Effects of an ultra-high magnetic field up to 25 T on the phase transformations of undercooled Co-B eutectic alloy

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

“…Closer observation (Figure 4c1) reveals that the matrix should only consist of a single phase, and that it exhibits features similar to the typical intermetallic compounds [23]. A similar microstructure was also found in our previous work [31], and the unknown matrix turned out to be the metastable Co 23 B 6 phase. At the maximal undercooling obtained in the present work (∆T = 277 K), the microstructures evolve into the finer globular FCC-Co phases, with smaller sizes and larger fractions, surrounded by the anomalous eutectic containing submicron-sized FCC-Co particles.…”

“…Figure 8 shows the EBSD phase map and the orientation maps for the Co-18.5at.%B eutectic alloys solidified with ∆T = 150 K. Figure 8a indicates that the single-phase matrix and the primary FCC-Co phase are both involved. The single-phase matrix is unsurprisingly indexed as FCC-Co, as is shown in Figure 8b, which was also encountered in our previous work [31]. The single-phase matrix should be the Co with a slight trace of the FCC-Co + Co 23 B 6 eutectic because some fine FCC-Co particles can be found in the matrix (Figure 8c).…”

“…The orientations of the FCC-Co and Co 2 B phases in the fragmented anomalous eutectic region are slightly deflected because of the remelting caused by the large recalescence, while the orientations of the FCC-Co and Co 2 B phases in the coarsened eutectic appear to have not been affected.Figure8shows the EBSD phase map and the orientation maps for the Co-18.5at.%B eutectic alloys solidified with ∆T = 150 K. Figure8aindicates that the single-phase matrix and the primary FCC-Co phase are both involved. The single-phase matrix is unsurprisingly indexed as FCC-Co, as is shown in Figure8b, which was also encountered in our previous work[31]. The single-phase matrix should be the Co23 B 6 phase.…”

“…The majority of the FCC-Co phase in the primary phase is transformed to HCP-Co. In addition, one should note that a number of Co 2 B grains can be found in the ellipsoidal primary phase, which is also observed in [31]. In such cases, no Co 3 B grains are detected.…”

“…However, in the present work, the eutectic is composed of the FCC-Co and tetragonal Co 2 B phases, rather than the FCC-Co/Co 3 B eutectic forms, following the primary FCC-Co phase when the ∆T < 100 K. Despite the fact that this phenomenon has been observed and verified in the Co-B system by many researchers [14,24], the formation mechanism is still controversial. Li et al [24] thought that the suppression of the peritectic reaction of liquid and Co In our previous work [31], the reason of the formation of the FCC-Co/Co 2 B eutectic is that the composition of the remaining liquid becomes extremely rich in boron because of the massive precipitation of the primary Co with the application of the magnetic field. In this work, although no magnetic field was applied, the volume fraction (f s ) of the precipitated FCC-Co phase is still large, e.g., f s (∆T = 60 K) = 27%, and f s (∆T = 95 K) = 39%, indicating that, in these cases, the composition of the remaining liquid remains rich in boron.…”