Microstructure and hardness of Ni-xSi alloys

The microstructure and hardness of conventionally solidified Ni-xSi (x = 21.4%, 22%, 24%, 26%) alloys were investigated. The solidification microstructures of different Ni-Si alloys were observed by optical microscope (OM) and scanning electron microscope (SEM) and the phase composition was indentified under the help of energy dispersive X-ray (EDX) analysis. The macro- and micro-hardness of the Ni-Si alloys at room temperature were also examined. The experimental results indicated that both the microstructure and hardness closely depended on the Si content. Due to the vast formation of primary ??-Ni₃₁Si₁₂ phase, the hardness of Ni-26.0%Si alloy was significantly improved compared with that of Ni-21.4%Si eutectic alloy. However, the fracture toughness was greatly weakened simultaneously. The (?? ₁-Ni₃Si+??-Ni₃₁Si₁₂) lamellar eutectoid structure formed in the primary ??-Ni₃₁Si₁₂ phase exhibited better ductility than single ??-Ni₃₁Si₁₂ phase at the cost of relatively small hardness reduction.