中文摘要：

通过Pb-6%Bi（质量分数）合金熔体的电阻率-温度行为,揭示在第一轮升温过程于813.3~1 135.9℃温度区间内发生熔体结构状态不可逆的变化;探索熔体状态对凝固行为与组织的影响,并利用牛顿热分析法（NTA）计算凝固潜热及固相分数随时间的变化。研究结果表明：当合金熔体经历结构转变后,凝固过冷度从5.4℃增大到8.4℃,凝固潜热从5.33×107 J/m3增加到7.08×107 J/m3,凝固所需时间由28 s增加到39 s,凝固组织显著细化。熔体结构变化是合金熔体中Bi—Bi共价键原子团簇分解所致,且这一过程是一个熵增的过程,新的熔体结构更加均匀无序;当温度降低时,这些团簇不会重新形成。这会使得凝固形核需要有更大的过冷度,形核率增加,晶体生长速度降低,从而致使组织细化。不同的熔体结构状态对凝固有明显的影响。

英文摘要：

The irreversible melt structure transition occurred at the temperature intervals of 813.3-1 135.9 ℃ during the first heating circle through the resistivity-temperature curve of Pb-6%Bi（mass fraction） alloy.Based on the results,the solidification experiments were carried out,and the latent heat of solidification and the solid fraction were calculated through the method of NTA（Newton thermal analysis）.The results demonstrate that the melt structure transition makes the undercooling degree increase from 5.4 to 8.4 ℃,the latent heat from 5.33×107 to 7.08×107 J/m3 and the freezing time from 28 to 39 s,and makes the microstructure become much finer.The melt structure transition is caused by the decomposition of the atom clusters which are formed by covalent bonds of Bi—Bi.The entropy of the melt increases with the decomposition of the clusters and the melt becomes more homogeneous and disorder.The clusters cannot generate when the temperature decreases.This transition makes the undercooling degree of nucleation increase,the rate of nucleate increases and the velocity of growth becomes lower,so the microstructure becomes finer.The state of the melt structure has influence on the solidification.