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J. Mater. Sci. Technol.  2017, Vol. 33 Issue (4): 352-358    DOI: 10.1016/j.jmst.2016.09.012
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Phase Selection in Solidification of Undercooled Co-B Alloys
Wei X.X.1, Xu W.2, Kang J.L.1, Ferry M.2, Li J.F.1,*()
1 State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2 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
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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.

Key words:  Undercooling      Rapid solidification      Metastable phase diagram      Structure evolution     
Received:  13 December 2015     
Corresponding Authors:  Li J.F.     E-mail:

Cite this article: 

Wei X.X., Xu W., Kang J.L., Ferry M., Li J.F.. Phase Selection in Solidification of Undercooled Co-B Alloys. J. Mater. Sci. Technol., 2017, 33(4): 352-358.

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Fig. 1.  Heating DSC curves of hypoeutectic, eutectic and hypereutectic alloy samples solidified at an undercooling below 5 K (heating rate = 20 K/min).
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).
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).
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).
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.
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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