temperature and enthalpy of the instrument were calibrated with pure In, Sn, Bi, Zn, Al and Au. A given sample weighing ~10 mg was placed in an alumina crucible and subject to two types of thermal processing: (i) the alloys were heated isochronally until completely melted, followed by cooling both at a rate of 20 K/min to determine the characteristic temperatures associated with any phase transformations that occur, and (ii) the molten alloys were cooled rapidly at a rate of 40 K/min from the liquidus temperature to a predetermined temperature, and then held isothermally for up to several hours to investigate the stability of Co23B6. 3. Results and Discussion 3.1. Solidification of Co–B alloys around the eutectic composition The L→α-Co + Co3B equilibrium eutectic reaction was first investigated. Fig. 1 shows the DSC curves of a hypoeutectic, eutectic and hypereutectic alloy, which were solidified under near-equilibrium conditions (<5 K undercooling). The curve of Co81.5B18.5 is composed of a single endothermic peak whereas the curves of both Co83.7B16.3 and Co80.4B19.6 have two partially overlapping peaks, indicating that Co81.5B18.5 is the eutectic composition, as confirmed in
Fig. 1. Heating DSC curves of hypoeutectic, eutectic and hypereutectic alloy samples solidified at an undercooling below 5 K (heating rate = 20 K/min).
the phase diagram[21]. However, all alloys started to melt at 1403 K, which corresponds to a higher temperature than the reported eutectic temperature of 1383 K[21]. As the superheating of a solid during melting is negligible, the equilibrium eutectic temperature of the Co–B alloys should therefore be 1403 K. The difference between the published eutectic temperature in 1966[22] and the present analysis may be due to improvements in the accuracy of analysis. Hereafter, we use 1403 K as the equilibrium eutectic temperature. The solidification microstructure of the Co–B alloys changes significantly with the melt undercooling prior to nucleation. Fig. 2 shows the microstructure of Co81.5B18.5 eutectic alloy solidified at various degrees of undercooling. A lamellar eutectic forms when the undercooling is less than 60 K (Fig. 2(a and b)); the dark phase and gray matrix phase are α-Co and Co3B, respectively. At larger undercooling (up to a maximum of 75 K in the present work), primary α-Co dendrites are surrounded by another phase, with a very fine fibrous eutectic filling the remaining space (Fig. 2(c and d)). To understand why an abrupt microstructural change occurs at some critical undercooling, the eutectic alloy was solidified at an undercooling of 55 and 65 K, respectively, and examined by XRD as shown in Fig. 2(e). It can be seen that the alloy consists of α-Co and Co3B at 55 K undercooling, whereas α-Co and Co23B6 form at 65 K undercooling. Hence, the unidentified gray phase surrounding the primary α-Co dendrites at large undercooling (Fig. 2(c and d)) is Co23B6, with the remaining space comprised of the α-Co/Co23B6 eutectic. That is to say, solidification of the Co81.5B18.5 eutectic alloy at an undercooling greater than 60 K generates metastable Co23B6 rather than stable Co3B. Temperature recalescence is a useful indicator of the solidification behavior of an alloy. Fig. 2(f) shows the cooling curves for the eutectic alloy at various degrees of undercooling. For an undercooling less than 60 K, there is a single temperature recalescence event during solidification with the highest recalescence temperature approaching the eutectic temperature (1403 K). However, for an undercooling greater than 60 K, there is a significant change in recalescence behavior with two recalescence events appearing on the cooling curves. During the first recalescence event, the alloy is reheated to a temperature far below the eutectic temperature. The alloy then starts to cool but is disrupted by a secondary recalescence event. Interestingly, the highest temperature reached during the secondary recalescence is essentially the same for all alloy compositions, which is followed by an extended temperature plateau before a more rapid decrease in temperature. Based on these results, a relationship between the degree of undercooling and solidification path is established for the Co81.5B18.5 eutectic alloy. For small undercooling (<60 K), the alloy solidifies into a classic α-Co/Co3B lamellar eutectic. At large undercooling, the metastable Co23B6 phase substitutes Co3B during solidification. In this situation, the first recalescence event is associated with the rapid growth of primary α-Co. As this phase grows in the liquid in the time interval between the two recalescence events, the B atoms rejected from the solid due to solute redistribution enrich the liquid ahead of the interface. The metastable Co23B6 phase then nucleates and rapidly grows in this B-rich liquid, resulting in the secondary temperature recalescence event, which eventually forms a solidification structure of primary α-Co dendrites surrounded by a halo of this metastable Co23B6 phase. This phenomenon, where halos of one eutectic phase encompass another eutectic phase, is widely observed in the solidification microstructure of off-eutectic composition alloys[23–26]. With the composition of the remaining liquid changing back to the eutectic composition owing to the Co23B6 phase growth, the remaining liquid transforms into a fibrous α-Co/ Co23B6 eutectic during the following period of the temperature plateau. The formation of primary α-Co + α-Co/Co23B6 eutectic in the deeply undercooled eutectic Co81.5B18.5 alloy melt indicates that the
Please cite this article in press as: X.X. Wei, W. Xu, J.L. Kang, M. Ferry, J.F. Li, Phase Selection in Solidification of Undercooled Co–B Alloys, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.09.012
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Fig. 2. Microstructures of the bulk Co81.5B18.5 eutectic alloy solidified at undercooling of (a) 5 K, (b) 55 K, (c) 65 K and (d) 75 K. (e) XRD patterns of 55 K and 65 K undercooled samples and (f) cooling curves of four samples (Te is the equilibrium eutectic temperature and Tme the metastable eutectic temperature).
