Nickel-based superalloy microstructure characterized by EBSD

图1:相抵销st image acquired with ARGUSTM imaging detector reveals at least 4 different phases. Note the presence of numerous fine precipitates at the phase boundary. The results of the simultaneous EBSD/EDS measurement are presented in Figure 2 and 5.

Nickel-based superalloys are known for their excellent mechanical strength, resistance to thermal creep deformation, fatigue, corrosion or oxidation. Therefore, they are often the material of choice for high-temperature structural applications with gas-turbine and aero-engine. Characterization of their microstructure is crucial to control with respect to the mechanical properties. In addition, solid solution and precipitation strengthening from secondary phase precipitates (ɣ’, nitrides, carbides) is necessary to achieve high temperature strength; hence the importance to determine unknown precipitates formed during the strengthening process.

In this application example, we reveal the importance of EDS assisted EBSD measurement in successfully identifying and indexing the different phases, including fine precipitates. The measured area is visible on the phase contrast image in figure 1 (acquired with the ARGUS™ BSE detectors). The presence of numerous fine precipitates (carbides) and 3 other different phases can be inferred from the ARGUS image. Combined EBSD/EDS measurement was done with a spatial resolution of 50 nm step size in order to resolve the carbides. EBSD results are presented in Figure 2, 5 and 6. Four phases were identified using combined EDS and EBSD measurements: nickel (the matrix), nickel aluminum, nickel tungsten and tantalum carbide.

The challenge in this analysis is to successfully distinguish the carbide from the nickel matrix phase. Both have cubic fcc structure and therefore produce very similar diffraction pattern (see figures 3 and 4). To this aim, the map was corrected offline by EDS assisted EBSD indexing. The results are presented in figure 5.

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Figure 2: Unprocessed EBSD phase map. All the phases were identified using the combintion of EDS and EBSD. Nickel matrix phase is displayed in red, NiAl in blue, TaNiC in yellow and NiW in green. The hit rate is 97%. The carbide and Nickel phases cannot be distinguished easily by EBSD only because they share similar lattice parameters, resulting in similar diffraction pattern (see figures 3 and 4). Therefore EDS assistance is required during EBSD indexing. Since EDS was measured simultaneously with EBSD indexing, the data can be corrected offline, without the need of the SEM. Result is presented in Figure 5.

Figure 3: Crystallographic information of Nickel matrix phase, cubic fcc structure, with the unit cell and the corresponding spherical Kikuchi pattern (kinematic simulation).

Figure 4: Crystallographic information of Tantalum Nickel carbide phase, cubic NaCl structure, with the unit cell and the corresponding spherical Kikuchi pattern (kinematic simulation).

Figure 5: EBSD phase map with EDS-assistance: all phases are distinguished with the assistance of EDS. Indexing rate of 98,6%, with no misindexing and the fine carbides are well resolved. EDS-assisted EBSD indexing can be done live at the SEM or offline, allowing to correct or complete the measurement at anytime, without requiring spending extra time at the SEM.