EBSD Analysis

The EBSD results provide crystallographic texture, grain orientation, misorientation between grains, and grain size, which are crucial in determining the mechanical behavior of the material. Figure 3.1 shows a schematic of grain orientations (including unit normals and angles) and an EBSD scan of typical U720 material. Prior to EBSD, the samples were electropolished utilizing a solution composed of 60 vol.% methanol, 34 vol.% butanol, and 6 vol.% perchloric acid at -20 °C. The raw data obtained were first analyzed using TSL OIM commercial software [245] to obtain grain orientations.

Figure 3.1. (a) Schematic of the orientations of grains in a polycrystal. (b) EBSD scan of U720, where the different colors represent orientations of individual grains. (c) Each color can be mapped to its projection on the

stereographic triangle.

The EBSD results showed that the billet, forged, and fatigue specimens were not textured, i.e. the orientations of the grains within the aggregate displayed nearly a random

distribution, as shown by the pole (Figure 3.2) and inverse pole figures (Figure 3.3). Therefore, the fatigue loading of the specimens did not result in grain rotation or texturing of the material at low and high strain ranges. Meanwhile, the monotonic tensile and compression specimens displayed a <111> and <110> texture, respectively. As expected, due to the large deformations each specimen experienced during testing, the grains in the polycrystalline material rotated and aligned to the loading axis. For each case, the primary slip caused the tensile and compression axes to rotate towards the [101] and [111] slip directions, respectively, as shown in Figure 3.4.

Figure 3.2. Pole figure displaying the texture of the scanned specimens. The samples did not display a strong texture with the exceptions of the tensile (<111> texture) and compression (<110> texture) specimens.

Figure 3.3. Inverse pole figure displaying the texture of the scanned specimens.

Figure 3.4. In FCC metals, primary slip in the

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From the EBSD scans of these seven specimens, a total of 29,035 grains were analyzed.

The grain area was measured during each EBSD scan. From this data, the grain size was calculated assuming a circular cross-section and the results are shown in Figure 3.5 and the

as expected since this material underwent a homogenization process but did not experience the standard heat treatment aforementioned. Hence, the grains were not able to grow during heat treatment. Further, the grain sizes of the forging and compression sample were much finer compared with the other specimens. All of the samples (except the billet material) were taken from specimens radially cut from a forging. The forging (as-received) and compression samples were cut from the grips of these specimens; hence they were taken from the rim of the forging, which is known to display a finer grain structure.

Figure 3.5. Grain size distributions of each specimen analyzed.

The mean grain size is 3.3 µm (averaged across all specimens cut from the forging) although grains can reach as large as 18.4 µm (Figure 3.5d). Thus, a small step size in the EBSD scans had to be used to accurately cover the small grains. At the same time, the bigger grains required large area scans for statistically meaningful data. Thus, in a typical scan, a few thousand grains were analyzed, as shown in Table 3.4. However, the good signal to noise ratio

of the EBSD patterns obtained from the electropolished samples allowed for relatively fast data acquisition for a single pattern, resulting in acceptable total measurements times. The long tail of each grain size distribution shown in Figure 3.5 suggests a bi-modal like distribution of grain sizes with large variations.

Table 3.4. Statistics from each EBSD scan, where the Taylor factor is determined for uniaxial loading.

Billet Forging

Fatigue- Low ∆ε

Fatigue- High ∆ε

Tensile- ET

Compress- RT

Number of Grains Scanned 4,265 7,424 3,664 2,373 3,457 3,662 Total Area of Grains Scanned 9,948 37,310 60,542 60,254 57,198 9,963

Average Grain Size (µm) 1.5 2.4 5.3 4.2 4.4 1.6

Standard Deviation - Grain Size (µm) 0.86 0.89 2.18 1.87 1.33 0.93

Maximum Grain Size (µm) 9.0 8.3 18.4 16.7 13.4 8.7

Number of Interfaces Detected 28,623 47,370 24,018 15,546 21,118 24,703

Average GBs per Grain 5.7 5.4 6.6 6.6 6.1 5.7

Taylor Factor 3.089 3.070 3.074 3.065 3.103 3.075

Further, due to the variation in grain sizes and neighboring grains, each grain can have a wide distribution of associated GBs. Given a sufficiently large sample size, a 2D EBSD measurement of the grain area is capable of producing accurate representation of the grain size compared to 3D data [247]. A histogram of the number of GBs belonging to each grain is shown in Figure 3.6. A typical 2D scan showed that each grain has a mean value of ~6 interfaces, but some grains had more than 25 GBs.

Figure 3.6. Histogram of the number of GBs (adjacent grains) per grain for each EBSD scan.

Given the adjacent grain information, we can define the misorientation between grains and establish its distribution for each EBSD scan, as shown in Figure 3.7. From these distributions, we see large number of ~60˚ misorientation, which is indicative of twin boundaries (TBs), as depicted in Figure 2.3. There are two types of twins, annealing and deformation twins.

We see that the twin density is high due to heat treatment (Figure 3.7b) and does not increase in the subsequent testing (deformation due to tensile, compressive, or fatigue loading). Thus, the twin density in this material is composed of annealing twins, which is typical of a nickel-based superalloy. The misorientation distibutions did not substantially change from forging to fatigue testing regardless of the applied strain amplitude. Hence, during fatigue deformation, the interfaces stayed intact. By contrast, tensile testing resulted in unidirectional deformation to the specimen and a change in the misorientation distribution, as shown in Figure 3.7e. As previously mentioned, the primary slip rotated the tensile axis to the [101] direction resulting in <111>

texture. In doing so (for the tension specimen and similarly the compression specimen), the misorientations were redistributed into mostly low-angle, thus annihilating many of the twins.

Figure 3.7. Misorientation distributions of each specimen analyzed.