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Electron Microscopy Technique for Imaging

Ultra Fine Scaled γγγγ´ Precipitates in Nickel Based SuperalloysIntroduction

Characterization Methodology

EFTEM imaging performed on a FEI Tecnai TF20 Transmission Electron Microscope

Technical Significance

Nickel Based Superalloys are usedextensively in gas turbine enginesbecause of their desirablemechanical properties at elevatedtemperatures and under loadbearing conditions.

Since many of the sought afterattributes of this alloy system areprimarily controlled by the γ´precipitates, it is essential toaccurately quantify their size,volume fraction, and morphologyso that this data may beincorporated into microstructuralevolution and mechanical propertymodeling.

For disk applications, the cuttingedge microstructure consists of abimodal distribution of secondary

and tertiary γ´ (Ni3Al,Ti)precipitates of the following sizescale:

Secondary γ´ (100-250 nm)Tertiary γ´ (5-30 nm)

Imaging of such ultra fine-scaledprecipitates proves to be a greatchallenge under conventional TEMand STEM characterizationtechniques.

Therefore, the objective of thiswork was to optimize an imagingtechnique that can be utilized torapidly and accurately acquireimages at a nano-scaled level ofresolution.

Research Goals`

Experimental Results

Energy Filtered Transmission Electron Microscopy (EFTEM) is an analyticalcharacterization technique used to produce elemental maps. Images are formed from electronsscattered at characteristic elemental ionization edges. Degrading artifacts and inelasticallyscattered electrons at all other energy levels are then filtered out and thus do not contribute tothe image construction. The result being a high resolution chemical map. In Ni-based

Superalloys, the γ matrix is enriched in elements such as Cr, Mo, Co, W, and Zr while the γ´precipitates are comprised of Ni3Al,Ti. Due to the differences in ionization edge potentials itis possible for the EFTEM imaging technique to distinguish γγγγ´ from the γγγγ matrix.

Figure 2. High resolution EFTEM micrographs that clearly resolve the ultra fine

scaled, bimodal distribution, of γ´ precipitates in Nickel Based Superalloy René 104.

a) Elemental Cr Map and b) Cr Jump Ratio Map

Superalloy René 104 was used to

substantiate the EFTEM imaging

technique. Thin TEM foils were

prepared by electropolishing in a

solution consisting of 10%HClO4

and 90% Methanol at -40°C/15V.

The Cr L2,3 ionization edge (575eV)

was used to obtain Pre and Post

Edge images under an acquisition