Halo Formation in Solidification of Off-eutectic Alloys
摘要
Halos of a second phase around primary phase often form in the solidification of off-eutectic alloys, but the underlying mechanism has not been clarified. In this paper off-eutectic Ag–Cu alloys were freely and directionally solidified, respectively, to investigate the halo formation. Halos of α-Ag phase constantly form around primary β-Cu phase in hypereutectic Ag–Cu alloys, while no halo forms around primary α-Ag phase in hypoeutectic Ag–Cu alloys regardless of in free or directional solidification, as well as how large the melt undercooling prior to nucleation is. Calculations indicate that the contact angle of α-Ag on β-Cu in hypereutectic Ag–Cu melts is approximately 85 deg, while that of β-Cu on α-Ag in hypoeutectic Ag–Cu melts is approximately 103 deg, i.e., α-Ag can wet β-Cu phase but β-Cu cannot wet α-Ag. Based on the findings in Ag–Cu alloys, it is proposed that halos form merely if the second phase can wet the primary phase. Otherwise, no halo forms. This argument also agrees well with the experimental findings in other eutectic systems. It is further suggested that biphasic eutectic systems can be divided into three categories, and the situation can be predicted by comparing the relative values of the liquid–solid interfacial energies and solid–solid interfacial energy.