Introduction
Electron backscatter diffraction (EBSD) data is used to develop new alloys and manufacturing techniques, determine failure modes in critical materials, and advance research in crystalline microstructures. Quality data is vital to making correct inferences and optimal decisions in all these scenarios. Quality data is sometimes defined as ensuring the correct orientations are measured from the EBSD patterns. In other cases, quality data is determined by the microstructure being sufficiently sampled to represent the material of interest correctly. With EBSD, a sufficient number of grains should be measured to represent the crystallographic texture of a material, and an adequate number of measurements per individual grain should be measured to characterize the grain size, shape, and grain misorientation profile. Collecting and analyzing this data efficiently makes EBSD a more practical and effective microanalysis technique for engineers and researchers.
Methods
To collect data quickly, the EDAX Velocity™ Ultra is the ideal EBSD detector. The Velocity Ultra is the world’s fastest EBSD detector, capable of collection speeds of up to 6,700 indexed points per second. EDAX’s Confidence Index algorithm gives users real-time feedback on indexing quality to ensure quality at these speeds. At these collection speeds, EBSD indexing success rates of greater than 99% indexing success can be achieved.
Results and discussion
The Gibeon meteorite is an iron meteorite with a fine octahedrite structure. The Gibeon meteorite has a two-phase microstructure, with a body-centered cubic ferritic phase and a face-centered cubic austenitic phase.
Figure 1. A grayscale PRIAS image combined with a colored IPF map showing the microstructure of the Gibeon meteorite sample.
The microstructure is shown in Figure 1, where a grayscale EDAX PRIAS™ image is combined with a colored inverse pole figure (IPF) map. The PRIAS image is generated using the center region of interest (ROI) channel defined on the phosphor screen of the Velocity Ultra detector. The IPF orientation map colors each pixel according to the crystallographic orientation aligned with the surface's normal direction, with the colored stereographic triangle to define the coloring scheme. To demonstrate the speed and performance of the Velocity Ultra detector, a section of the Gibeon meteorite was selected for EBSD analysis. This particular region was chosen for analysis due to the fine features well suited for electron microscopy study.
The EBSD data was collected from a single field of view from a 1608 µm x 1256 µm area using a sampling step size of 500 nm. By using a hexagonal sampling grid for a higher density of measurement points per area results, this step size produces approximately 9.3 million data points. This sample area was selected to capture the fine-grained region of the microstructure, while the step size was chosen to resolve the smaller austenitic grains. With live analysis of the two phases, this data was collected at 6,400 indexed points per second and a final indexing success rate of 99%, as defined by measurements with a Confidence Index greater than 0.1. The data was collected in less than 25 minutes at this acquisition rate. This speed is key, as this high-quality data can be collected within a typical time window used in microanalysis.
| Time to (seconds) |
|
OIM Analysis 8.6 |
|
OIM Analysis 9 |
| Open data |
|
46 |
|
26 |
| IPF map |
|
13 |
|
2 |
| Confidence Index standardization |
|
114 |
|
9 |
| Unique grain color map |
|
92 |
|
21 |
| Total time |
|
265 |
|
58 |
Table 1. OIM Analysis 9 has a >4.5x speed improvement compared to OIM Analysis 8.6.
Of course, collecting this data is one-half of the story. Analyzing the data is the other. Driven by the acquisition speeds enabled by the Velocity Ultra, significant improvements have been engineered into the EDAX OIM Analysis™ software to reduce analysis time and improve time to results metrics. In this example, the new OIM Analysis 9 software is compared with the previous OIM Analysis 8.6 software for tasks required to analyze the Gibeon meteorite microstructure. The results in Table 1 show an over 4.5x improvement in time required to analyze and create IPF and Unique Grain Color maps, with the latter shown in Figure 2.
Figure 2. A unique grain color map of the Gibeon meteorite sample.
In a grain color map, measured orientations are grouped as grains as defined using a grain tolerance angle, with a 5° tolerance used here, and then the determined grains are randomly colored to show grain size and shape. This grain determination algorithm is also used in the Confidence Index Standardization routine. These results show that OIM Analysis 9 has significant speed improvements for file opening, map rendering, and grain determination. In addition, it is now easier to move and scale large maps without waiting for the maps to be re-rendered, removing some of the friction involved in manual data analysis.
Figure 3. A phase map of the Gibeon meteorite sample.
The phase map and the grain size distribution comparing the grain sizes for the ferritic and austenitic phases are shown in Figures 3 and 4, respectively. The average grain size in equivalent diameter for the austenitic phase is 3.2 µm, while the average grain size for the ferritic phase is 11.2 µm. Note that over 10,000 grains were measured, with 249 grains at the edge of the scan area excluded from these grain size calculations. The overall shape of the grain size distributions shows that the 500 nm step size selected was sufficient to resolve the grain structure with enough fidelity for grain size measurements.
Figure 4. The grain size distribution comparing the grain sizes for the ferritic and austenitic phases of the Gibeon meteorite sample.
Conclusion
This example shows how combining the Velocity Ultra EBSD detector, APEX EBSD pattern acquisition and indexing, and OIM Analysis 9 processing allows users to quickly collect and reliably analyze EBSD data with improved time-to-results. This functionality lets users utilize their time and resources more efficiently, enabling their EBSD results to advance their research and materials development requirements further.