Microstructure and hardness of undercooled Ni78.6Si21.4 eutectic alloy

The maximum undercooling up to 550 K was achieved in the bulk eutectic Ni78.6Si21.4 alloy melt through glass fluxing, which was combined with cyclic superheating. Transmission electron microscope (TEM), scanning electron microscope (SEM) and optical microscopy techniques were adopted to investigate the microstructure, and identified the phase composition. The directional growth, quasi-regular and refined microcrystalline microstructures were achieved in the as-solidified structure with the increased undercooling. With the increase of undercooling, the relationship between the microstructure and the hardness including the size effect of crystal grain were systematically investigated. The experimental results show that directional growth, quasi-regular and refined microstructure produced large strengthening effect, and improved the hardness to 56.5 on the Rockwell hardness C (HRC) scale. The fact is also proved by the Vickers hardness.