The role of Silicon on Microstructure and High Temperature Performance of Aluminide Coating on Superalloy In-738LC

Abstract

The microstructures of slurry Si-modified aluminide coatings on Ni-base superalloy In-738LC have been investigated using SEM, TEM, EDS and XRD. The corrosion performance of the coatings has been also investigated by hot corrosion and cyclic oxidation tests. In order to evaluate the hot corrosion performance, the anodic polarization curves were measured in Na 2 SO 4 -25 mol%NaVO 3 -5 mol%NaCl at 1053 K. The cyclic oxidation test was carried out by exposure to alternate conditions of air atmosphere at 1373 K and room temperature. The analyses of SEM, EDS and XRD were also used to characterize attack morphologies of the coatings exposed to the test conditions. The results showed that the heat-treated slurry Si-modified aluminide coating was composed of β-NiAl phase including fine precipitates, which were found to be complex silicides. The results of hot corrosion tests indicated that at least 10-13 mass%Si in the coating layer is needed to improve the corrosion resistance of the aluminide coating. The coatings that met this criterion also showed a good cyclic oxidation resistance.

Internal surface protection of gas turbine blade by Si‐aluminide coating

“…A thin, single phase inner layer ($10 mm in thickness) and a thick layer ($30 mm in thickness) containing precipitates were distinguished. From the viewpoint of the microstructure, the silicon-enriched nickel aluminide coating formed on GTD-111 superalloy was found to be consistent with previous work, which dealt with the same coating on In-738 superalloy [6]. Accordingly, it can be predicated that the fine particles precipitated in the top coat of the Si-AL coating are silicides (mostly as chromium silicides), as has been indicated in previous publication [8].…”

“…Up to 3-4 wt%, which is the solubility limit of silicon in the nickel aluminide phase [4], of the measured silicon content presents that in the coating matrix in solid solution while the remaining is present as compounds with refractory elements, which are strong silicide formers to form silicides precipitates [4]. It has been demonstrated by several researchers, including the senior author, that the presence of silicon in solid solution and/or silicides as silicon donors achieves improved performance of the diffusion aluminide layer, primarily by improved adherence of the protective aluminum oxide to the internal surface of the substrate [3][4][5][6][7].…”

“…The mass loss of AL-and Si-AL-coated specimens was approximately 8 and 0 mg/cm 2 , respectively, after 90 cycles of oxidation. Our previous work has shown that this superiority of the Si-AL coating over the AL coating is owing to the formation of silica, which is more resistant than alumina, on the coating surface when exposing to high temperature oxidative environments [6]. The numbers in brackets are in atomic percent.…”

“…Among potential coating modifying elements, Si has been demonstrated by several researchers, including the senior author, to be effective in improving the oxidation and Type II hot corrosion performance of the aluminide coatings [3][4][5][6][7][8][9]. Our previous work [6] demonstrated that Si may be added easily into the aluminide structure through a slurry silicoaluminizing technique.…”

“…Our previous work [6] demonstrated that Si may be added easily into the aluminide structure through a slurry silicoaluminizing technique. In addition, it was found that the silicon content of the coating and the distribution of precipitates can be controlled by the amount of Si relative to the total solid amount in the slurry, as well as by controlling coating parameters, such as coating time and temperature.…”