Discussion on nucleation mechanism of undercooled Ni_80.3B_19.7 melts

Ni_80.3B_19.7 alloy was undercooled using the glass-fluxing method. Applying high speed video (HSV) and metallographic methods, the recalescence behavior and as-solidified microstructure were observed. In combination with the analysis of the experimental results and the calculation on the basis of time dependent nucleation theory, the nucleation mechanism in undercooled Ni_80.3B_19.7 melt was investigated. Owing to the time required to form the critical nucleus is much smaller than the time required for solid/liquid (S/L) interface to touch the nucleus, the critical nucleuses ahead the S/L interface can form and develop into the nucleation events, which make the copious nucleation ahead the S/L interface be operative. The calculation is in quantitative agreement with the experimental results.