The Inhibiting Effect of Reactive Element Oxides on the Pack Cementation Aluminide Coating Formation
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E well-known beneficial effect of reactive elements (REs) such as Y, La, Ce, and Zr on the oxidation resistance of high-temperature alloys has stimulated numerous researches concerning the utilization of REs in high-temperature coatings.[1–4] The popular MCrAlX overlay coatings (M and X denote Ni/Co and RE, respectively) epitomize the outcome of such studies, although exploiting the effect in the competitor coating—the aluminide coating—has not experienced a similar progress. Modification of pack cementation aluminide coatings by elements such as Pt,[5] Cr,[4,6], and Si[7] has been a common practice to attain compatibility with different high-temperature applications; however, RE-modified pack cementation aluminide coatings have not been provided commercially. Attempts have been made to introduce REs into the aluminide coating by pre-depositing a RE-containing layer;[8–12] however, addition of a RE-containing
F. EBADI, F. SHAHRIARI NOGORANI, and F. FATEMI are with the Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz 71557-13876, Iran. Contact e-mail: [email protected] Manuscript submitted May 9, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
compound (usually a reactive element oxide; REO) to the pack powder mixture[13–15] would be practically preferred due to eliminating the pre-depositing step and obtaining a defect-free microstructure. Our previous studies[16,17] have demonstrated perhaps for the first time that adding RE oxides to the conventional powder mixture of the pack cementation aluminizing (Al-NH4Cl-Al2O3) induces an inhibiting effect on the coating formation. For example, addition of only 3 to 4 wt pct of CeO2 nanoparticles to the aluminizing powder mixture halved the thickness of the aluminide coating.[16] This strong effect if adequately recognized may be utilized in controlling kinetics of RE-modified aluminide coating. In the current study the effect of different amounts of Y2O3 in the aluminizing powder mixture on the aluminide coating formation is investigated.
II.
EXPERIMENTAL PROCEDURES
Rectangular specimens (10 9 10 9 3 mm3) were cut by electric-discharge machining from the nickel-based cast alloy IN-738 LC (61Ni-16Cr-8.5Co-3.4Al-3.4Ti-2.6W-1.75Mo-1.75Ta-0.9Nb-0.11C-0.01B-0.05Zr in wt pct) and used as the substrate. The specimens were prepared for coating application by grinding to average roughness of 0.07 lm (cut-off: 0.8 mm), and then
rinsing with acetone. Aluminizing was carried out at 760 C under flowing argon in powder mixtures of 15Al, 2NH4Cl, xREO, (83 x)Al2O3 (in wt pct), where x denotes the amount of a reactive element oxide (REO). The value of x was selected as 0, 1, 3, and 5 wt pct for Y2O3 and the relevant specimens were removed from the furnace after 0 (immediately after arriving at 760 C), 0.25, 0.5, and 2 hours. The powder mixtures were mixed thoroughly in a glass bottle for 15 minutes, and then poured around the specimens in a semi-sealed retort made of AISI 321 austenitic stainless steel with 25 g capacity. Sealing the retort lid was c
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