Re-Examination of the Microstructural Evolution in Undercooled Co-18.5at.%B Eutectic Alloy

Abstract

The undercooling (∆T) dependencies of the solidification pathways, microstructural evolution, and recalescence behaviors of undercooled Co-18.5at.%B eutectic alloys were systematically explored. Up to four possible solidification pathways were identified: (1) A lamellar eutectic structure consisting of the FCC–Co and Co3B phase forms, with extremely low ΔT; (2) The FCC–Co phase primarily forms, followed by the eutectic growth of the FCC–Co and Co2B phases when ΔT < 100 K; (3) As the ΔT increases further, the FCC–Co phase primarily forms, followed by the metastable Co23B6 phase with the trace of an FCC–Co and Co23B6 eutectic; (4) When the ΔT increases to 277 K, the FCC–Co phase primarily forms, followed by an FCC–Co and Co3B eutectic, which is similar in composition to the microstructure formed with low ΔT. The mechanisms of the microstructural evolution and the phase selection are interpreted on the basis of the composition segregation, the skewed coupled zone, the strain-induced transformation, and the solute trapping. Moreover, the prenucleation of the primary FCC–Co phase was also detected from an analysis of the different recalescence behaviors. The present work not only enriches our knowledge about the phase selection behavior in the undercooled Co–B system, but also provides us with guidance for controlling the microstructures and properties practically.

The Activation Energy of Viscous Flow and Liquid–Liquid Structure Transition in Co-B Alloys

“…Table 2 shows that the Co 23 B 6 compound has a high molar volume. It was previously noted that when the eutectic melt is heated above 1650 K, the undercooling ∆T increases significantly, and the metastable Co 23 B 6 phase is formed [18,20]. The boundary temperature of 1650 K just coincides with the onset temperature of LLST in the eutectic alloy in Figure 5.…”

“…Upon cooling, a stable Co 2 B phase [16] or metastable phases Co 3 B and Co 23 B 6 [17] are formed in an alloy with a boron content of less than 35 at.%. The phase composition of the alloy after solidification strongly depends on the overheating temperature of the melt [18] and solidification pathways [19,20].…”