composition of the metastable α-Co/Co23B6 eutectic is actually hypereutectic in the equilibrium phase diagram. 3.2. Solidification of Co80.4B19.6 alloy Based on the findings that Co81.5B18.5 is the equilibrium eutectic composition, but is a hypoeutectic composition for the metastable α-Co/Co23B6 eutectic system, a series of hypereutectic alloys in the equilibrium phase diagram with composition steps of 0.5 at.% B were solidified at various degrees of undercooling to determine this metastable eutectic composition. According to the generated solidification microstructures, cooling curves and XRD analyses, the metastable eutectic composition was determined to be Co80.4B19.6.
Fig. 3(a and b) shows the changes in microstructure with degree of undercooling. Below a critical undercooling of 60 K, the alloy solidifies according to the equilibrium phase diagram, whereby primary Co3B forms followed by the eutectic reaction that generates α-Co/ Co3B. The XRD pattern in Fig. 3(c) confirms that only α-Co and Co3B are generated. Above this critical undercooling, the solidification microstructure is composed entirely of a fibrous eutectic, Fig. 3(b), with the two phases in the eutectic identified by XRD to be α-Co and Co23B6 (Fig. 3(c)). The temperature recalescence behavior of the Co80.4B19.6 alloy (see Fig. 3(d)) is in good agreement with the microstructural evolution: below the critical undercooling, there are two recalescence events corresponding to the formation of primary Co3B and α-Co/Co3B eutectic, respectively, whereas only a single
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Fig. 3. Microstructures of the bulk Co80.4B19.6 alloy solidified at an undercooling of (a) 20 K and (b) 68 K, and (c) their corresponding XRD patterns and (d) cooling curves (Tl is the equilibrium liquidus temperature, Te the equilibrium eutectic temperature and Tme the metastable eutectic temperature).
recalescence event occurs above the critical undercooling that corresponds to the metastable eutectic reaction. Hence, the metastable eutectic composition is indeed Co80.4B19.6
3.3. Solidification of Co79.3B20.7 alloy We have shown that the metastable Co23B6 phase forms during solidification of some deeply undercooled Co–B alloys. To better understand the formation of this phase, an alloy with the same nominal composition, Co79.3B20.7, was solidified at different degrees of undercooling. For an undercooling less than the critical value of ~60 K, the equilibrium Co3B phase still nucleates as the primary phase and grows into coarse plates, with the α-Co/Co3B eutectic forming between them (Fig. 4(a)). Above this critical undercooling, the solidification microstructure consists only of coarse plates, exhibiting features similar to a typical intermetallic compound (Fig. 4(b)), with Fig. 4(c) confirming that the only phase present is Co23B6. Overall, below a critical undercooling the Co79.3B20.7 alloy solidifies according to that expected in the phase diagram. The rapid growth of primary Co3B triggers the first temperature recalescence event, while the subsequent rapid growth of α-Co/Co3B eutectic leads to the secondary event (Fig. 4(d)). However, above a critical undercooling, the alloy solidifies to generate a microstructure consisting entirely of Co23B6 phase. Hence, there is only one temperature recalescence on the cooling curve, and the recalescence temperature never exceeds the melting temperature of this phase (Fig. 4(d)).
