In Situ Observations of Early Stage Oxidation of Ni-Cr and Ni-Cr-Mo Alloys

Abstract

Results of in situ transmission electron microscopy experiments on the early stage oxidation of Ni-Cr and Ni-Cr-Mo alloys are reported. An epitaxial rock-salt oxide with compositions outside the conventional solubility limits initiated at the surface of both alloys, progressing by a layer-by-layer mode. Kirkendall voids were found in Ni-Cr alloys near the metal/oxide interface, but were not seen in the Ni-Cr-Mo. The voids initiated in the oxide then diffused to the metal/oxide interface, driven by the misfit stresses in the oxide. A sequential oxide initiation was observed in NiCr alloys: rock-salt → spinel → corundum; however, for NiCrMo alloys, the metastable Ni 2-x Cr x O 3 (corundum structure) phase formed shortly after the growth of the rock-salt phase. Chemical analysis shows that solute atoms were captured in the initial oxide before diffusing and transforming to more thermodynamically stable phases. The results indicate that Mo doping inhibits the formation of Kirkendall voids via an increase in the nucleation rate of corundum, which was verified by density functional theory calculations.

Identifying heating rate dependent oxidation reactions on a nickel-based superalloy using synchrotron diffraction

“…This was in agreement with studies on the early stages of oxidation on binary alloys where it was shown that the first oxides formed were nickel, nucleated as islands distributed across the surface of the specimen [4][5][6][7]. It has also been shown that a metastable chromium-oxygen phase formed on the surface of binary alloys, surrounding the NiO islands [5][6][7][8][9]. Examination of the surface of the specimens after the heating stage, Fig.…”

“…The influence of other alloying elements on the stability of the oxide also needs also to be considered, e.g. molybdenum has been shown to stabilise this oxide on simple nickel alloys [9] and is present in the current alloy, Table 1.…”

“…It is noted that in studies where a range of temperatures were investigated, a more continuous layer of spinel formed at the lower temperatures investigated, [25,26,29,31]. The concentration of chromium in the alloys was also shown to have an effect [7,19,25,29] as did low concentration elements such as molybdenum [9].…”

“…Ni -Cr binary alloys [5][6][7], demonstrated the formation of NiO islands interspersed with a Cr complex; when at very early stages of Cr 2 O 3 formation. The studies all show that the nucleation and growth rates depend on a number of factors including crystal orientation at the exposed surface of the metal [2,3,4,5,8] and alloying additions [5][6][7][8][9]. These processes may occur rapidly in time periods of a few minutes or may extend to 90 min90 minutes.…”

“…These processes have been studied for some pure metals and simple alloy systems using exacting experimentation techniques; e.g. [1][2][3][4][5][6][7][8][9]. These studies involved monitoring the changes occurring to clean metallic surfaces when exposed to oxidising environments, using a number of high spatial resolution techniques.…”