Microstructure and properties of Ni-Ni ₃Si composites by directional solidification

Ni-Ni ₃Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni ₃Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni-Ni ₃Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni-Ni ₃Si 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-Ni ₃Si hypoeutectic composites present lower micro-hardness than pure Ni ₃Si, which indicate Ni-Ni ₃Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni-Ni ₃Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni ₃₁Si ₁₂ phase in the Ni-Ni ₃Si eutectic composites.