3.4. Metastable phase diagram To determine the characteristic temperatures associated with the metastable Co23B6 phase, the Co81.5B18.5, Co80.4B19.6 and Co79.3B20.7 alloys solidified at large undercooling into metastable Co23B6 phase were reheated at a rate of 20 K/min, and the DSC curves are shown in Fig. 5(a). The Co81.5B18.5 alloy starts to melt at 1343 K, with a shoulder following the main exothermic peak on the DSC curve. The Co80.4B19.6 alloy also starts to melt from 1343 K, but there is no other exothermic signal after the main peak. For the Co79.3B20.7 alloy, the onset temperature of melting shifts to 1348 K, and only one exothermic peak appears on the DSC curve. During performing the DSC measurement, it was found that Co–B alloys can be easily undercooled beyond 60 K in the alumina crucible of the DSC under the protection of high purity argon atmosphere, and the resultant metastable Co23B6 phase can be remained at room temperature if the cooling rate is higher than 25 K/min (the cooling rate of the bulk sample in the undercooling experiment is ~60 K/min). Therefore, the Co79.3B20.7 alloy was melted and then cooled at a rate of 40 K/min to different temperatures followed by isothermal annealing in the DSC based on the premise that Co23B6 has formed during solidification. Fig. 5(b) shows the isothermal DSC curves where it can be seen that no transformation occurs after 4 h at 1208 K or above. However, holding at 1203 K results in the onset of an exothermic peak after ~161 min. Decreasing the annealing temperature results in a progressive shortening of this onset time to a minimum of 4.3 min at 1023 K, but further decreasing the
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Fig. 4. Microstructures of the bulk Co79.3B20.7 (Co23B6) alloy solidified at an undercooling of (a) 17 K and (b) 70 K, and (c) their corresponding XRD patterns and (d) cooling curves (Tl is the liquidus temperature, Te the equilibrium eutectic temperature and Tml the melting temperature of metastable Co23B6 phase).
temperature lengthens this time. Finally, no transformation was detected after 4 h at 923 K. The alloy annealed to the end of the exothermic peak is composed of α-Co and Co3B (Fig. 5(c and d)). Obviously, the metastable Co23B6 phase decomposes via a eutectoid reaction Co23B6→α-Co + Co3B in the temperature range of 923–1208 K. Based on the foregoing results, the Co-rich side of the Co–B phase diagram is given in Fig. 6. The Co81.5B18.5 alloy melt solidifies into α-Co + Co3B eutectic through an equilibrium eutectic reaction at 1403 K. Under the equilibrium eutectic line, there exists a metastable eutectic reaction, L→α-Co + Co 23 B 6 , with the eutectic composition of Co80.4B19.6 and eutectic temperature of 1343 K. The Co23B6 phase is stable between the eutectoid (1208 K) and melting (1348 K) temperatures but, at lower temperatures, decomposes via the eutectoid reaction Co23B6→Co + Co3B. With the knowledge of this phase diagram, one can control the microstructure and finally the properties of Co–B alloys by undercooling the alloy melts to different degrees. Zr50Cu50 alloy is a good glass former with a high reduced glasstransition temperature of 0.56. Aiming at exploring why the alloy exhibits good glass-forming ability, Wang et al.[27] measured the crystal growth velocity over a wide range of undercooling up to 325 K, and observed a maximum growth velocity at an undercooling of 200 K instead of the monotonic increase with undercooling. They then concluded that diffusion-controlled mechanism dominates the crystal growth at all undercoolings. Metastable phases form from undercooled alloy melts generally because of their lower solid/ liquid interface energy and correspondingly lower nucleation work
than the stable phase formation[2,10,11]. If it is noted that Co23B6 phase contains less B than stable Co3B phase, and less Co is rejected by the growing Co23B6 phase in the undercooled alloy melts investigated, it is clear that solute diffusion is another favorable factor for the Co23B6 phase formation at large undercooling. Wang et al.[28] examined the undercooling change of a hypoeutectic Co83B17 alloy with the overheating temperature of the alloy melt. They found that the undercooling could get up to 200 K if the overheating temperature exceeded 1663 K. Otherwise, an undercooling as low as about 80 K was obtained. In order to avoid contaminating the alloy composition owing to the possible reaction between the alloy and crucible, we controlled the overheating degree not more than 200 K in the experiment. This might be the reason why the undercooling achieved in our work is smaller. Considering that the highest recalescence temperature is very near to the metastable eutectic temperature of Co/Co23B6 on the recalescence temperature curve of Co83B17 alloy at 200 K undercooling[28], it is very probable that there is no other new phase to be involved in the solidification.
