Novel insight into mechanism of secondary phase's morphology evolution in hypomonotectic Cu-Pb-Sn alloy during solidification

In the present study, hypomonotectic Cu-Pb-Sn alloys with Pb-rich secondary phase particles (SPPs) in different morphology were prepared at specific casting temperature (Tq) corresponding to different molar fraction (fS0) of the existing primary α-Cu (S0) before pouring. The results show that the SPPs transform from continuously network to independently rod-like morphology and then become abnormally coarsening with increasing Tq. Phase-field calculation was carried out to discuss the morphology evolution mechanism of the secondary phase droplets (L2), which can effectively overcome the experimental difficulty of the investigation about the monotectic reaction with an extremely high reaction rate. It is found that fS0 can strongly affect such morphology evolution. A relatively high fS0 corresponding to a lower Tq is proven to promote the formation of network L2. In contrast, a lower fS0 corresponding to a higher Tq can promote the formation of rod-like L2, which is mainly benefitted from the immiscibility of the Cu-rich residual liquid (L1) and L2. However, if Tq continuously increases, abnormally coarsening of L2 will occur mainly through collision and coagulation driven by the Marangoni force and the solidification front's pushing. The coarsening process of L2 during heating was observed through synchrotron radiation. This work reveals the morphology evolution mechanism of the SPPs in hypomonotectic Cu-Pb-Sn alloy during the monotectic reaction.