Formation of the microstructure of rapidly solidified alloys for the system $\mathrm{Sn}$–$\mathrm{Bi}$

The results of microstructural study of rapidly solidified $\mathrm{Sn}$–$\mathrm{Bi}$ alloys obtained at the melt cooling rate of $10^5$ K/s with the compositions of $\mathrm{Sn}$–$\mathrm X$ wt. % $\mathrm{Bi}$ ($\mathrm X=13,20,30,43$) are presented. Microstructural studies are carried out using scanning electron microscopy; a grain structure is analyzed by an electron backscatter diffraction technique. It is found out that the crystallization of all investigated alloys proceeds by a chemically partionless mechanism which results in the formation of a supersaturated solid solution of bismuth in a tin lattice with the original composition. Observations of the solid solution decomposition process at room temperature shows that decomposition proceeds by both continuous and discontinuous mechanisms in alloys with bismuth concentration not higher than the limit of solubility of bismuth in a tin ($20$ wt. %). Needle-like coherent bismuth inclusions are formed in the volume of a tin grain as a result of continuous decomposition. Discontinuous decomposition rate increases with the increasing concentration of bismuth in the alloy. In hipoeutectic alloys with bismuth concentration higher than the solubility limit, decomposition occurs by discontinuous mechanism. Complete decomposition proceeds by several stages and results in formation of areas with different degrees of microstructure fineness.