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Single crystal superalloy alloy: γ’ precipitates

Fig. 1 shows a TEM bright field overview of a single crystal creep resistant superalloy with coherent cuboid γ’-precipitates. The size, shape and aspect ratio of these secondary precipitates are characterized.


Figure1: TEM bright field image of γ’ precipitates in a Ni-base single crystal alloy.


The use of electron diffraction and dark field imaging reveals tertiary precipitates in the channels between the secondary cuboids (see Fig. 2).

Figure2: TEM dark field image of Ni-base single crystal alloy with nano-sized tertiary γ’ particles in channels between the secondary cuboids.


Polycrystalline superalloy: γ’’ precipitates

Fig. 3 shows three grains (different contrast values) with precipitates in a polycrystalline superalloy.


Figure3: TEM bright filed image of polycrystalline Ni-base alloy with platelet shaped γ'' precipitates in three neighboring grains.


Using electron diffraction (Fig. 4) the precipitates are identified as γ’’. The strong diffraction spots belong to the Ni matrix, the weaker spots result from the γ’’ precipitates.


Figure4: Diffraction pattern of the dark grain in Fig. 3. Weak diffractions spots between the strong ones (Ni matrix) are related to the γ'' precipitates.


Fig. 5 shows a TEM dark field image using the diffraction spot marked in Fig. 4. All precipitates have bright contrast and allow the obtaining of local number densities and volume fractions.


Figure5: TEM dark field image of γ'' precipitates (bright lines).


High Resolution TEM (HRTEM) imaging in Fig. 6 is used to detect coherency, strain fields on the interface between matrix and precipitate and crystal defects.

Figure6: HRTEM image of a semi-coherent γ'' precipitate.