4. Conclusions The phase selection in solidification of undercooled Co–B alloys around the equilibrium eutectic composition Co81.5B18.5 was investigated by using glass-flux processing techniques and DSC. A bulk metastable phase Co23B6 was successfully obtained from the deeply undercooled melt, and its thermal stability corresponding to the
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Fig. 5. (a) DSC curves of the Co81.5B18.5, Co80.4B19.6 and Co79.3B20.7 alloys consisting fully of metastable Co23B6 phase at a heating rate of 20 K/min, (b) DSC curves of the Co79.3B20.7 alloy consisting fully of metastable Co23B6 phase during isothermal annealing at various temperatures, (c) microstructure of the alloy isothermally annealed to the end of the decomposition and (d) the corresponding XRD patterns.
metastable phase diagram of Co–B was clarified. The following conclusions are drawn. (1) Phase selection is involved in the solidification of undercooled Co–B alloy. The alloys around the equilibrium eutectic
composition Co81.5B18.5 solidify according to the equilibrium phase diagram at undercooling below 60 K, resulting in a solidification structure consisting of stable Co and Co3B phases. Metastable Co23B6 phase forms instead of the stable Co3B phase at larger undercooling, since the solidification structure changes significantly. (2) There is a metastable eutectic reaction L→Co + Co23B6 under the equilibrium eutectic line. The corresponding eutectic temperature is 1343 K and the eutectic composition is Co80.4B19.6. (3) The metastable Co23B6 phase exhibits good stability in the temperature range from 1208 K up to its melting point of 1348 K. In the temperature range of 953–1203 K, this phase transforms by a eutectoid reaction into α-Co + Co3B. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51227001 and 51471108) and the SJTU-UNSW Cooperative Research Fund (16X120030005). References
Fig. 6. Co-rich region of the Co–B phase diagram. The solid lines correspond to the equilibrium diagram[21], while the dashed lines show the estimated metastable phase diagram. The symbols denote the phase constitution of typical bulk alloy samples solidified at different undercoolings.
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Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Phase Selection in Solidification of Undercooled Co–B Alloys X.X. Wei 1, W. Xu 2, J.L. Kang 1, M. Ferry 2, J.F. Li 1,* 1
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Australian Research Council Centre of Excellence for Design in Light Metals, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia 2
A R T I C L E
I N F O
Article history: Received 13 December 2015 Received in revised form 25 January 2016 Accepted 27 January 2016 Available online Key words: Undercooling Rapid solidification Metastable phase diagram Structure evolution
A series Co-(18.5–20.7) at.% B melts encompassing the eutectic composition (Co81.5B18.5) were solidified at different degrees of undercooling. It is found that the metastable Co23B6 phase solidifies as a substitute for the stable Co3B phase in the alloy melts undercooled above a critical undercooling value of ~60 K. The Co23B6 and α-Co phases make up a metastable eutectic. The corresponding eutectic composition and temperature are Co80.4B19.6 and 1343 K, respectively. On exposure of the metastable Co23B6 phase at a given temperature above 1208 K, it does not decompose even after several hours. But it transforms by a eutectoid reaction to α-Co + Co3B at lower temperature. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Solidification structure changes with the melt undercooling prior to solidification. A high undercooling can be achieved by either rapidly quenching the melt or excluding heterogeneous nucleation substrates from it. In the first case, one is difficult to trace the thermo-physical properties associated with crystal nucleation and growth. In the latter, however, it becomes possible to monitor the temperature recalescence behavior. With the obtained information, a number of high-undercooling induced phenomena, such as the grain refinement in single phase alloys[1–5], the transition from lamellar to anomalous eutectic in eutectic alloys[6–9] and metastable phase formation[10–13] have been well interpreted. More importantly, bulk non-equilibrium materials can be produced by the latter method. M-B (M = Fe, Ni, Co) alloys are widely used as soft magnetic materials. A comprehensive understanding of their structure evolution with undercooling is very important for improving the soft magnetic properties. So far explorations were mainly performed on the solidification behavior of Ni–B[14–16] and Fe–B[17–19] alloys, which resulted in the discovery of metastable Ni23B6 and Fe3B phases at high undercooling. Unfortunately, the Ni23B6, phase is so unstable that it decomposes and transforms into stable Ni and Ni3B phases during the subsequent cooling, which hinders the structural investigation
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of metastable Ni23B6 phase. Recently, Ohodnicki et al.[20] simulated the crystal structure and energy of possible phases in M-B alloys, and predicted that metastable M23B6 phase is relatively stable in Co–B binary alloys than that in Fe–B and Ni–B binary alloys. Thus, a special solidification structure evolution associated with metastable Co23B6 phase formation may be present in Co–B alloys. In this paper, we report the experimental results of the solidification behavior of undercooled Co–B alloys.
