Month: February 2014

PRIAS Imaging – New approaches to visualizing microstructure

Matt Nowell, EBSD Product Manager EDAX

As part of the group that helps develop ideas for EDAX products, it is always exciting and rewarding when those ideas become features within our software that our customers can use for microstructural characterization.  This month, we have announced a new feature named PRIAS, or Pattern Region of Interest Analysis System.  This is a feature that is near and dear to my heart, as we have been discussing it and developing it internally for a number of years.  It wasn’t until recently when our Hikari XP camera became fast enough to support it that we felt that we could release this feature with its full characterization potential.  In this blog post, I want to introduce PRIAS, and show some of its exciting new capabilities.

Two of the most common OIM map types are the image quality map and IPF orientation map.  Examples of these from an Inconel 600 nickel-based superalloy are shown below.  These images are common in part because they show a nice, high-contrast visualization of the microstructure, and communicate information about grain size and shape easily.

OIM Image Quality Map OIM IPF Orientation Map
OIM Image Quality Map OIM IPF Orientation Map

Generally traditional SEM imaging with Secondary Electron (SE) or Backscatter Electron (BSE) detectors with EBSD samples does not provide great contrast or images because of the sample preparation requirements, the sample tilt requirements, and the detector geometry relative to the specimen.  Often on a well-polished surface, users hope for a small piece of dust to help facilitate focusing.  The traditional remedy for this problem has been a Forward Scatter Detector, which is a solid-state diode typically positioned around the bottom of the phosphor screen.  While it would be possible to implement multiple FSD diodes (as some other EBSD vendors have done), each diode requires amplification and analog-to-digital conversion circuitry, which effectively limit the maximum number of diodes that can be used.

The PRIAS approach is an innovative method to use the EBSD camera and phosphor screen synergistically as both an EBSD pattern collection detector and an array of positional electron detectors.  In the PRIAS mode, region of interest (ROI) electron detection areas are defined on the EBSD phosphor screen and the average intensity is measured for each ROI as the electron beam is rastered over the area of interest.  The intensity variations for the various ROIs can then be used to create gray scale or colored micrographs showing orientation, atomic number, and topographic contrast.

PRIAS imaging data can be collected from 3 different modes of operation:

  • PRIAS Live is a traditional imaging technique.  In this mode, the Hikari XP camera is highly binned to produce very fast frame rates which enable acceptable imaging speeds.  With PRIAS Live, 25 ROIs are pre-defined as electron detector regions.  The array of PRIAS live images collected from the same Inconel 600 sample is shown below.
  • The second mode, PRIAS Collection, runs simultaneously with standard OIM mapping.  In this mode, 3 ROIs are pre-defined and these imaging channels are stored with the collection orientation data.  These 3 signals can then be mapped in OIM Analysis.
  • The third mode, PRIAS Analysis, requires saving EBSD patterns during OIM mapping but offers the most flexibility in post-processing analysis.  In this mode, users can position and size the different ROIs to be imaged, as well as perform background and dynamic background corrections to the saved patterns.
Array of PRIAS Live images collected from 25 positional ROIs
Array of PRIAS Live images collected from 25 positional ROIs

If the same or similar ROIs are selected, each mode produces similar images as shown below.

PRIAS Live Image 4_PRIAS Collection 4_PRIAS Analysis
PRIAS Live Image PRIAS Collection Image PRIAS Analysis Image

The advantage of the PRIAS Live mode is that qualitative microstructural imaging can be obtained in a much shorter time period than standard OIM mapping.  PRIAS images can also be collected at high speeds at lower voltage and current conditions.  This might lead to interesting applications in characterizing materials like plastics and glasses that are not typically associated with EBSD work.  These are areas we are currently investigating.

For both the PRIAS Live and PRIAS Analysis modes, an array of up to 25 images is available for further processing.  This processing includes weighted arithmetic operations and RGB coloring of multiple ROI signals.  In the image below, RGB coloring was used with 13 ROIs to produce an image showing both grain contrast as well as deformation contrast within the grains.  While the coloring cannot be used for quantitative orientation analysis, as is typical for IPF Orientation maps, the image does provide good visualization of the underlying microstructure in a matter of minutes.

Colored PRIAS image using RGB coloring of 13 ROIs
Colored PRIAS image using
RGB coloring of 13 ROIs

PRIAS images of combined gray scale and colored contrasts can also be generated, similar to combined image quality and orientation maps.  This image can often highlight interesting structural features for further analysis as shown below.

4_Combined Gray Color PRIAS 4_Combined IQ IPF
Combined gray scale and colored PRIAS image Combined gray scale image quality
and colored orientation OIM map

To this point these examples have focused on orientation contrast to generate the PRIAS images.  However, PRIAS imaging can also provide atomic number and topographic contrast information.  From the same Inconel 600 sample at a higher magnification, EDS elemental maps of Chromium and Carbon are shown below.  The images show the position of a larger carbide precipitate within a grain and numerous Cr-rich carbides decorating the grain boundaries.

