Effect of Metastable Phase Content Controlled by Heat-Treatment on High Temperature Oxidation Performance of Directionally Solidified Ni-Si Hypereutectic Composites

Different annealing heat treatment processes were performed on Ni-Si hypereutectic composites at the solidification rate of 40 µm/s to eliminate the metastable phase and the best heat treatment process was selected (annealing temperature 1 000 °C, holding time 4 h). The oxidation weight gain and oxide rate, oxide film morphology, and oxidation kinetics of Ni-Si hypereutectic composites were studied. Moreover, the formation mechanism of the oxide film was investigated through a thermodynamic analysis, specifically by calculating the change of Gibbs free energy associated with the oxidation reactions. It is found that the oxide resistance of the Ni-Si hypereutectic composite without metastable phase is better than that of the 67.9% content of the metastable phase. The surface of oxidized film is composed of granular NiO, while the underlying layer of oxidized film is composed of platelet-shaped SiO₂ and spinel-like nickel silicate Ni₂SiO₄. Directionally solidified Ni-Si hypereutectic composites have potential applications in high temperature fields. However, Ni₃₁Si₁₂ metastable phase is inevitably formed due to the non-equilibrium solidification, which makes the overall properties of the material unstable.