2. Experimental Procedures The Co-rich side of the Co–B phase diagram contains both a eutectic reaction L → α-Co + Co3B at 18.5 at.% B, and a peritectic reaction L + Co2B → Co3B at 25 at.% B. A series of alloys around the eutectic composition (Co81.5B18.5) were selected for investigation. The alloys were synthesized from 99.999 wt% Co and B blocks. To minimize errors in alloy composition caused by the volatilization of B during melting, a Co75B25 master alloy was initially produced. Ingots of various compositions around the eutectic composition were prepared by arc melting a mixture of high purity Co and the master alloy under a high purity argon atmosphere on a water-cooled copper hearth. Each ingot was remelted several times and turned over after each solidification stage to ensure chemical homogeneity. For the undercooling experiments, about 3 g of a given alloy and a small quantity of B2O3 glass flux were put together into a fused silica crucible and inserted in the induction-heating coil. The flux was dehydrated for 6 h at 1273 K before use. After evacuating the
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vacuum chamber to a pressure of 2 × 10−3 Pa, ultra-high purity argon was back-filled into the chamber, followed by induction melting of each alloy. Covered by the molten flux, each alloy was cyclically superheated and cooled until a desired undercooling was obtained. Each alloy was then cooled to room temperature. An infrared pyrometer with an accuracy of 1 K and a response time of 1 ms was utilized to monitor the thermal history of each alloy, with the subsequent temperature data recorded in a computer. To avoid interfering with nucleation, solidification was designed to take place spontaneously rather than triggered manually. Each solidified alloy was sectioned, polished, and etched with a mixture of glacial acetic acid, hydrochloride acid, nitric acid and water. The solidification microstructures were analyzed using a JSM7600F field emission gun scanning electron microscope (SEM), and the composition of the phases determined by energy dispersive spectroscopy (EDS). The phase constitution of the alloys was determined using a Thermo ARL X-ray diffractometer (XRD). Thermal analysis was performed in a Netzsch (404F3) differential scanning calorimeter (DSC) under a flow of high-purity argon. The temperature and enthalpy of the instrument were calibrated with pure In, Sn, Bi, Zn, Al and Au. A given sample weighing ~10 mg was placed in an alumina crucible and subject to two types of thermal processing: (i) the alloys were heated isochronally until completely melted, followed by cooling both at a rate of 20 K/min to determine the characteristic temperatures associated with any phase transformations that occur, and (ii) the molten alloys were cooled rapidly at a rate of 40 K/min from the liquidus temperature to a predetermined temperature, and then held isothermally for up to several hours to investigate the stability of Co23B6. 3. Results and Discussion 3.1. Solidification of Co–B alloys around the eutectic composition The L→α-Co + Co3B equilibrium eutectic reaction was first investigated. Fig. 1 shows the DSC curves of a hypoeutectic, eutectic and hypereutectic alloy, which were solidified under near-equilibrium conditions (<5 K undercooling). The curve of Co81.5B18.5 is composed of a single endothermic peak whereas the curves of both Co83.7B16.3 and Co80.4B19.6 have two partially overlapping peaks, indicating that Co81.5B18.5 is the eutectic composition, as confirmed in
Fig. 1. Heating DSC curves of hypoeutectic, eutectic and hypereutectic alloy samples solidified at an undercooling below 5 K (heating rate = 20 K/min).
