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Grain Size Analysis in Metals and Alloys


Background

In the metallographic laboratory, analyzing grains in metallic and alloy samples, such as aluminum or steel, is important for quality-control. Most metals are crystalline in nature and contain internal boundaries, commonly known as "grain boundaries". When a metal or alloy is processed, the atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain will eventually impact others and form an interface where the atomic orientations differ. It has been established that the mechanical properties of the sample improve as the grain size decreases. Therefore, alloy composition and processing must be carefully controlled to obtain the desired grain size.

After metallographic sample preparation, grains in a specific alloy are often analyzed via microscopy, where the size and distribution of these grains can demonstrate the integrity and quality of the sample. For example, because human life may be at stake, automotive manufactures study the size and distribution of grains in a particular alloy to determine if a newly designed automotive component will hold up under extreme circumstances. Aerospace component manufactures need to pay strict attention to the grains characteristics of an aluminum component used in the landing gear of a commercial aircraft. In addition to analyzing these grains size and distribution trends, rigorous internal quality-control procedures may require that these results be well documented and archived for future reference.

Image of Grains in Steal at 100x Magnification

Image of Grains in Steel at 100x Magnification

Challenge

Although a wide-variety of international standards exist1, ASTM E112 is the dominant standard that grains are analyzed under in North and South America. In former times, and even still in practice today, most quality-control laboratories would analyze grains via the "Chart Comparison" method. Here, operators perform a visual estimation of the grain size by comparing a live image under an optical microscope to a micrograph chart, often posted on the wall near the microscope.

Example of a microscope eyepiece reticle used to compare against a live image.
Example of a microscope eyepiece reticle used to compare against a live image.

Or, instead of comparing to a micrograph poster, one can insert an eyepiece reticle containing images of predefined grain size patterns, directly into the microscope's optical path. This way, the comparison is performed directly in the microscope, where the operator can see both the sample at question as well as a "golden" image simultaneously.

Since the grain size is being estimated by the operator, these methodologies can produce inaccurate and unrepeatable results, often not reproducible between different operators. Further, quality-control technicians are required to manually enter their results into a computer-based spreadsheet or report providing an additional opportunity for errors.

How can a metallurgical quality-control laboratory implement a turn-key, fully-automated grains analysis solution, eliminating potential inaccuracies and subjectivity introduced by the human factor, while complying with ASTM E112 or other international standards? Additionally, how can the data be automatically archived and reports automatically generated-all while saving valuable time and money?

Solution

Enter the modern digital metallurgical QC laboratory. Thanks to advancements in material-science microscopy specific software, operators can leverage image-analysis to analyze grains, in compliance with ASTM E112 as well as a wide-variety of international standards.

One popular digital solution in which this is accomplished is known as the "Intercept Method". Here, a pattern (i.e. circles, cross-and-circles, lines, etc) is overlaid atop the digital image (live or captured). Each time the overlaid pattern intercepts with a grain boundary, an intercept is drawn on the image and recorded (hence the name "Intercept Method"). Taking the system calibration into consideration, the image-analysis software automatically calculates the ASTM "G-Number" and mean intercept length, as a function of the intercept count and pattern length.

Grain Analysis via the Intercept method.
Grain Analysis via the Intercept method.

Grain analysis via the Planimetric method.
Grain analysis via the Planimetric method.


Another popular method for calculating grain size in the digital metallurgical laboratory is known as the "Planimetric" method. Unlike the Intercept method, the Planimetric method determines the grain size on an image (live or captured) by calculating the number of grains per unit area.

Since the results are calculated internally within the image-analysis software, the guesswork attributed to the human element is removed. In many instances, overall accuracy and repeatability, as well as reproducibility has shown improvement. What's more, many metallurgical-specific microscope image-analysis software packages can be configured to archive the grains results automatically into a spreadsheet or optional integrated database.

Results of an ASTM E112 Analysis
Results of an ASTM E112 Analysis

Reports, containing relevant analysis data and associated images, can also be generated with the push of a button-all with minimal training.

Configuration

A typical equipment configuration for analyzing grains via digital image-analysis consists of:

Inverted Metallurgical Microscope:
An inverted microscope is typically preferred over an upright model because the flat, polished sample lays flat on the mechanical stage, ensuring consistent focus as one maneuvers the scanning stage.

Material Science Specific Image-Analysis Software:
Material Science microscope specific image-analysis software packages often offer optional add-on modules that allow users to analyze grains directly in compliance with ASTM E112, as well as various International standards. At the time of purchase, the user should determine if the Intercept or Planimetric method is more appropriate.

Typical equipment configuration: Inverted Metallurigical Microscope, 10x Metallurigical Objective Lens, Microscope-Specific High-Resolution Digital Camera.
Typical equipment configuration: Inverted Metallurigical Microscope, 10x MetallurigicalObjectiveLens, Microscope-Specific High-Resolution Digital Camera.

