Electron backscatter diffraction (EBSD) is routinely employed as a characterization tool to obtain individual grain orientations, local texture and phase identification. However, in the case of γ – γ’ Ni-based superalloys, the EBSD technique allows mapping the orientations but fails discriminating the two phases because their diffraction signature is too similar. A coupling with EDX analysis for instance helps to identify the two phases but suffers from the lack of spatial resolution of the EDX maps.  Another way to discriminate the two phases is to use the BSE images, sensible to the chemistry, but because of the difference in the geometry of acquisition of the EBSD images and the BSE images, superposition of the two information is complicated by strong  spatial distortions. In this context, any new technique that can lead to an easier phase and orientation mapping would be welcome, especially to resolve the fine secondary γ’ precipitates (typically few tens of nanometers).

We proposed recently the iCHORD method (for ion CHanneling ORientation Determination), aiming at constructing orientation maps based on the well-known channeling contrast phenomenon observed in a rotation series of ionic images (see figure 1) [1]. The proof-of-concept of the iCHORD method was established using a titanium nitride specimen (TiN) and further tests were conducted on different metallic materials. In the case of Ni-based superalloys, the channeling contrast is very strong and allows obtaining quite easily orientation maps comparable in all aspects to the one obtained by the EBSD technique. Furthermore, when using the ion beam to scan the surface, a secondary ion signal can be detected and used for imaging the sample. In the case of γ – γ’ superalloys, this secondary ion signal provides a strong contrast between the γ phase and the γ’ phase (see figure 2). Because of the identical geometry of acquisition for the iCHORD orientation maps and the secondary ion images, both information can be easily superimposed. It allows discriminating between the two phases and giving their crystallographic orientations (see figure 3). Moreover, the spatial resolution of the secondary ion images is around few tens of nanometers, which is far better than the resolution of ~1 µm of EDX maps.

To conclude, the advantages of using ion images to study Ni-based γ – γ’ superalloys are discussed, particularly the benefits taken from the higher spatial resolution and the ease of mixing orientation and phase information.

References:

[1] Crystal Orientation Mapping via ion channeling: an alternative to EBSD

C. Langlois, T. Douillard, H. Yuan, N.P. Blanchard, A. Descamps-Mandine, B. Van de Moortèle, C. Rigotti, T. Epicier, Ultramicroscopy 157 65-72 (2015)

Figures:

Figure 1: geometry of acquisition – ez is the rotation axis in our experiments.

Figure 2: secondary ion image of a Ni-based gamma-gamma’ superalloy.

Figure 3: superposition of a secondary image containing chemical information about gamma and gamma’ phases, and the corresponding orientation map obtained by the iCHORD method.

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EMC Abstracts - https://emc-proceedings.com/abstract/ni-based-superalloy-crystalline-orientation-mapping-and-gamma-gamma-phases-discrimination-with-the-ichord-method/