the phase diagram[21]. However, all alloys started to melt at 1403 K, which corresponds to a higher temperature than the reported eutectic temperature of 1383 K[21]. As the superheating of a solid during melting is negligible, the equilibrium eutectic temperature of the Co–B alloys should therefore be 1403 K. The difference between the published eutectic temperature in 1966[22] and the present analysis may be due to improvements in the accuracy of analysis. Hereafter, we use 1403 K as the equilibrium eutectic temperature. The solidification microstructure of the Co–B alloys changes significantly with the melt undercooling prior to nucleation. Fig. 2 shows the microstructure of Co81.5B18.5 eutectic alloy solidified at various degrees of undercooling. A lamellar eutectic forms when the undercooling is less than 60 K (Fig. 2(a and b)); the dark phase and gray matrix phase are α-Co and Co3B, respectively. At larger undercooling (up to a maximum of 75 K in the present work), primary α-Co dendrites are surrounded by another phase, with a very fine fibrous eutectic filling the remaining space (Fig. 2(c and d)). To understand why an abrupt microstructural change occurs at some critical undercooling, the eutectic alloy was solidified at an undercooling of 55 and 65 K, respectively, and examined by XRD as shown in Fig. 2(e). It can be seen that the alloy consists of α-Co and Co3B at 55 K undercooling, whereas α-Co and Co23B6 form at 65 K undercooling. Hence, the unidentified gray phase surrounding the primary α-Co dendrites at large undercooling (Fig. 2(c and d)) is Co23B6, with the remaining space comprised of the α-Co/Co23B6 eutectic. That is to say, solidification of the Co81.5B18.5 eutectic alloy at an undercooling greater than 60 K generates metastable Co23B6 rather than stable Co3B. Temperature recalescence is a useful indicator of the solidification behavior of an alloy. Fig. 2(f) shows the cooling curves for the eutectic alloy at various degrees of undercooling. For an undercooling less than 60 K, there is a single temperature recalescence event during solidification with the highest recalescence temperature approaching the eutectic temperature (1403 K). However, for an undercooling greater than 60 K, there is a significant change in recalescence behavior with two recalescence events appearing on the cooling curves. During the first recalescence event, the alloy is reheated to a temperature far below the eutectic temperature. The alloy then starts to cool but is disrupted by a secondary recalescence event. Interestingly, the highest temperature reached during the secondary recalescence is essentially the same for all alloy compositions, which is followed by an extended temperature plateau before a more rapid decrease in temperature. Based on these results, a relationship between the degree of undercooling and solidification path is established for the Co81.5B18.5 eutectic alloy. For small undercooling (<60 K), the alloy solidifies into a classic α-Co/Co3B lamellar eutectic. At large undercooling, the metastable Co23B6 phase substitutes Co3B during solidification. In this situation, the first recalescence event is associated with the rapid growth of primary α-Co. As this phase grows in the liquid in the time interval between the two recalescence events, the B atoms rejected from the solid due to solute redistribution enrich the liquid ahead of the interface. The metastable Co23B6 phase then nucleates and rapidly grows in this B-rich liquid, resulting in the secondary temperature recalescence event, which eventually forms a solidification structure of primary α-Co dendrites surrounded by a halo of this metastable Co23B6 phase. This phenomenon, where halos of one eutectic phase encompass another eutectic phase, is widely observed in the solidification microstructure of off-eutectic composition alloys[23–26]. With the composition of the remaining liquid changing back to the eutectic composition owing to the Co23B6 phase growth, the remaining liquid transforms into a fibrous α-Co/ Co23B6 eutectic during the following period of the temperature plateau. The formation of primary α-Co + α-Co/Co23B6 eutectic in the deeply undercooled eutectic Co81.5B18.5 alloy melt indicates that the
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Fig. 2. Microstructures of the bulk Co81.5B18.5 eutectic alloy solidified at undercooling of (a) 5 K, (b) 55 K, (c) 65 K and (d) 75 K. (e) XRD patterns of 55 K and 65 K undercooled samples and (f) cooling curves of four samples (Te is the equilibrium eutectic temperature and Tme the metastable eutectic temperature).
composition of the metastable α-Co/Co23B6 eutectic is actually hypereutectic in the equilibrium phase diagram. 3.2. Solidification of Co80.4B19.6 alloy Based on the findings that Co81.5B18.5 is the equilibrium eutectic composition, but is a hypoeutectic composition for the metastable α-Co/Co23B6 eutectic system, a series of hypereutectic alloys in the equilibrium phase diagram with composition steps of 0.5 at.% B were solidified at various degrees of undercooling to determine this metastable eutectic composition. According to the generated solidification microstructures, cooling curves and XRD analyses, the metastable eutectic composition was determined to be Co80.4B19.6.
