The Effects of Chemical Vapor Aluminizing Process Time and Post-processing for Nickel Aluminide Coating on CMSX-4 Alloy

In this study, a nickel aluminide coating was obtained on CMSX-4 via a chemical vapor aluminizing (CVA) technique. The CVA coating process was carried out for 4 and 6 hours at 1070°C to examine the effects of CVA process time on the properties of the nickel aluminide coating. Heat treatment processes were subsequently applied to nickel aluminide-coated samples for 2 hours at 1050 and 1100°C to examine the effects of heat treatment temperature on the coating. The characterization of coated samples was carried out via SEM, EDS, XRD, GDOES and Vickers hardness analyses. The results showed that the increment in CVA process time induced an increase in inter-diffusion zone thickness due to the increased Ni outward diffusion from the CMSX-4 substrate. The additive layer of the four-hour CVA coating is composed of the δ-Ni₂Al₃ phase due to the dominance of Al inward diffusion. The phase transformation from δ-Ni₂Al₃ to the desired β-NiAl was observed in four-hour CVA coating structure subsequent to the heat treatment processes. However, the additive layer of six-hour CVA coating consists of the β-NiAl phase both before and after the heat treatment processes. Heat treatment processes increase the thickness of the coatings because they provide increased Al inward and Ni outward diffusion. The results of the Vickers hardness tests demonstrated that the heat treatment processes cause a decrease in the hardness of the additive layer of the four-hour CVA coating from 754 to 578 HV for heat treatment at 1050°C, and to 522 HV for heat treatment at 1100°C due to phase transformation phenomena.