Effect of Undercooling on the Microstructure and Mechanical Properties of Hyper-eutectic Ni-Sn Alloy

Abstract

In this study, container-less solidification of hyper-eutectic Ni-Sn alloy has been performed by using the electromagnetic levitation technique. The effect of undercooling on the formed microstructure and on the mechanical properties have been investigated. Growth velocities were determined by high-speed video-imaging of the solidification process. A step change in the growth velocities that are increasing with increasing undercooling is observed. This aligns with an observed first change in the microstructure between low and intermediate undercoolings. At the lower undercoolings, a pro-eutectic Ni3Sn phase along with lamellar eutectic structure in the inter-dendritic region is found. At intermediate undercooling of 100-150 K, a divorced eutectic microstructure is observed whereas at undercoolings above 165 K $\alpha$-Ni precipitates are observed within $\beta$-Ni3Sn dendrites. Microhardness testing revealed higher strength for the lamellar phase as compared to the non-lamellar phase. Nano-indentation has been performed to determine the hardness and strength of individual phases in the microstructure.