Fig. 3(a and b) shows the changes in microstructure with degree of undercooling. Below a critical undercooling of 60 K, the alloy solidifies according to the equilibrium phase diagram, whereby primary Co3B forms followed by the eutectic reaction that generates α-Co/ Co3B. The XRD pattern in Fig. 3(c) confirms that only α-Co and Co3B are generated. Above this critical undercooling, the solidification microstructure is composed entirely of a fibrous eutectic, Fig. 3(b), with the two phases in the eutectic identified by XRD to be α-Co and Co23B6 (Fig. 3(c)). The temperature recalescence behavior of the Co80.4B19.6 alloy (see Fig. 3(d)) is in good agreement with the microstructural evolution: below the critical undercooling, there are two recalescence events corresponding to the formation of primary Co3B and α-Co/Co3B eutectic, respectively, whereas only a single
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Fig. 3. Microstructures of the bulk Co80.4B19.6 alloy solidified at an undercooling of (a) 20 K and (b) 68 K, and (c) their corresponding XRD patterns and (d) cooling curves (Tl is the equilibrium liquidus temperature, Te the equilibrium eutectic temperature and Tme the metastable eutectic temperature).
recalescence event occurs above the critical undercooling that corresponds to the metastable eutectic reaction. Hence, the metastable eutectic composition is indeed Co80.4B19.6
3.3. Solidification of Co79.3B20.7 alloy We have shown that the metastable Co23B6 phase forms during solidification of some deeply undercooled Co–B alloys. To better understand the formation of this phase, an alloy with the same nominal composition, Co79.3B20.7, was solidified at different degrees of undercooling. For an undercooling less than the critical value of ~60 K, the equilibrium Co3B phase still nucleates as the primary phase and grows into coarse plates, with the α-Co/Co3B eutectic forming between them (Fig. 4(a)). Above this critical undercooling, the solidification microstructure consists only of coarse plates, exhibiting features similar to a typical intermetallic compound (Fig. 4(b)), with Fig. 4(c) confirming that the only phase present is Co23B6. Overall, below a critical undercooling the Co79.3B20.7 alloy solidifies according to that expected in the phase diagram. The rapid growth of primary Co3B triggers the first temperature recalescence event, while the subsequent rapid growth of α-Co/Co3B eutectic leads to the secondary event (Fig. 4(d)). However, above a critical undercooling, the alloy solidifies to generate a microstructure consisting entirely of Co23B6 phase. Hence, there is only one temperature recalescence on the cooling curve, and the recalescence temperature never exceeds the melting temperature of this phase (Fig. 4(d)).
3.4. Metastable phase diagram To determine the characteristic temperatures associated with the metastable Co23B6 phase, the Co81.5B18.5, Co80.4B19.6 and Co79.3B20.7 alloys solidified at large undercooling into metastable Co23B6 phase were reheated at a rate of 20 K/min, and the DSC curves are shown in Fig. 5(a). The Co81.5B18.5 alloy starts to melt at 1343 K, with a shoulder following the main exothermic peak on the DSC curve. The Co80.4B19.6 alloy also starts to melt from 1343 K, but there is no other exothermic signal after the main peak. For the Co79.3B20.7 alloy, the onset temperature of melting shifts to 1348 K, and only one exothermic peak appears on the DSC curve. During performing the DSC measurement, it was found that Co–B alloys can be easily undercooled beyond 60 K in the alumina crucible of the DSC under the protection of high purity argon atmosphere, and the resultant metastable Co23B6 phase can be remained at room temperature if the cooling rate is higher than 25 K/min (the cooling rate of the bulk sample in the undercooling experiment is ~60 K/min). Therefore, the Co79.3B20.7 alloy was melted and then cooled at a rate of 40 K/min to different temperatures followed by isothermal annealing in the DSC based on the premise that Co23B6 has formed during solidification. Fig. 5(b) shows the isothermal DSC curves where it can be seen that no transformation occurs after 4 h at 1208 K or above. However, holding at 1203 K results in the onset of an exothermic peak after ~161 min. Decreasing the annealing temperature results in a progressive shortening of this onset time to a minimum of 4.3 min at 1023 K, but further decreasing the
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Fig. 4. Microstructures of the bulk Co79.3B20.7 (Co23B6) alloy solidified at an undercooling of (a) 17 K and (b) 70 K, and (c) their corresponding XRD patterns and (d) cooling curves (Tl is the liquidus temperature, Te the equilibrium eutectic temperature and Tml the melting temperature of metastable Co23B6 phase).
