Development of an aluminide coating layer on Alloy 617 by Al sputtering and inter-diffusion heat treatments

To improve the oxidation resistance in high temperature environments, an aluminide coating layer was developed on the surface of Alloy 617 by Al sputtering, aluminizing and inter-diffusion heat treatment (IDHT). After the IDHT at 1000 °C and 1150 °C, β-NiAl and γ′-Ni3Al layers were formed on Alloy 617, respectively. Also, an inter-diffusion zone (IDZ) was formed between the aluminide layer and Alloy 617 substrate, where Cr-rich M23C6 carbide and Cr and Mo-rich σ phase were extensively precipitated. The formation of IDZ and precipitates were explained with the low solubility of alloying elements in aluminide phases and diffusion of Ni. Isothermal oxidation tests in air at 900 °C showed transient oxidation kinetics for the aluminized + IDHTed Alloy 617, resulting in much lower oxidation rate than the as-received Alloy 617. A thin Al-rich oxide layer was formed on top of aluminide coating layer and re-precipitation of σ phase and M23C6 carbide was observed below the IDZ. However, despite the formation of surface oxide layer and re-precipitation below the IDZ, the overall size and composition of aluminide layer were not significantly altered during the oxidation tests for Alloy 617 with outer γ′-Ni3Al layer.