10x Metallurgical Objective Lens: The required objective magnification for grains analysis.

Microscope-Specific High-Resolution CCD or CMOS Digital Camera: When considering a digital camera for Grains analysis, more important than digital resolution is the pixel size, or resulting pixel density. To ensure enough pixels are provided to sample and digitally reconstruct the smallest detail, many microscopists follow "Nyquist Theorem", which states that 2 to 3 pixels are required to sample the smallest detail, or optical resolution. Considering that Grains analysis will be always performed with a 10x objective lens (coupled with 10x eyepieces = 100x total magnification), the optical resolution of a typical mid-grade objective lens would be approximately 1.1μm. This means that the actual, calibrated pixel size must be smaller than 366nm (providing the required 3 pixels per smallest distinguishable feature). For example, a 5MP camera with 3.45μm pixel size will yield a calibrated pixel size of 345nm (dividing the actual pixel size by the 10x objective lens, using a 1x camera adapter). Dividing the lens resolution (1.1μm) by the calibrated pixel size (345nm) = 3.2. In this example, 3.2 pixels are present to sample the smallest distinguishable feature, meeting the Nyquist criteria of 2 to 3 pixels per distinguishable feature. Although this may sound confusing, a general rule of thumb is that most common material-science microscopy specific cameras 3MP or greater (considering the pixel size of most common CCD and CMOS sensors) are recommended for Grains analysis.

Because Grain Size analysis can be performed reliably in gray scale mode (where setting threshold parameters is simpler than color mode), the chosen camera should be capable of imaging in gray scale mode as opposed to color only. Also, choosing a camera that can achieve a fast refresh-rate in live mode will help when focusing or positioning the sample.

A coded manual or motorized revolving objective nosepiece is recommended. The chosen image-analysis software should be capable of automatically reading the objective lens magnification at all times. This ensures the highest level of measurement accuracy as the potential of manually entering the incorrect objective lens magnification into the software is eliminated.

A manual or motorized XY scanning stage is required to manipulate the sample and position at the area of interest for observation and analysis.

A PC meeting the minimum system requirements of the camera and image-analysis software, and high-resolution monitor are required.

Procedure

  1. Choosing the 10x objective lens, under reflected-light, brightfield conditions, maneuver the sample on the XY stage to view the area of interest to be analyzed.
  2. Capture the digital image via the image-analysis software. Note: Alternatively many software platforms offer the ability to analyze a live image in addition to a captured image.
  3. Within the Grains Analysis software, apply required filters to ensure that intercepts are accurately represented on the image. In many software packages, this ability is provided interactively so that the operator can view the effects of the filter on the resulting intercepts.
  4. The image is analyzed in compliance within the chosen standard. Resultant data is written into a spreadsheet directly within in image-analysis software.
  5. It is not uncommon for grains analysis to be performed over 5 random fields. If so, repeat steps 1 through 4 five consecutive times.
  6. Based on a user's pre-defined template, a report is automatically generated incorporating the analysis results, supporting grains images and relevant data.

Summary

Unlike former techniques where operators performed a visual estimation of the grain size, or "G-Number" manually by eye, modern material-science microscope specific image-analysis software allows the grain size to be calculated accurately and repeatedly, as human intervention is minimized. Many software packages are designed to comply with ASTM E112 and a wide-range of international standards, and can be implemented with minimal efforts. Going beyond the scope of the analysis, many software packages additionally offer the ability to automatically generate reports based on the analysis data, and even go so far as providing an integrated database for archiving and quick-and-easy searching of images and related data. When considering a turn-key solution for automatic grains analysis, working directly with an experienced material-science specific microscope manufacturer is of utmost importance, as they can assist you in every step of this process, from equipment selection to full deployment.

References

Carmo Pelliciari, Dr. Eng., Metallurgical Consultant

American Society for Testing and Materials (ASTM) E112-88 Standard
ASTM International, 100 Barr Harbor Drive, PO Box C700,
West Conshohocken, PA, 19428-2959 USA

"Committee E-4 and Grain Size Measurements: 75 years of progress."
ASTM Standardization News, May, 1991, George Vander Voort

Olympus IMS

Products used for this application


GX53NEW

The GX53 inverted microscope features exceptional image clarity and excellent resolution at high magnifications. With accessories including a coded revolving nosepiece and software, the microscope's modular design makes it easy to customize for your requirements.

MPLFLN-BD

The MPLFLN-BD lens has semi apochromat color correction and is suitable for the widest range of applications. Especially designed for darkfield observation and the examination of scratches or etchings on polished surfaces.

OLYMPUS StreamNEW

OLYMPUS Stream image analysis software offers user guidance through all process steps for image acquisition, quantitative measurements, reporting and advanced materials science inspections tasks.
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