Role of Ni in high elastic modulus Al-Si-Ni alloys: Solidification and microstructure evolution

Abstract

A novel Al-Si-Ni casting alloy with an ultra-high elastic modulus exceeding 100 GPa was developed by compositional tailoring of a hypereutectic Al-18Si binary system through incremental Ni addition from 4 to 28 wt %. The solidification behavior and microstructural evolution were systematically investigated with respect to Ni content, focusing on the formation and morphology of Ni-rich intermetallic compounds (IMCs) and the refinement of primary Si (Sip) particles. When the Ni content exceeded 8 wt%, Sip and primary Al3Ni formed sequentially in the first stage of solidification, followed by the Al–Si-Al3Ni eutectic reaction. At Ni content exceeding 20 wt%, the Al6Ni3Si (τ3) was formed as a primary phase, and it was observed to exist as a binary mixture of Al3Ni-Al6Ni3Si with a core-shell structure. In addition, the nucleation temperature of Sip rises, weakening the refinement effect of AlP and resulting in the coarsening of Sip. With further addition to 24 wt% Ni, the Al3Ni2 was formed as a primary phase, and a ternary mixture with a distinct layered morphology appeared, consisting of Al3Ni2, Al6Ni3Si, and Al3Ni phases. Transmission electron microscopy (TEM) analyses revealed crystallographic orientation relationships between these IMCs: (110)[113] Al6Ni3Si // (031)[113] Al3Ni and (0113)[1211] Al3Ni2 // (100)[031] Al6Ni3Si. Based on these observations, the phase formation sequence and microstructural evolution mechanisms during solidification were elucidated as a function of Ni content, which provides fundamental guidance for designing high-modulus lightweight alloys with optimized microstructures.