The nucleation mechanism of alloys and control methods are important research areas for solidification theory. The triggered nucleation method can autonomously control the undercooling degree of nucleation, which is highly useful for studying nucleation theory and microstructure evolution. However, the melt is metastable. In the actual treatment, the introduction of exogenous impurities into the melt will be used as a heteroamorous point for triggered nucleation, changing the undercooling degree ΔT and remelting degree ΔTR, and further significantly affecting the subsequent solidification behavior, which is more common. In this paper, two types of Co80B20 alloys solidified via different nucleation modes were obtained by glass coating combined with cyclic superheating and undercooling. In the spontaneous nucleation mode, ΔT and ΔTR are 280 and 199 K, respectively, and the microstructure is composed of a fine, regular eutectic Co/Co3B. In the exogenously triggered nucleation mode, ΔT and ΔTR decrease to 122 and 21 K, respectively. The microstructure is transformed into primary Co dendrites and intergranular Co23B6 matrix phases. Exogenous impurities reduce f(θ), the critical nucleation radius and ΔT, increasing the inhomogeneity of the structure, which should be controlled during actual melt treatment.