Pb—Bi亚共晶和包晶合金的快速凝固

RAPID SOLIDIFICATION OF UNDERCOOLED Pb-Bi HYPOEUTECTIC AND PERITECTIC ALLOYS

采用硅油净化法使大体积Pb－50％Bi亚共品合金和Pb－29．2％Bi包晶合金分别获得了45K（0．11TL）和117K（0．23TL）的过冷度，并对这两种合金快速凝固过程中ε金属间化合物相的组织形态及随过冷度的变化规律进行了探讨，对于Pb－50％Bi亚共晶合金，实验发现ε相以枝晶方式生长，随着过冷度的增大，ε相枝晶细化显著，最大过冷度处一次和二次枝晶间距细化达一个数量级，同时在过冷度达45K的试样中发现了ε相的表面形核，在试作内部ε枝晶相互支叉形成网状，对于Pb－292％Bi包晶合金，当过冷度较小时包晶反应持证明显，但当过冷度达到117K时包品区应被抑制，ε相直接从液相中形成，凝固组织为单相ε金属间化合物为了进一步揭示亚共晶合金中ε相技晶生长动力学特征和包晶合金中的相选择及组织变化规律。

Bulk samples of Pb-50%Bi hypoeutectic and Pb 29.2%Bi peritectic alloy were undercooled up to 45 K(0.11TL) and 117 K(0.23TL) respectively with fluxing technique by using silicone oil as denucleating ageat. The microstructure and growth characteristics of E intermetallic compound in these two alloys were investigated. In Pb-50%Bi hypoeutectic alloy, E phase grows in dendrite manner.With the increase of undercooling, the refinement phenomenon occurs to E dendrite, whose first and second dendrite arm spacings were reduced by almost an order of magnitude at the maximum undercooling. Meanwhile, surface nucleation of E phase was found, and E dendrites cross each other inside the sample. In Pb-29.2%Bi peritectic alloy, E phase grows around the coarse primary (Pb) in traditional manner of peritectic transformation at small undercooling. However, the peritectic transformation is supPressed and E phase forms directly from liquid when undercooling attains 117 K. Calculations were also carried out according to the current dendrite growth model and the classic nucleation theory in order to shed further light on the growth kinetics of E phase in Pb-50%Bi hypoeutectic alloy and the phase selection in Pb-29.2%Bi peritectic alloy.