Size-dependent solubility and phase transformation behavior of Sn–Cd nanoparticles in an Al matrix

Abstract

Nanoscaled Sn–Cd alloy particles with a nominally equiatomic composition and size in the range of 5–120 nm, embedded in an Al matrix, have been synthesized by rapid solidification processing. The orientation relationship between tin, cadmium and aluminum is given by (10̅1)_Al||(0̅10)_Sn||(1̅210)Cd[111]A_Al||[101]_Sn||[0001]Cd. Particles in the size range 5–25 nm display a single-phase hcp structure, while particles with size range of 25–120 nm size are found to be phase-separated into a tin-rich phase with bct structure and a cadmium-rich phase with hcp structure. In situ heating studies in the transmission electron microscope indicate that particles having a size range of 5–18 nm always display a single-phase microstructure right up to the melting point and subsequent cooling. The in situ heating experiments revealed complex pathways for transformation for particles having size between 18 and 30 nm. The as-solidified microstructures of over 75% of the particles analyzed, exhibit a two-phase structure in the melt-spun samples. While more than 75% of the particles analyzed by in situ heating and cooling experiments reveal formation of a single-phase structure during resolidification, some of the particles on cooling from molten state solidify into a two-phase microstructure of cadmium and tin solid solution. These particles, on cooling in the microscope, transform to single-phase particles of cadmium-rich solid solution around 110 °C suggesting a kinetically dominated microstructure evolution. In contrast, the particles in the size range 30–120 nm exhibit a two-phase microstructure both during heating and cooling. Size-dependent alloying behavior of the particles could be explained based on models that consider capillary pressure and interfacial energies.