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

Systematic investigations of the microstructure of Ni₅₀Ti₃₃Zr₁₆Si₁, Ni₄₅Cu₅Ti₃₃Zr₁₆ Si₁ and Ni₄₀Cu₁₀Ti₃₃Zr₁₆ Si₁ 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, Ni₄₅Cu₅Ti₃₃Zr₁₆ Si₁ and Ni₄₀Cu₁₀Ti₃₃Zr₁₆ Si₁, 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.