Abstract: Ni-0.39%B-6.58%Si alloy is denucleated by cyclical superheating within a special boron-silicate glass 50%Na2SiO3 +30%SiO2 + 11.82%Na2B4O7 + 8.18%B2-O3. Large undercoolings up to 363K(0.229TL)are achieved. The crystal nucleation and rapid growth behav iour of the undercooled melt is examined by the high speed cinematography and the infrared pyrometry. It is found that the heterogeneous nucleation dominates melt undercoolability even though the undercooling has exceeded the previously accepted critical value of 0.2Tm for homogeneous nucleation. The Recalesceuce occurs twice during the solidification of the highly undercooled alloy. The first is a major recalescence process and corresponds to the rapid crystallization of primary a-Ni phase, whereas the second is a minor one caused by the formation of ternary eutectic (a-Ni+Ni3B + -Ni6Si2B). The growth velocity of primary ?Ni dendrites varies markedly with time and attains a transient maximum of 1043mm/s at an undercooling of 363K. Cinematography study also reveals that it takes only a small fraction of the total recalescence time for the primary a-Ni dendrites to spread across the whole melt.

Key words: undercooling, rapid solidification, crystal nucleation and growth, high speed cinematography