Using the Velocity™ EBSD Camera Series for High-Speed EBSD Mapping

Since Electron Backscatter Diffraction (EBSD) mapping first became commercially available 25 years ago the technique has made use of new technological developments to become increasingly faster and efficient.

The analog video cameras that were used by the first EBSD systems could only operate at speeds of 30 fps, and the images that were taken were often averaged to give an effective speed of 1 fps. After this, the first digital CCD-cameras could achieve 40 fps with later models able to achieve 200 fps. Following the eventual development of high-speed systems frame rates of 1,500 fps were possible.

Velocity™ Plus EBSD Detector from EDAX

EDAX’s Velocity™ Plus EBSD Detector broke these records. The detector uses an extremely sensitive, low-noise CMOS sensor to achieve rates of up to 3,000 indexed points per second. The latest system in the range, the Velocity™ Super EBSD Detector, has pushed these limits even further with a rate of up to 4,500 indexed points per second.

It is only possible to achieve such rapid acquisition rates if the detector is highly optimized. The Velocity™ Super EBSD Camera uses a custom lens designed for sensitive applications, a custom EBSD CMOS-based camera, and software that improves the rate of indexing and the system performance. Data captured using this system is shown in Figure 1.

Figure 1 is a combination of an EBSD Image Quality (IQ) greyscale map and a colored Inverse Pole Figure (IPF) map, also known as an IQ + IPF map, of an Inconel 600 superalloy imaged at a rate of 3,000 indexed points per second. The coloring on the map relates to the orientation of crystal structures in the sample with respect to the sample surface normal direction. An 11 nA beam current was used for this measurement, indicating how sensitive the Velocity™ camera is, and indexing occurred at a 99.6% success rate, demonstrating the speed and accuracy of the system.

EBSD IQ + IPF map from a Ni superalloy collected at 3,000 indexed points per second at 11 nA with 99.6% indexing success.

Figure 1. EBSD IQ + IPF map from a Ni superalloy collected at 3,000 indexed points per second at 11 nA with 99.6% indexing success.

Two Versions of the Velocity™ EBSD System

The Velocity™ EBSD System has two different versions – the rapid Velocity™ Plus that can collect at a rate of up to 3,000 indexed points per second, and the super-fast Velocity™ Super that can collect at a rate of up to 4,500 indexed points per second. To be able to collect at such a fast rate the Velocity™ Super uses a dedicated high-speed mode. Beam currents of 25 nA or higher are needed to achieve these collection speeds with 99% indexing success on standard samples. Lower bean currents can be used if this indexing success is now required.

An IQ + IPF map of a sample of Inconel 718 alloy that has been 3D printed is shown in Figure 2. The data was collected using a beam of around 30 nA current at a rate of 4,500 indexed points per second with an indexing success rate of 98.2%. The orientation information helps users to understand the solidification rates and mechanisms during the additive manufacturing process.

EBSD IQ + IPF map from an additively manufactured IN718 alloy collected at 4,500 indexed points per second.

Figure 2. EBSD IQ + IPF map from an additively manufactured IN718 alloy collected at 4,500 indexed points per second.

These microstructures can contain information both over a large area and with fine detail. The high-speed collection capability of the Velocity™ Super is ideal for characterizing these 3D printed structures.

CMOS-Based EBSD Detectors in Velocity™ Series

The CMOS-based EBSD detectors used in high-speed collection in the Velocity™ series have an EBSD pattern resolution of 120 x 120 pixels. This is a greater resolution than other systems. For example, the EDAX Hikari CCD-based detector has a resolution of 30 x 30 pixels when collecting 1,500 points a second, and when working at high resolution (120 x 120 pixels), it can only collect around 500 patterns per second.

The high resolution and acquisition speed of the Velocity™ allow it to be used on a wide variety of different samples and materials, regardless of their crystal structure or material state, without having to optimize the band detection or indexing settings.

For the following images the Velocity™ was configured to index at a rate of approximately 2,500 points per second and collect patterns at a rate of approximately 3,000 points per second, using the default Hough parameters in the EDAX TEAM™ software, with a beam current of around 30 nA. These high-speed collections can be achieved by non-experts, meaning operators can get more out of their instrument, without having to develop the extensive knowledge that they would need to use conventional instruments.

An IQ + IPF map of a sample of deformed ferritic steel is shown in Figure 3. Subtle color changes in each grain show the extent of its deformation. Deformation can result in the orientations within a grain changing by up to 30°. The orientations within a grain change as much as 30°, but the precision of the measurements at these conditions allows detection of the small rotations within the microstructure.

EBSD IQ + IPF map from a deformed ferritic steel sample.

Figure 3. EBSD IQ + IPF map from a deformed ferritic steel sample.

Deformed materials have more defects within the crystal lattice, meaning their EBSD patterns are not as sharp, which can negatively impact the indexing performance and band detection efficiency. Despite this, the Velocity™ achieved an indexing rate of 98.3%, demonstrating that the instrument is capable of analyzing deformed materials at high speed.

The examples discussed so far have all involved materials with a single phase. In the case of multiple phase materials, the system must be able to determine both the correct phase for a given EBSD pattern and the correc orientation. These additional demands mean that the data collection process requires additional computational power.

Figure 4a shows an IQ + IPF map from a dual-phase (BCC Ferrite and FCC Austenite) steel sample and Figure 4b shows an IQ + Phase map, with an indexing success rate of 97.3%. In both images the orientation has been determined correctly and the phases are clearly resolved. The type of steel that was imaged is corrosion resistant, allowing it be to used in harsh environments. Developing an understanding of the phase distribution within the steel can assist in improving the performance of the steel.

EBSD (a) IQ + IPF map and (b) IQ + Phase map from a duplex phase steel sample.

Figure 4. EBSD (a) IQ + IPF map and (b) IQ + Phase map from a duplex phase steel sample.

All of the examples discussed so far have had cubic crystal structures. These structures are highly symmetrical, which makes determining their orientation simpler. Results collected from a titanium medical implant produced via 3D printing are shown in Figure 5. The IQ + IPF map is shown in Figure 5a and the IQ + Phase map is shown in Figure 5b. These maps were collected at an indexing success rate of 94.3%.

EBSD (a) IQ + IPF map and (b) IQ + Phase map from an additively manufactured titanium sample.

Figure 5. EBSD (a) IQ + IPF map and (b) IQ + Phase map from an additively manufactured titanium sample.

The sample is mainly composed of alpha titanium, which possesses a hexagonal crystal structure, with small localized regions of beta titanium, which has a body centered cubic crystal structure. Following cooling during the 3D printing process the beta phase (BCC) is retained. Specific orientation relationships measured here confirm the phase transformation mechanism. Measuring the size and fraction of the beta phase can give insight into the cooling rates within the material.

Conclusion

The examples covered in this article demonstrate that the Velocity™ cameras can collect high-quality data at high-speeds using reasonable beam currents, without the need for expert level software optimization. This makes the cameras useful for traditional EBSD mapping and also for applications where a short acquisition time is important, such as in-situ experiments and 3D serial sectioning.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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