Determining Distribution of Creep Damage in Materials Using EBSD Characterization

Service life and potential failure mechanisms of several materials can be revealed with microanalysis. This article discusses the application of electron backscatter diffraction (EBSD) and energy dispersive spectrometry (EDS) to analyze the microstructure and damage distribution analysis of a nickel superalloy subjected to creep deformation.

Ni Superalloys

Ni superalloys have superior creep resistance and mechanical strength, thanks to the coupling of a gamma (γ) matrix with a gamma prime (γ’) intermetallic phase. This intermetallic phase, formed by chemical incorporation of alluminium or titanium, serves as a barrier to dislocation.

Creep resistance relies on slowing speed of dislocations within the crystal structure. The γ’ particles are prone to raft under creep conditions, causing deformation largely at grain boundaries and the γ’/γ interface. This analysis uses EBSD to explore the microstructure and damage distribution developed in the Inconel alloy IN 738 subsequent to creep deformation.

Instrumentation

This analysis uses an AZtecSynergy system from Oxford Instruments to collect both the EBSD and EDS data for examining deformation in the sample. The NordlysNano detector is used to acquire the EBSD data. The NordlysNano detector is optimized for acquiring high quality, distortion free EBSPs. An X-MaxN150 large area SDD detector is used for acquiring the EDS data.

BLG CrossCourt 3, a third party software package, is also used to process EBSD patterns utilizing a cross-correlation technique to quantify residual strain. It uses the raw, unprocessed EBSPs saved in TIFF format during the acquisition process. This application requires the high quality, distortion free patterns acquired from the NordlysNano detector.

Experimental Results

Figure 1 depicts a secondary electron image of the polished sample, revealing the microstructure composed of large grains containing gamma prime particles in a matrix of gamma. In this area, two crack tips are seen. The gamma prime particles seem to have rafted in the grain above the lower crack tip. Some large particles are also seen around the crack tip.

Secondary Electron image of the Inconel 738 sample

Figure 1. Secondary Electron image of the Inconel 738 sample

Figure 2 shows the X-ray maps acquired from the same area, revealing the elemental distribution in the sample. The γ phase contains large amount of chromium, while the γ’ phase has a higher proportion of titanium, aluminum, and nickel. The crack tip particles consist of carbides of chromium and molybdenum.

X-ray maps collected from the crack tip.

Figure 2. X-ray maps collected from the crack tip.

Figure 3 delineates the EBSD patterns, corresponding solutions and spectra from the three different phases (γ, γ’ and carbide particles). The composition of the γ’ and γ phases is different, but they have the same crystal structure (Table 1).

Hence, it possible to index both the phases with the fcc Ni phase, as illustrated in the EBSD maps shown in Figure 4. Ni gamma (γ) phase, Ni3AlTi gamma prime (γ’) phase, and Carbide M23C6 boundary particles are shown in Figures 3a, 3b, and 3c, respectively.

EBSD patterns, corresponding solutions and spectra from γ, γ’ and carbide particles

Figure 3. EBSD patterns, corresponding solutions and spectra from γ, γ’ and carbide particles

Table 1. Match Unit details for γ’, Ni and carbides

Phase Space group Laue group Lattice parameter a=b=c nm
Ni3AlTi (γ’)i 225 11, m3m 0.587
Ni γ 225 11, m3m 0.357
Carbide boundary particles 225 11, m3m 1.06

The pattern quality map depicted in Figure 4a reveals the boundaries between the γ and γ’ phases and the rafting of the γ’ phase. Figures 4d, 4 e and 4f are local misorientation maps, which illustrate the existence of the misorientation at the γ phase grain boundaries and the interfaces between γ/ γ’ phases.

The higher degree of misorientation is delineated by the bright green through yellow color, depicted on the key (Figure 5). The highest degree of misorientation measured (up to 2 degrees) is between the γ’ particles in grain A, where the highest rafting is noticed.

EBSD maps

Figure 4. EBSD maps

Colour key showing local misorientation for Figs. 4d, e, and f

Figure 5. Colour key showing local misorientation for Figs. 4d, e, and f

This data can be further analyzed by tracking subtle relative changes between the EBSPs. Here, Crosscourt 3 is utilized for measuring these pattern distortions and quantifying the full relative distortion matrix at each point of the scan.

This analysis makes use of the export of the EBSPs as ‘raw’ unprocessed 12 bit TIFF images and the high angular resolution and distortion free images acquired using the NordlysNano.

Crosscourt 3 yields several outputs, but only the high resolution Kernel Average Misorientation map is shown here (Figure 6). The processing involves only the patterns from grains A, B and C not the smaller grains and carbides. All grains exhibit a complicated pattern of deformation with the development of clearly visible 2μm-size sub cells.

HR Kernel Average Map output from CrossCourt 3

Figure 6. HR Kernel Average Map output from CrossCourt 3

Conclusion

From the results, it is evident that collecting EBSD with X-ray data in the SEM is a robust means of characterizing materials to gain insights into the distribution of creep damage.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments NanoAnalysis.

For more information on this source, please visit Oxford Instruments NanoAnalysis.

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