Introduction: The increasingly rigorous demands of the aerospace industry for new superalloys with good material properties at high temperatures drives the development of new nickel-base superalloys. Unfortunately, the development of new alloys lags behind the engineering demands due to the long development process. One such nickel-base superalloy is Rene88DT, commonly used in the disks of jet engines. This project seeks to characterize the carbides in Rene88DT in order to facilitate the modeling of the microstructural development based on composition and processing. Three different samples of Rene88DT were analyzed: as received from manufacturer (sample P) and heat treated (samples N and O).

Data Electron Microscopy

Scanning Electron Microscopy (SEM)

The Scanning Electron Microscope provides high magnification images for qualitative and quantitative analysis. Electrons pass through a 5-30 kV potential gradient and are focused with a pair of magnetic lenses. The interaction between the electron beam and sample is complex, producing several types of radiation through a number of physical reactions. The complexity of this reaction is the basis for the usefulness of electron microscopy and the reason for the wide selection of electron microscopy analytical tools.

Scanning electron microscopy primarily uses secondary and backscatter electrons in imaging. Secondary electrons are formed from incident electrons colliding with the atomic electrons in the sample, knocking out loosely bound electrons near the surface (~10nm). The electrons have only about 5eV of energy, making them susceptible to collection via Coulombic attraction. The intensity of the secondary electron beam indicates surface topography, and raster scanning across the surface generates an image. Backscatter electrons elastically scatter of the surface, retaining ~80% of their energy. These high energy electrons are collected with a strong magnetic field in Ultra High Resolution (UHR) SEM detectors.

Scanning electron microscopy, especially with UHR detectors, provided morphological information. High resolution images of planar cross sections andelectrolytically extracted carbides were the basis for aspect ratio analysis.

Energy Dispersive Spectroscopy (EDS)Energy Dispersive Spectroscopy (EDS) provides semi-quantitative compositional

information. EDS detectors measure the energy of x-ray photons and match the peak location to the characteristic energy levels in atoms. The x-rays are generated when incident electrons excite atomic core electrons, leaving a vacancy in a low energy level. An outer electron quickly fills the vacancy and releases a high energy photon in the process. The area under the peaks correlate to relative abundance, yielding the compositional information.

Unfortunately, EDS compositional information is riddled with problems, especially when examining submicron objects. EDS has difficulty detecting and quantifying low atomic number elements (usually Z < 11, sometimes as good as Z < 6). Thus, using EDS to differentiate between carbides and borides using Carbon (Z = 6) and Boron (Z=5) content is extremely difficult and error prone. In addition, certain characteristic energies of different atoms lie very close together, making it difficult to detect or quantify certain elements. Such was the case with Molybdenum, Niobium and Zirconium, which are all present in Rene 88DT. Further, the interaction volume of the EDS is a few cubic microns, much larger than sub-micron carbides that were examined. This leads to interference from the background matrix in the planar cross section samples and an inability to distinguish individual compositions of carbides in the extracted samples (not to mention possible interference from the carbon tape adhesive).

EDS indicated a strong presence of Tungsten and Titanium in the extracted N and O carbide samples. Extracted P samples were heavy in Nickel and Niobium. It also indicated a strong presence of carbon and boron, by atomic percentage. This verifies that the extracted objects are indeed carbides and borides, but exact abundances are highly suspect given the insensitivity to low Z atoms.

Auger Electron Spectroscopy (AES)Auger Electron Spectroscopy (AES) provides low volume semi-quantitative compositional information. Similar to EDS,

AES identifies elements through characteristic energy levels. AES is based off a slightly different physical reaction, wherethe energy is released through the expulsion of an outer atomic electron rather than a photon. The electrons have lowenergy (tens or hundreds of eV) and can’t escape the sample deeper than a few nanometers, creating an extremelysmall interaction volume. This allows analysis of single carbides.

Due to the limited availability of the AES, I was not able to make a comprehensive analysis of the carbides. Further, surface contamination waspresent in all samples, making quantitative analysis impossible. However, in thesamples examined, it appeared that two compositions existed between distinct butmorphologically similar carbides: either Tungsten heavy or Titanium heavy.

AnalysisAspect Ratio

Aspect Ratio, the ratio between the length and width of a two dimensional object, provides a quantitative measure of the size distributions, providing a means of identifying distinct morphologies.

