Morphology and microstructure of Yb2O3 layer formed over aluminide coating produced by pack cementation of Haynes® 263 alloy

Intermetallic coatings from the Ni-Al system modified with rare earth elements (REE) additions are developed to protect the Ni-based superalloys against the increasingly demanding operating conditions, like those at ultra-supercritical (A-USC) power plants. The pack cementation process is generally limited to treatment of smaller parts, but simultaneously it allows to produce thick, well adhering and easily alloyed coatings. In this experiment, the Haynes® 263 alloy was subjected to pack cementation (PC) aluminizing process. The powder bed used in this experiment was prepared in two stages, i.e., first, the Al (8 wt.%), AlCl3 (4 wt.%), and Al2O3 (88 wt.%) powders were thoroughly mixed and only then, the Yb (1.5 wt.% of the all former) was added. The coating obtained through this process was investigated using scanning and transmission electron microscopy (SEM/TEM) methods. It helped to prove that the diffusive layer consisted mostly of Al14Cr3Ni2 ζ-phase, with significant amount of Al13(Co, Cr)4 and (Ni, Co, Cr)3Al4 larger precipitates as well as numerous smaller ones, composed of (Al, Ni, Co, Cr, Ti) or (Al, Ni, Co, Cr, Mo). The additive layer consisting of Al3Ni2 phase with up to 6 at. % Co and 2 at. % Cr was covered by a discontinuous layer of α-Al2O3 occasionally decorated with particles of the same phase. Only the very surface was found to be occupied by an amorphous Yb2O3 oxide, which formed a thin (~ 5 nm), but simultaneously a continuous layer. The latter finding is the first experimental documentation of the location of REE in the coating obtained through PC process using a powder bed modified with elements from that group.

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Opracowanie rekordu ze środków MNiSW, umowa nr POPUL/SP/0154/2024/02 w ramach programu "Społeczna odpowiedzialność nauki II" - moduł: Popularyzacja nauki (2025).