Effect of Sn addition on morphology evolution of secondary phase in hypomonotectic Cu-Pb-Sn alloy during solidification

Abstract

In the present study, the Cu-24Pb-xSn (wt.%) alloys with Pb-rich secondary phase particles (SPPs) in different morphology were prepared by intermediate frequency induction heating technology. The experimental results show that the SPPs transform from network to rod-like morphology with Sn content increasing from 0 wt% to 6 wt%. The relevant function mechanism of Sn addition was revealed through synchrotron X-ray radiography and thermodynamic analysis. It is indicated from the thermodynamic calculation that Sn addition can weaken the demixing tendency in Cu-Pb system. This can also lead to intensity drop of the monotectic reaction which was characterized by differential scanning calorimetry (DSC). Simultaneously, a triple-phase-coexistence (TRPC) region is introduced with Sn addition during solidification, which contains a Cu-rich liquid (L1), a Pb-rich liquid (L2) and a solid α-Cu (S). Benefitted from the mobility of these two immiscible liquids, L2 tends to show regular rod-like morphology and finally transform to rod-like SPPs, which is similar to some immiscible alloys containing an immiscibility gap, such as Al-Pb, Al-Bi and Ti-Ag alloys. The width of TRPC region in the pseudobinary phase diagram of Cu-Pb-Sn alloy increases with increasing Sn content. This is attributed to the fact that increasing Sn content can lead to the drop of interfacial tension between L1 and L2. Simultaneously, the interfacial tension drop will decrease the Marangoni migration velocity of the SPPs, and the Marangoni migration serves as the main driven force of SPPs' collision and coagulation in this case. Therefore, Sn addition is helpful to obtain fine and dispersed SPPs in this case. This work possesses guiding significance to the composition design and production of relevant hypomonotectic alloy with homogeneous and refined microstructure.