Microstructure evolution of directionally solidified Pb-Bi-Sn quasi-peritectic alloy

The Pb-30 wt.%Bi-18 wt.%Sn quasi-peritectic alloy was prepared and directionally solidified with liquid metal cooling and quenching technology under solidification rate of 0.5-100 μm/s and temperature gradient up to 18 K/mm. The microstructure evolution and solute distribution were investigated, which were based on solidification path, phase composition, solute distribution and microstructure characteristics. The DSC analysis was employed to investigate the solidification path, and the results indicate that the solidification is: L → L+α-Pb → L+α-Pb+β-Pb ₇Bi ₃ → L+α-Pb+β-Pb ₇Bi ₃+Sn → (β-Pb ₇Bi ₃+Sn) eutectic. The XRD results show that the phase compositions of the directionally solidified samples consist of primary α-Pb phase(f.c.c), β-Pb ₇Bi ₃ phase(h.c.p) and (Sn) phase(b.c.t). The primary α-Pb phase grew in cellular solid/liquid interface with solidification rate 0.5-2 μm/s, but it grew in dendritic S/L interface as the solidification rate increased to 5 μm/s or more. The solute distribution in the directionally solidified sample was measured by using EDS. The results show that solute segregation occurs in local position, which results in the formation of eutectic structure (β-Pb ₇Bi ₃+Sn).