Conclusions•P sample carbides seem to have larger areas and aspect ratios than either N and O carbides•N and O carbides have similar size and aspect ratio distribution. •N and O carbides have Tungsten and Titanium as their primary metallic ingredients.•P carbides might have Nickel and Niobium as their primary metallic ingredients.•N carbides might have two distinct compositions: one heavy in Titanium, and the other heavy in Tungsten.

Jeffrey P. Hutchinson, Dr. Kip FindleyWashington State University

This work was supported through the National Science Foundation: Division of Materials Research REU site program under grant number 0453354 and Pacific Northwest National Laboratory

Limitations:The individual biases in this study are too numerous to list here, so this

section will restrict itself to problems of compositional identification. The unusual morphology of the extracted P samples suggests that they might not be carbides at all. EDS analysis of the P samples did detect unusually high nickel content and nearly no Tungsten.

Histogram of Planar Aspect Ratios

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Planar Cross Sections Morphology

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Elliptical Area Approximation (um^2)

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0.171.26 AuL

5.4137.14 W L

1.573.45 NiK

0.300.58 V K

0.691.34 CrK

4.267.62 TiK

2.242.97 ClK

1.665.96 MoL

14.528.68O K

69.1631.02C K

At%Wt%Element

Figure 2. EDS spectrum and resulting composition of N extracted carbides.

Figure 1. SEM images of a) Extracted N carbides b) Extracted P carbides, and c) planar N carbides.

The P sample (purple in Figure 4) had on average a much higher aspect ratio than either samples N or O (blue and green, respectively). Similarly heat treated, samples N and O are similar in their distribution of aspect ratio and sizes .

Figure 4d illustrates the various morphologies of the extracted P sample. The figure shows that there is a set of P samples with low aspect ratio and high area. These samples are the large plate structures seen in Fig. 1b. The vertical column around ~1um2 represents a group of thin rods. Whereas simple visual examination of images like Fig. 1b would suggest two distinct morphologies (plate like and rod like), aspect ratio analysis does not show a clear division.

N and O have very tight ranges of aspect ratios, centered around ~1.3, and a small range of areas. The P samples, on the other had have a broader range of aspect ratios, centered at a higher value, and typically larger areas. The increase in aspect ratio and decrease in area from the extracted to planar P sample is likely due to the appearance of plates in a randomly oriented plane.

Figure 3. An Auger spectrum of two carbides. Notice the Tungsten (W) peaks in the red sample and the Titanium peaks (TI) in the blue.

Extracted Precipitates Morphology

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Fig. 4c Fig. 4dFigure 4. Various graphical representations of the

aspect ratios of carbides and borides in Rene88DT.

Histogram of Extracted Sample Aspect Ratios

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Fig. 1a Fig. 1b Fig. 1c

This may indicate that some of the plates are metal shavings (Rene88DT being nickel-base superalloy), borides, or merely that the composition of the carbides differs between the samples. Both the extracted samples and the planar sections indicated heavy carbon content, suggesting carbides were present, but heavy boron content suggested borides were as well. P samples were not analyzed with AES, which would provide a more specific compositional analysis of a smaller volume and clearly identify carbides versus impurities.

Thus, the aspect ratio data may represent both carbides and impurities.

Recommendations for Further Research•Further AES analysis of all samples to detect individual carbide compositional trends and verify P sample identity.•Volume fraction analysis and investigation into the spatial relationship between grain boundaries and carbide formation.•Expansion of study to other heat treatments or processing techniques.

Sample PreparationAll samples were prepared through metallographic polishing and etching or electrolytic extraction. Kallings’ etch and a 95 HCl :3 H202:: 2 HNO3 solution were used as etchants for the planar section samples. The etchants attacked the gamma prime phase, leaving carbides and

borides in relief since gamma prime will preferentially nucleate on them. The electrolytic extraction removed the background matrix, releasing carbides and borides into the solution to be collected via filtration. The electrolytic extraction process was performed as directed by ASTM standard E-963. Approximately 1cm3 samples were cut, ground with 120

grit paper, ultrasonically washed and rinsed with methanol. These samples were set in a Platinum wire basket connected to the anode of an electrolytic cell. The cathode was a stainless steel mesh and the electrolyte 10% HCl by volume. After running for ~4 hours at .9 A, the electrolyte and rinsings were collected and filtered with .2 um filter paper.