1 State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2 Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Cite this article:
Yun LI, Lianjie LIU, Xinming LI, Jinfu LI. Solidification of Undercooled Co75B25 Alloy. Acta Metall Sin, 2018, 54(8): 1165-1170.
The method to deeply undercool alloy melts far below the liquidus temperature by eliminating heterogeneous nucleation sites inside is frequently used in studying non-equilibrium solidification behavior, preparing quasi-crystal, amorphous alloy and other metastable materials. Previous work on the solidification of Co-(18.5~20.7)%B (atomic fraction) alloys indicated that metastable Co23B6 phase instead of stable Co3B phase was formed as the primary phase from the melts undercooled by larger than 60 K. To know whether Co23B6 phase can still primarily form from the deeply undercooled melt of Co75B25, the nominal composition of Co3B phase, the Co75B25 alloy melt was undercooled to different degrees using the glass fluxing technique, and the solidification path was identified by analyzing the microstructures and cooling curves of the samples. There was nothing other than α-Co and Co2B phases to form during solidification, indicating that not only the peritectic reaction of L (liquid) and Co2B into Co3B, predicted by the Co-B phase diagram, but also the formation of Co3B as primary phase at large undercooling were inhibited. The peritectic reaction did not occur even though the solidification was designed to occur at a very small undercooling and a cooling rate decreased to 5 K/min.
Fig.1 XRD spectra of Co75B25 alloy solidified at typical undercoolings (ΔT)
Fig.2 Cooling curves of Co75B25 alloy solidified at typical undercoolings (The insets on top right are magnifications of the transformation from the first recalescence to the second one for ΔT=210 K and 284 K, respectively; TL is the liquidus temperature)
Fig.3 Microstructures of Co75B25 alloy solidified at typical undercoolings 7 K (a), 85 K (b), 210 K (c) and 284 K (d) (Insets are magnification of local microstructure for relevant undercoolings)
Fig.4 XRD spectrum (a) and microstructure (b) of Co75B25 alloy undercooled by 5 K and solidified at a cooling rate of 5 K/min
Fig.5 DSC curves of repeatedly heating (a, c and e) and cooling (b and d) of Co75B25 alloy (with all the heating rates of 20 K/min)
Fig.6 Partial phase diagram of Co-B system on the Co-rich side, with dash lines exhibiting the L→α-Co/Co2B eutectic reaction[22]
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