Effect of Cu on local amorphization in bulk Ni–Ti–Zr–Si alloys during solidification

Systematic investigations of the microstructure of Ni50Ti33Zr16Si1, Ni45Cu5Ti33Zr16Si1 and Ni40Cu10Ti33Zr16Si1 slowly cooled bulk specimens reveal the local occurrence of an amorphous phase in these alloys. The sites and the morphology of the amorphous phase strongly depend on the presence of Cu. The formation of a nanoscale amorphous phase in the Cu-free Ni50Ti33Zr16Si1 alloy is observed within the primary solidified Ni50(Ti,Zr)50 intermetallic compound with an isolated morphology. In contrast, in the Cu-containing alloys, Ni45Cu5Ti33Zr16Si1 and Ni40Cu10Ti33Zr16Si1, local amorphization occurs at negatively curved interfacial areas between the primary Ni50(Ti,Zr)50 grains and the Ni3Ti and NiTiZr matrix phases. Chemical analysis of the different phases in these alloys reveals that the amorphous phase is more Zr-rich compared to the primary Ni50(Ti,Zr)50 phase, indicating that the Zr content is very crucial for enhancing the glass-forming ability. However, Cu is decisive in controlling the volume fraction, the morphology and the sites of the amorphous phase in these alloys.