temperature lengthens this time. Finally, no transformation was detected after 4 h at 923 K. The alloy annealed to the end of the exothermic peak is composed of α-Co and Co3B (Fig. 5(c and d)). Obviously, the metastable Co23B6 phase decomposes via a eutectoid reaction Co23B6→α-Co + Co3B in the temperature range of 923–1208 K. Based on the foregoing results, the Co-rich side of the Co–B phase diagram is given in Fig. 6. The Co81.5B18.5 alloy melt solidifies into α-Co + Co3B eutectic through an equilibrium eutectic reaction at 1403 K. Under the equilibrium eutectic line, there exists a metastable eutectic reaction, L→α-Co + Co 23 B 6 , with the eutectic composition of Co80.4B19.6 and eutectic temperature of 1343 K. The Co23B6 phase is stable between the eutectoid (1208 K) and melting (1348 K) temperatures but, at lower temperatures, decomposes via the eutectoid reaction Co23B6→Co + Co3B. With the knowledge of this phase diagram, one can control the microstructure and finally the properties of Co–B alloys by undercooling the alloy melts to different degrees. Zr50Cu50 alloy is a good glass former with a high reduced glasstransition temperature of 0.56. Aiming at exploring why the alloy exhibits good glass-forming ability, Wang et al.[27] measured the crystal growth velocity over a wide range of undercooling up to 325 K, and observed a maximum growth velocity at an undercooling of 200 K instead of the monotonic increase with undercooling. They then concluded that diffusion-controlled mechanism dominates the crystal growth at all undercoolings. Metastable phases form from undercooled alloy melts generally because of their lower solid/ liquid interface energy and correspondingly lower nucleation work
than the stable phase formation[2,10,11]. If it is noted that Co23B6 phase contains less B than stable Co3B phase, and less Co is rejected by the growing Co23B6 phase in the undercooled alloy melts investigated, it is clear that solute diffusion is another favorable factor for the Co23B6 phase formation at large undercooling. Wang et al.[28] examined the undercooling change of a hypoeutectic Co83B17 alloy with the overheating temperature of the alloy melt. They found that the undercooling could get up to 200 K if the overheating temperature exceeded 1663 K. Otherwise, an undercooling as low as about 80 K was obtained. In order to avoid contaminating the alloy composition owing to the possible reaction between the alloy and crucible, we controlled the overheating degree not more than 200 K in the experiment. This might be the reason why the undercooling achieved in our work is smaller. Considering that the highest recalescence temperature is very near to the metastable eutectic temperature of Co/Co23B6 on the recalescence temperature curve of Co83B17 alloy at 200 K undercooling[28], it is very probable that there is no other new phase to be involved in the solidification.
4. Conclusions The phase selection in solidification of undercooled Co–B alloys around the equilibrium eutectic composition Co81.5B18.5 was investigated by using glass-flux processing techniques and DSC. A bulk metastable phase Co23B6 was successfully obtained from the deeply undercooled melt, and its thermal stability corresponding to the
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Fig. 5. (a) DSC curves of the Co81.5B18.5, Co80.4B19.6 and Co79.3B20.7 alloys consisting fully of metastable Co23B6 phase at a heating rate of 20 K/min, (b) DSC curves of the Co79.3B20.7 alloy consisting fully of metastable Co23B6 phase during isothermal annealing at various temperatures, (c) microstructure of the alloy isothermally annealed to the end of the decomposition and (d) the corresponding XRD patterns.
metastable phase diagram of Co–B was clarified. The following conclusions are drawn. (1) Phase selection is involved in the solidification of undercooled Co–B alloy. The alloys around the equilibrium eutectic
composition Co81.5B18.5 solidify according to the equilibrium phase diagram at undercooling below 60 K, resulting in a solidification structure consisting of stable Co and Co3B phases. Metastable Co23B6 phase forms instead of the stable Co3B phase at larger undercooling, since the solidification structure changes significantly. (2) There is a metastable eutectic reaction L→Co + Co23B6 under the equilibrium eutectic line. The corresponding eutectic temperature is 1343 K and the eutectic composition is Co80.4B19.6. (3) The metastable Co23B6 phase exhibits good stability in the temperature range from 1208 K up to its melting point of 1348 K. In the temperature range of 953–1203 K, this phase transforms by a eutectoid reaction into α-Co + Co3B. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51227001 and 51471108) and the SJTU-UNSW Cooperative Research Fund (16X120030005). References
Fig. 6. Co-rich region of the Co–B phase diagram. The solid lines correspond to the equilibrium diagram[21], while the dashed lines show the estimated metastable phase diagram. The symbols denote the phase constitution of typical bulk alloy samples solidified at different undercoolings.
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