Bi-20%Sb合金不同熔体状态下的凝固行为及组织

前期研究发现,升温过程中Bi-20%Sb合金熔体在829～1107℃区间存在不可逆的电阻率温度异常行为。据此,分别选取700℃和900℃进行等温电阻率试验,在700℃保温电阻率随时间无变化,而在900℃保温过程中电阻率在37～54min发生了明显降低,揭示熔体结构状态发生了改变。根据这一结果进行凝固试验,发现在相同的冷却条件下经历过熔体结构转变试样的冷却速度较慢,凝固组织明显细化;两种熔体状态下的过冷度与冷却速率分别满足分段线性关系;两种熔体状态下单位面积晶粒数和冷却速率均满足良好的线性关系。凝固过冷度增加,是因为合金熔体在结构转变后原子团簇变得更加细小均匀,需要降到更低温度时才可以发生形核。冷却速度变慢,是因为热导率随着混乱度的增加而减小,经历过熔体结构转变的熔体在降温的过程中热导率减小。

Solidification Behaviors and Structures of Bi-20%Sb Alloy Under Different Melt States

In our prior work,an irreversible liquid-liquid structure transition(LLST) was observed in the range of 829-1107℃ in Bi-20%Sb alloy.Basing on this,the holding temperatures,700℃ and 900℃.were chosen to test the resistivity change during isothermal holding.The results show that there was no change at 700℃,while at 900℃ the resistivity decreased obviously during 37 min to 54 min,which indicated that there was a LLST in the melt.According to the results,solidification experiments were carried out.The solidification results show that the samples experienced LLST had lower cooling rate,finer structures under the same cooling conditions;undercooling and cooling rate met sectional functions;moreover,grain number in unit area and cooling rate were both in fine linear relation.Because atom clusters in the melt which experienced LLST were much smaller,the nucleation started at lower temperature and undercooling increased.Thermal conductivity decreases with the increase of confusion,so thermal conductivity of the melt which experienced LLST is reduced during cooling process,giving rise to slower cooling rate.