采用熔融玻璃净化法+循环过热法, 考察了Ni－30%Sn (质量分数) 亚共晶合金在深过冷快速凝固条件下的再辉行为和组织演化规律. 结果表明, 当过冷度较小时, 合金熔体无明显再辉过程, 随着过冷度增大, 合金熔体凝固逐渐呈现由α－Ni枝晶和(α－Ni+Ni₃Sn)层片状共晶或羽毛状共晶向完全(α－Ni+Ni₃Sn)反常共晶转变, 在再辉曲线上呈现出由两次再辉向单次再辉的转变. 通过分析Ni－30%Sn亚共晶合金中两相的形核行为、生长速度随过冷度的变化规律以及再辉过程中两相的结晶分数变化, 解释了反常共晶的形成机制. Ni－30%Sn亚共晶合金深过冷凝固反常共晶的形成主要体现在两个方面: 较大过冷度下, 率先形成的细化的α－Ni枝晶骨架在再辉过程中充分熟化、熔断,被依附析出的Ni₃Sn相包围, 形成反常共晶; 随着过冷度的增大, 单相枝晶生长向双相枝晶生长转变, 细化的双相枝晶组织在大的再辉度下充分熟化、熔断, 也会形成反常共晶.

Eutectic solidification involves many important metals and inorganic non－metallic materials. To date, there still exist a large controversy on the formation mechanism of anomalous eutectic under the nonequilibrium rapid solidification. In this work, adopting glass flux method combined with cyclical superheating, the recalescence behaviors and microstructure evolution in highly undercooled solidification of Ni－30%Sn hypoeutectic alloy were investigated. It is found that, there is not an obvious recalescence process in the alloy melts with the low undercooling. With increasing the melt undercooling, the solidification microstructure experienced a gradual phase evolutions from α－Ni dendrite + (α－Ni+Ni₃Sn) lamellar or feathery eutectic to completely (α－Ni+Ni₃Sn) anomalous eutectic, which led the recalescence process to occur from twice to once in the cooling process. Through analyzing the nucleation behaviors of α－Ni and Ni₃Sn phases and the relationships between their growth velocities and the melt undercoolings, and the variation of their crystalline fraction in rapid recalescence process, the formation mechanism of anomalous eutectic was explained. The formation of anomalous eutectic in highly undercooled Ni－30%Sn hypoeutectic alloy should be attributed to the following two reasons: in the large melt undercooling, the complete coarsening and remelting of the pre－formed refined α－Ni dendrite skeleton occured in the subsequent recalescence process, and then the dendrite fragments were surrounded by the precipitated Ni₃Sn phase, which eventually led to the formation of anomalous eutectic. With the undercooling increased, the primary single－phase dendritic growth will change to the two－phase dendritic growth, the coarsening and remelting of two－phasic dendrites occured in the subsequent larger recalescence process, which also led to the formation of anomalous eutectic.