材料期刊网

研究了两种镍基高温合金上的渗Al层的退化过程。两种合金用两种方法施加的涂层组织相似:外层以NiAl+Ni_2Al_3为母体,上面弥散着Ti(C,N)和Al_3Ti;内层由NiAl,Ni_3Al,Ti(C,N),M_(23)C_6和M_6C等相组成。渗Al层高温氧化时的退化与外层相变紧密相关,其过程如下: Ni_2Al_3+NiAl(富Al)→NiAl(富Ni)→NiAl+Ni_3Al→Ni_3Al→Ni_3Al+γ Ni_2Al_3和NiAl(富Al)相极不稳定,而NiAl(富Ni)相十分稳定。Ni_3Al沿NiAl晶界生核标志着渗层退化的开始,渗层NiAl完全变成Ni_3Al+γ是退化的终了。

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℃.