4_EDS Cr 4_EDS C
EDS Cr Map EDS C Map

A PRIAS image generated from a single selected ROI shows strong atomic number contrast highlighting the position of the larger carbide phase.  Selecting 4 ROIs maintains atomic number contrasts while adding more grain boundary contrast.  Selecting 2 ROIs localizes the topographic grain boundary contrast at the precipitates.  With the ability to select and manipulate up to 25 ROIs, PRIAS provides great power and flexibility to image specific microstructural components.

4_PRIAS 1 ROI 4_PRIAS 4 ROIs GB Contrast 4_PRIAS 2 ROIs Precipitate Contrast
PRIAS image using 1 ROI
showing atomic number contrast
PRIAS image using 4 ROIs
showing atomic number and
grain boundary contrast
PRIAS image using 2 ROIs
showing atomic number and
precipitate contrast

PRIAS images can also be compared and differences determined.  As shown below, these differences correspond to grain boundaries within the material.

4_PRIAS Difference 4_
PRIAS difference image PRIAS difference image with EBSD-detected
grain boundaries colored red

I hope I have shown some of the interesting capabilities of PRIAS imaging for faster visualization of microstructures.  Certainly, since it has been introduced to our EDAX applications team, we have found it to be a very fun and addictive tool to experiment with.  I certainly would not be surprised if future blog posts highlight using this functionality on a wide range of materials.

Solid Angle Tool

Dr. Patrick Camus
Principal Product Developer, EDAX

It recently was brought to my attention that there is a solid angle (SA) calculator on the web. It is provided by Dr. Nestor Zaluzec of Argonne National Lab. It can be found at :
http://tpm.amc.anl.gov/NJZTools/XEDSSolidAngle.html

This web site is of interest to me and many of our customers and potential customers because the SA of your EDS detector is a measure of its efficiency in detecting X-rays. A large SA detector will provide a higher detection rate of X-rays than a smaller SA detector.

Historically, the SA of a detector was approximated by the simplistic equation SA = A / d2 where A is the active area of the detector and d is the detector to sample distance. For small detectors at large distances, this approximation is quite good. However, for large area detectors (which are currently quite popular) and for most TEM applications (which use very short values for d), this simple equation is not valid. This web site is very useful because it provides a much more accurate calculation for the solid angle for any detector size and position. In addition, it can accurately predict values when the additional geometric complication of an off-axis detector is required.

I will use this web site to compare a few representative geometries of SA calculations for round EDS detectors. The web site also has the capability of calculating the SA for rectangular and annular detectors, but I will leave those examples for the user.

In the first example, a typical SEM geometry will be examined where 10 mm2 and 30 mm2 detectors are compared. Most designs for these detectors have the same d, for instance 45 mm in this example.Figure 1

Using these values, simplistic and calculated SA values can be found.

d (mm) A (mm2) Simple SA (msR) Calculated SA (msR)
45 10 5 5
45 30 15 15

In this example, both SA methods provide the same values. There is no primary benefit to using the calculation method.

The next example is for a TEM geometry for 10 mm2 and 30 mm2 detectors are compared. The primary differences to the SEM example are that (1) d=10 which is significantly shorter than for the SEM geometry, and (2) the axis of the detector does not point to the sample. This geometry is very typical for TEM that have a side entry mount location. In addition, this geometry also needs a take-off angle, for which 20 degrees is used.Figure 2_TiltedCircularDetectorAngleDefined

Using these values, simplistic and calculated SA values can be found.

d (mm) A (mm2) Simple SA (msR) Calculated SA (msR)
10 10 94 88
10 30 282 249

In this example, the simple SA method over estimates the true SA values by 7% and 13% for the 2 detectors. SA values for TEM geometries should not be reported using the simple method but should always use the more accurate calculated method.

A final example shows a practical application of the SA calculations in comparing potential EDS detector geometries. When comparing detectors and considering SA values, the A is not always the primary consideration for the efficiency because the d term can have a significant effect. This usually occurs when the tubing for the detectors is not the same and the d must increase for a fatter mounting tube to avoid hitting any item with in the SEM chamber, usually the pole piece. This example compares a 60 mm2 in a small diameter tube mounted at 40 mm and an 80 mm2 detector in a larger diameter tube that must be mounted at a larger d of 50 mm.

Using these values, simplistic and calculated SA values can be found.

d (mm) A (mm2) Simple SA (msR) Calculated SA (msR)
40 60 36 36
50 80 32 31

Firstly, the values provided by the 2 calculation methods are essentially the same (within round-off errors) for both detectors. Intuition would indicate that the larger 80 mm2 detector should provide the greater SA value. However, the increased d actually reduces its SA value to less than that for the smaller 60 mm2 detector.

Solid angle values for EDS detectors are an indication of the X-ray detection efficiency of the system. A web site is available to calculate the values very accurately once the detector geometry is defined. Comparisons of detector geometry can aid a customer in selecting the best detector for their application.