Rapid solidification of undercooled Pb-Bi hypoeutectic and peritectic alloys

Bulk samples of Pb-50%Bi hypoeutectic and Pb-29.2%Bi peritectic alloy were undercooled up to 45 K(0.11T_L) and 117 K(0.23T_L) respectively with fluxing technique by using silicone oil as denucleating agent. The microstructure and growth characteristics of ε intermetallic compound in these two alloys were investigated. In Pb-50%Bi hypoeutectic alloy, ε phase grows in dendrite manner. With the increase of undercooling, the refinement phenomenon occurs to ε dendrite, whose first and second dendrite arm spacings were reduced by almost an order of magnitude at the maximum undercooling. Meanwhile, surface nucleation of ε phase was found, and ε dendrites cross each other inside the sample. In Pb-29.2%Bi peritectic alloy, ε phase grows around the coarse primary (Pb) in traditional manner of peritectic transformation at small undercooling. However, the peritectic transformation is suppressed and ε 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 ε phase in Pb-50%Bi hypoeutectic alloy and the phase selection in Pb-29.2%Bi peritectic alloy.