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| DETERIORATION OF ALUMINIDE COATING ON NICKEL-BASE SUPERALLOYS |
| by Guan Hengrong; Lou Hanyi; Mao Xiaoyu and Du Zhanjun(Institute of Metal Research; Academia Sinica) |
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Cite this article:
by Guan Hengrong; Lou Hanyi; Mao Xiaoyu and Du Zhanjun(Institute of Metal Research; Academia Sinica). DETERIORATION OF ALUMINIDE COATING ON NICKEL-BASE SUPERALLOYS. Acta Metall Sin, 1981, 17(4): 412-493.
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Abstract The deterioration of aluminide coating formed on two cast nickel-base superalloys by pack cementation or by vapour calorizing under reduced pressure has been investigated.To evaluate the coating stability, the oxidation tests were made by exposure to static air under 800-1000℃ for 3000-5000 h or 1100℃ for 100 h.The microstructures of these coatings formed not only by both processes but also on both alloys are quite similar. It is observed that the outer zone of coating consists mainly of NiAl and Ni_2A1_3 phases on which the Ti (C, N) and Al_3Ti are dispersed; and the inner zone is composed of NiA1, Ni_3Al, Ti(C, N), M_(23)C_6 and M_6C near the underlying metal. The deterioration of the aluminide coating after a long-term exposure at elevated temperature seems to be closely related to the phase transformation of the outer zone, the sequence of whic(?) may be as follows:Ni_2Al_3+NiAl(enriched in Al)→NiAl(enriched in Ni)→→-NiAl+Ni_3Al→Ni_3Al→Ni_3Al+γ The Ni_2Al_3 and NiAl(enriched in Al) phases are extremely unstable and disappear after a short exposure below 1000℃, while NiAl(enriched in Ni) phase is very stable and not even altered to Ni_3Al after oxidized at 900℃ for 5000 h. The nucleation of the Ni_3Al phase along the grain boundary of NiAl indicates the initiation of coating deterioration. An end of its deterioration is that the transformation of NiAl into Ni_3Al+γ is just finished. The granular phases in the inner zone are carbides,mainly as M_(23) C_6 or M_6C differing with either alloy, all of them disappear at1100℃. It is also found that the σ phase in the diffusion zone may be vanished after 100 h exposure at 1000℃.
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Received: 18 April 1981
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