Solidification Pathways in Highly Undercooled Co79.3B20.7 Alloy

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

“…According to the phase diagram of the Co-B system [26], the Co-18.5at.%B alloy is at the equilibrium eutectic point, which should be constituted by the FCC-Co and orthogonal Co 3 B phases. 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 the present work, since the FCC-Co/Co 2 B eutectic is formed from the residual liquid, the theories on the suppression of the peritectic reaction (L + Co 2 B → Co 3 B) [24] and the remelting and resolidification of the Co 23 B 6 phase [14] are supposed to be inappropriate. Therefore, the concept of a eutectic coupled zone is employed herein to obtain a decent explanation.…”

“…Li et al [24] studied the ∆T dependence of the microstructural evolution of a Co 75 B 25 alloy and found that only the α-Co and Co 2 B phases were observed over all of the achieved ∆T ranges. In the recent work of Liu et al [14], up to five solidification pathways were revealed in the solidification of an undercooled Co 79.3 B 20.7 alloy, and the primary phase changes from Co 3 B to Co 2 B to Co 23 B 6 , and then to a α-Co + Co 3 B eutectic as the ∆T increases. However, the mentioned pathways above were not observed in the alloy with the composition of Co 80 B 20 [13], which is similar to the composition of Co 79.3 B 20.7 .…”

“…There are mainly four types of intermetallic compounds in M-B systems (M = Fe, Co, Ni): MB, M 2 B, M 3 B, and M 23 B 6 , whose formations are dependent on the solidification routes [13,14], which are affected by the composition and technology parameters, such as the cooling rate, the undercooling, the holding time, etc., and which show different magnetic properties because of the different contents of boron. It is worth noting that the Co 23 B 6 metastable phase is usually regarded as a good candidate for enhancing the soft properties [15], and, in addition, the Co 2 B phase with the higher boron content has an intrinsic hardness of about 816 HV, which is comparable to the hard chromium (860 HV).…”

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

“…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].…”