Microstructure and properties of Ni-Ni3Si composites by directional solidification

Ni-Ni3Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni3Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni-Ni3Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni-Ni3Si eutectic composites display regular lamellar eutectic structure at the solidification rate R=6.0-40.0 μm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni-Ni3Si hypoeutectic composites present lower micro-hardness than pure Ni3Si, which indicate Ni-Ni3Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni-Ni3Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni31Si12 phase in the Ni-Ni3Si eutectic composites.