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

• Published in: Materials Vol. 15; no. 4; p. 1315
• Main Authors: He, Yixuan, Wu, Yuhao, Bu, Fan, Zhang, Yiyuan, Zhang, Yifan, Hei, Bo, Zhang, Jianbao, Wang, Haifeng
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 10.02.2022; MDPI
• Subjects: Alloys; Boron; Candidates; Composition; Cooling; Co–B system; Eutectic alloys; Evolution; Heat treating; Intermetallic compounds; Lamellar structure; Microstructure; phase selection; pseudoeutectic regions; Recalescence; recalescence degree; Solidification; solidification pathway; Supercooling; United States > US; recalescence degree; phase selection; Co–B system; pseudoeutectic regions; solidification pathway
• ISSN: 1996-1944; 1996-1944
• DOI: 10.3390/ma15041315

The undercooling (∆ ) 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 Co B phase forms, with extremely low Δ ; (2) The FCC-Co phase primarily forms, followed by the eutectic growth of the FCC-Co and Co B phases when Δ < 100 K; (3) As the Δ increases further, the FCC-Co phase primarily forms, followed by the metastable Co B phase with the trace of an FCC-Co and Co B eutectic; (4) When the Δ increases to 277 K, the FCC-Co phase primarily forms, followed by an FCC-Co and Co B eutectic, which is similar in composition to the microstructure formed with low Δ . 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.