Electron backscatter diffraction (EBSD) pattern analysis is ideal for detecting both crystallographic orientation and structure. However, the detection of the correct structure can be a difficult task when the crystallographic structures present are similar.
Figure 1. Constituent phases with the same or similar crystallography.
By combining the EBSD measurements with Energy Dispersive Spectroscopy (EDS) chemical compositional data which has been simultaneously collected, the crystallographic structures can be effortlessly distinguished, and a much more absolute phase description can be determined, using both structural and compositional data. It is imperative to accurately detect constituent phases in order to perform precise microstructural analysis of all phases presents individually as well as collectively.
Comparison with Existing Solutions
EBSD can be used to detect and differentiate crystallographic structures present in multiphase materials. The following drawbacks can be encountered by using only EBSD for differentiation:
- Similar inter-planer angles are produced by the constituent phases with the analogous cubic crystal structure and diffracting planes. This makes differentiation impossible when taking into account only these angles while indexing.
- It is possible to enhance the differentiation of phases with the same crystal structure by including diffraction line bandwidth analysis while indexing. However, this approach needs a precise determination of the band edges, which needs EBSD patterns of higher resolution. It takes a longer time to collect high-resolution patterns that are extremely sensitive to changes in pattern quality due to extrinsic and intrinsic factors, such as sample preparation artifacts and residual plastic deformation.
- Constituent phases with analogous crystal structures are capable of creating similar inter-planar angles, and the way that the bands are identified restricts the capability to distinguish these structures. EBSD pattern resolution and quality will again directly influence the performance of phase differentiation.
- The entire task of indexing every single point with each candidate crystallographic structure and then establishing the most ideal solution can take a significant amount of time as the number of potential structures increases.
Chemical Indexing Scan (ChI-Scan) ™ offers a solution for these drawbacks by integrating the structural data determined with EBSD with the entire spectrum chemical data quantified with EDS. The list of candidate crystallographic structures at each measurement position is decreased to only those matching the measured chemistry by utilizing the EDS data that was simultaneously collected. This approach has the following benefits:
- Constituent phases with similar or the same crystallography can be easily distinguished chemically using the EDS data that was simultaneously collected (Figure 1).
- Diffraction line bandwidth detection and high-resolution EBSD patterns are not needed for structural discrimination. Data collection using rapid lower resolution EBSD patterns help to obtain high-quality results within a short time span and the differentiation is not as sensitive to preparation quality or sample deformation.
- Reduction in the overall data collection time occurs as the number of structures to be examined at each point is reduced by the chemical filtering. This becomes extremely significant as the number of structures present increases. Microstructures containing as many as 12 phases have been examined.
- Automatic determination of compositional phases is made possible by the investigation of the full chemical spectrum of EDS data. It is possible to rebuild specific EDS elemental maps at any time because the complete EDS spectrum is saved at each analysis point. Prior knowledge is also not necessary for precise analysis.
Figure 2 depicts EDS elemental maps of iron and copper as well as a ChI-Scan ™ generated phase map with an iron-nickel phase and a copper phase and from a polished printed circuit board. In this sample, the electrochemical deposition was used to deposit metallic interconnects.
Using the EDS spectral information, the phases present are detected as Kovar, an iron-nickel-cobalt alloy, and copper. Both of these phases are Face Centered Cubic with matching diffracting planes and identical lattice constants, making conventional discrimination with EBSD extremely difficult. The EDS elemental maps highlight the spatial distribution of the phases. This data is used by ChI-Scan ™ to choose the ideal candidate structure at each analysis point. Determining the correct phase and orientation leads to the possibility of performing further microstructural analysis.
Figure 2. EDS elemental maps for copper (top) and iron (center) and ChI-Scan phase map (bottom) for printed circuit board metals.
Figure 3 displays grain maps from the Kovar phase (right) and the copper phase (left), where grains are colored in a random way to reveal morphology and size. The copper phase exhibits a bimodal grain size distribution comprising of larger grains adjacent to the Kovar interface and smaller grains that are away from it. This suggests that the deposition process involved two varied deposition and grain growth mechanisms that were both active.
The Kovar phase exhibits a more uniform grain distribution. Figure 4 shows the grain size distributions from both phases. Analysis of the grain misorientations suggests that the Kovar phase has considerable twinning (about 50% of the grain boundaries within the phase), while very few twin boundaries are exhibited by the copper phase (about 7%). This type of in-depth analysis is possible only with the precise phase differentiation provided by ChI-Scan ™.
Figure 3. EBSD grain maps for copper phase (left) and Kovar phase (right) showing a bimodal grain structure for the copper phase.
Figure 4. Grain size distributions for copper and Kovar phases.
Recommended EDAX Solution
Pegasus Analysis systems with the ChI-Scan ™ feature allow precise phase differentiation and phase mapping for engineers and scientists characterizing multi-phase materials. ChI-Scan ™ enables automatic chemical phase determination by using total EDS spectral data to remove structurally based ambiguities during the course of EBSD pattern analysis. This advanced technology enhances the quality of the final data and decreases the time taken to collect the data, thus providing users with rapid and precise microstructural analysis.
ChI-Scan ™ has the potential to be used on semiconductor, ceramic, metallic, and geological multi-phase samples. ChI-Scan is successfully used for mineral analysis in copper ore bearing rocks, inclusion analysis in aerospace alloys, oxide phase identification in rare earth magnets, and carbide analysis in steels. ChI-Scan ™ needs simultaneously collected EBSD and EDS data which can be acquired using the Octane series of EDS detectors and the Hikari or DigiView EBSD detectors.
This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.
For more information on this source, please visit EDAX Inc.