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ChI-Scan — the Shortcut to a Quantitative Understanding of the Microstructure in Multi-Phase Samples

Introduction

Characterizing the microstructure of compounds and/or phases present in a sample enables microanalysts to determine the relationship between processing history and material properties. Over the past 20 years, Electron Backscatter Diffraction (EBSD) pattern analysis and Energy Dispersive Spectroscopy (EDS) have proven invaluable tools for developing materials with improved properties.

However, all too frequently, these techniques are performed in isolation, dramatically reducing the effectiveness of analysis, significantly increasing the time before answers are revealed, or requiring greater technical expertise. With Chemical Indexing Scan (ChI-Scan™), the elemental information gained from EDS guides the crystallographic analysis from EBSD measurements, automatically enabling accurate microstructural analysis of all phases present, collectively and individually.

Advantages of ChI-Scan

  • Distinguish constituent phases with the same cubic crystal structure and similar lattice constants
  • Decrease data collection times from hours to minutes when differentiating phases with the same crystal structure and dissimilar lattice constants
  • Avoid ambiguity in your analysis from sample preparation and/or plastic deformation
  • Diminish processing time ten-fold with guided analysis based on eligible crystallographic structures
  • Analyze all phases that are present in a sample

ChI-Scan analysis enabled the texture of a polyphase mineral to be exposed. Using ChI-Scan, two distinct rhodochrosite phases were discovered through variation in the iron content (left). The phase map (center) shows low-iron rhodochrosite (cyan), high-iron rhodochrosite (yellow) and other phases including barium sulfate (blue), quartz (red), and pyrite (green). IPF Orientation combined with Image Quality map of the same sample (right).
Figure 1. ChI-Scan analysis enabled the texture of a polyphase mineral to be exposed. Using ChI-Scan, two distinct rhodochrosite phases were discovered through variation in the iron content (left). The phase map (center) shows low-iron rhodochrosite (cyan), high-iron rhodochrosite (yellow), and other phases, including barium sulfate (blue), quartz (red), and pyrite (green). IPF Orientation combined with Image Quality map of the same sample (right).

 

ChI-Scan

EBSD identifies the crystallographic structure and orientation of analysis points within a material, revealing detailed microstructural information. However, when two or more phases with similar crystallographic structures are present in a sample, it can be difficult to uniquely identify the correct structure. ChI-Scan leverages the compositional information from EDS to reduce the candidate crystallographic structures at each measurement position to only those with appropriate chemistry, thereby resolving ambiguities. This allows phases of similar crystal structure to be discriminated and faster data collection as the requirements for high-quality EBSD data (signal-to-noise ratio and resolution) are relaxed. Using ChI-Scan, microstructures with more than 10 unique phases have been analyzed.

Microanalysis Results

In this example, the importance of ChI-Scan in the microstructural analysis of the metallic interconnects of a printed circuit board is demonstrated. The interconnects were deposited using a multi-step electrochemical deposition of copper and Kovar. Kovar is an iron-nickel-cobalt alloy with a coefficient of thermal expansion similar to borosilicate glass. It is used in the electronics industry for metal parts bonded to hard glass envelopes. To understand and control the effects of processing on the properties of the two materials, it is important to understand their microstructures, particularly the grain size distribution.

When performed in isolation, EBSD analysis using the Velocity™ Plus EBSD Detector indexed 99.8 % of the points successfully, providing high-quality orientation maps that reveal detailed microstructural information such as grain size, crystal orientation, and twin boundaries. However, it was impossible to understand each phase's microstructure and their relationships due to the similarity in crystal structure between these two phases. The phase map reveals the random selection of two Face-Centered Cubic (FCC) phases corresponding to copper and Kovar but with no correlation to the observed microstructural features. Copper and Kovar are FCC materials with identical diffracting planes and similar lattice constants, making differentiation with EBSD very difficult.

  EBSD   EDS   Phase Map
EBSD EBSD Orientation map collected at >3,000 indexed points per second using a Velocity Plus EBSD detector + = EBSD phase map
 
ChI-Scan EBSD Orientation map collected at >3,000 indexed points per second using a Velocity Plus EBSD detector + Elemental map by EDS collected simultaneously with EBSD data using Octane Elect Super detector. = ChI-Scan phase map
  Table 1. EBSD orientation maps were collected at >3,000 indexed points per second using a Velocity Plus EBSD Detector.   Elemental map by EDS was collected simultaneously with EBSD data using an Octane Elect Super Detector. Colorized to display copper (red), iron (green), and nickel (blue).   Phase maps were determined from data. Colorized to display copper (top - blue, bottom - orange) and Kovar (top - yellow, bottom - green).

 

However, the simultaneous collection of elemental maps and analysis with ChI-Scan enables copper and Kovar to be distinguished and accurate phase maps generated. Quantitative microstructural analysis is now possible, and the grain size distributions from both phases are shown in Figure 2.

The copper phase has a bimodal grain size distribution with larger grains adjacent to the Kovar interface and smaller grains away from it, suggesting two different deposition and grain growth mechanisms were active during the deposition process. The Kovar phase has a more homogeneous grain distribution. Analysis of the grain misorientations indicates that the Kovar phase has significant twinning (approximately 50% of the grain boundaries within the phase). The copper phase has far fewer twin boundaries (approximately 7%). This type of detailed analysis would not be possible without the accurate phase differentiation provided by ChI-Scan.

EBSD grain maps for the copper phase (left) and Kovar phase (right) showing a bimodal grain structure for the copper phase.
Figure 2. EBSD grain maps for the copper phase (left) and Kovar phase (right) showing a bimodal grain structure for the copper phase.

 

Grain size distributions for the copper and Kovar phases.
Figure 3. Grain size distributions for the copper and Kovar phases.

 

Conclusion

The ChI-Scan feature in EDAX Pegasus Systems enables:

  • Accurate phase mapping for scientists and engineers characterizing multi-phase materials by eliminating ambiguities in EBSD pattern analysis
  • Rapid determination of critical data by enabling lower quality EBSD patterns to be used
  • Improved analytics for quantitative analysis of all phases in a material

ChI-Scan can be used on metallic, ceramic, semiconductor, and geological samples, including (but not limited to) carbide analysis in steels, oxide phase identification in rare earth magnets, inclusion analysis in aerospace alloys, and mineral analysis in copper ore bearing rocks.

ChI-Scan requires simultaneously collected EDS and EBSD data obtained using EDAX EDS and EBSD detectors.