深过冷熔体中的晶体形核与快速长大

CRYSTAL NUCLEATION AND RAPID GROWTH IN HIGHLY UNDERCOOLED ALLOY MELT

以特制的无机盐玻璃作为净化剂去除液态Ni-0.39％B-6.58％Si合金中的异质晶核,获得363K(0.229T_L)过冷度,采用高速摄影及快速红外测温技术研究了深过冷熔体的快速凝固行为。

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.