Advancements in backscatter electron diffraction instrumentation, software and X-ray microanalysis helps analysts to study crystalline materials rapidly and accurately. With the use of electron microscope chambers having ideal port geometries, where all detectors point at the same tilted sample area, it is possible to collect EDS, EBSD and WDS data simultaneously without any loss in acquisition performance.
Collection of EDS data simultaneously with EBSD helps in interpreting the sample structural characterization. The more the data obtained from the sample, the greater will be the analyst’s confidence in the interpretation and characterization.
The speed at which data can be obtained from a sample depends on software and hardware design as well as operational settings. For EBSD, these include, sample backscatter coefficient, SEM beam current and voltage, camera sensitivity, optic efficiencies, camera exposure and gain settings. Multiple detection schemes must be simultaneously deployed for complete sample characterization.
For each pixel location, it is possible to synchronize the data with the help of hardware or software triggering. With the use of software triggering, delay from command initiation until each hardware reaction or response must be precisely characterized. It is preferable that these delays are short and consistent for each detector. The ideal method is to have a number of detectors initialize simultaneous acquisitions through hardware triggering from a real-time operating system to all detection sub-systems.
In order to precisely and completely characterize the samples, simultaneous collection of EDS and EBSD signals are very useful. It is possible to use the conclusions arrived at from the elemental analysis to fine-tune the interpretation of crystallographic analyses for confident reporting. The ability to perform these analysis at the full speed of the acquisition hardware is advantageous in improving lab efficiency and meeting customer requests.
Carbides in A Nickel Based Superalloy
Carbides are believed to beneficially impact the high temperature performance of most metallic alloy systems. Characterizing the carbide type is important for fine-tuning the structure to properties and characterizing carbide orientations may prove advantageous to properties. Presently the distribution and size of carbides are such that in order to obtain meaningful statistics, many analyses must be done. Aquisition speed is hence the key consideration while designing characterization experiment.
Alloy 2205 is a dual-phase stainless steel having good corrosion resistance and a high-yield strength. The two phases are body-centered and face-centered and body-centered cubic with different chemistries between the two phases. Collecting a full EDS spectrum and EBSD pattern at each point at the same time as the beam rasters over the sample enables the microstructures and chemistries of the two distinct phases to be determined very rapidly. Since spectral images are collected during EBSD analysis there is no need for predefining elements for mapping and a comprehensive picture of the sample chemistry is obtained. Further processing of the spectral images is possible by removing the background and performing peak deconvolution to obtain quantitative maps. Additionally COMPASS software can be used to clearly define maps of the chemical phases.
In order to balance the best properties of both the phases, dual-phase materials are produced. This material has a very fine FCC particle structure within the BCC matrix material. In order to improve the properties of this material, it is important to understand the shape, size, composition and relative orientation of the phases.
The experimental procedure is listed below:
- The samples were polished down to 1 µm diamond, then final polished using a vibratory polish for 15 to 60 mins.
- The samples were then mounted in a FESEM under high vacuum using a 70 degree pre-tilted sample holder. The beam current was changed for each sample and the beam energy was 20 keV.
- The Thermo Scientific NORAN System 7 was connected to an UltraDry EDS silicon drift detector and a QuasOr EBSD detector.
- There were different camera exposure times for each sample but they were typically 1-5 ms.
- The EDS pixels dwell times were preset by the acquisition software with a value high enough to collect and transfer the EBSD camera data.
- The camera binning value was set to 8 × 8, but certain acquisitions were collected using a binning value of 4 × 4.
- The EBSD and EDS mapping resolution was typically 256 × 198 pixels using beam tilt correction for the sample tilt.
- Multiple crystal structures were chosen for crystallographic differentiation of the phases in the samples.
- While performing the acquisition process at high speeds, the number of graphic display elements in the software was reduced to bring down the computer CPU loading.
- Typically Indexing Quality (IQ), Pattern Quality (PQ), and Euler color orientation maps for each phase were displayed.
- The individual diffraction patterns of each pixel were saved for some of the acquisitions.
- EDS spectral imaging data are presented in formats such as elemental gross or region-of-interest (ROI) count maps and elemental quantitative maps, which are more accurate representations of the elemental distributions. Complementing the diffraction phase analysis are the preferred EDS spectral imaging analysis methods MSA and phase mapping.
Results and Discussion
The results obtained for carbides are listed below:
- There is a very little contrast within the matrix material grains but carbides can be seen in the electron image. Secondary electron imaging shows they are prominantly seen on the polished surface proving their high hardness.
- EDS spectra showed a peak for both Nb and Mo in the particles, but it was found that the Mo contribution came from matrix contribution during analysis of the small particles.
- An X-ray peak was visible near 280 eV inicates the chance of boron enrichment. By using WDS, it was confirmed that this peak was only the Nb-M X-ray line and that carbon was truly present only in the particles.
- Next the EBSD, EDS acquisitions were done.
- Phase and grain boundaries are seen in the Pattern Quality (PQ) and Indexing Quality (IQ) maps.
- The IQ values were minimal at the carbide particle borders due to the noted topology which caused some shading effects. The matrix material had significantly larger grains that the particles and contained a fair number of twins.
Large energy range EDS spectrum with WDS spectrum
Small energy range EDS spectrum with WDS spectrum
Figure 1a. EDS and WDS results for the carbides sample
Figure 1b. EBSD results for the carbides sample
The results for A2205 are listed below:
- There is no contrast in polished sample electron image for backscattered or secondary electron imaging.
- Grain and phase contrast was seen in both the PQ and IQ maps. The orientation phase maps showed a nearly random orientation of the grains and the FCC phase contained a high percentage of twin boundaries.
- It is seen that both phases comprise the same Fe-Cr-Ni elements so the elemental maps contrast was quite low.
- However, quantitative maps of the elements increased the contrast enough to show a coordination of the Ni-enrichment and Cr-depletion in the FCC phase, as expected.
Figure 2. EBSD and EDS results for A2205 dual-phase steel
Figure 3. EBSD and EDS results from dual-phase steel
The results for dual-phase steel are listed below:
- There is very slight contrast in the electron image of the polished samples for backscattered or secondary electron imaging.
- Phase and grain contrast was readily seen in both the IQ and PQ maps.
- The orientation phase maps showed a highly aligned BCC matrix phase and a nearly random orientation of the grains in the FCC phase which contained a limited percentage of twin boundaries.
- Identical Fe-Cr-Ni elements are present in both phases, hence the contrast in the elemental maps was quite low, confirmed by the phase spectra of the phases. MSA was used to isolate the pixels of each chemical phase within the analysis region.
- The chemical and orientation phase maps matched very well.
In order to have increased confidence in the interpretation presented to customers, analysts prefer collecting as much information as they can. Collecting a range of complementary information simultaneously is a highly desired trait in an analysis system. The ability to collect that data at the highest speed possible improves laboratory efficiency. WDS, EDS, and EBSD contain complementary compositional and crystallographic information required to precisely determine unique phases in materials.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.