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Grain Size Analysis in Metals and Alloys


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 using 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 manufacturers 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 manufacturers need to pay strict attention to the grain characteristics of aluminum components used in the landing gear of a commercial aircraft. In addition to analyzing grain size and distribution trends, the inspectors may be required by strict internal quality control procedures to thoroughly document the results and archive them for future reference.

Image of grains in steel at 100× magnification
Image of grains in steel at 100× magnification

Challenge

Although a wide variety of international standards exist, ASTM E112 is the dominant standard that grains are analyzed under in North and South America. Quality-control laboratories used, and continue to use, the ASTM chart comparison method to analyze grains. With this method, 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.

Or, instead of comparing to a micrograph poster, the operator inserts 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 in question as well as the “golden” image simultaneously.

Since the grain size is estimated by the operator, these methods can produce inaccurate and unrepeatable results, often not reproducible by 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 turnkey, fully automated grains analysis solution, helping eliminate 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 reducing cost?

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

Solution

Enter the modern digital metallurgical quality-control (QC) laboratory. Thanks to advancements in materials-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 (circles, cross-and-circles, lines, etc.) is overlaid on 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 (see an example of the markings in the image on the right).Taking the system calibration into consideration, the image-analysis software automatically calculates the ASTM G, or grain size, number and mean intercept length as a function of the intercept count and pattern length.


Grain analysis using the intercept method
Grain analysis using the intercept 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.


Grain analysis using the planimetric method
Grain analysis using the planimetric method

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 improve. Furthermore, the metallurgical-specific image-analysis software of some microscopes can be configured to archive the grain results automatically into a spreadsheet or optional integrated database.

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


Results of an ASTM E112 analysis
Results of an ASTM E112 analysis

Configuration

A typical equipment configuration for analyzing grains through digital image-analysis consists of the following components:

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 the user maneuvers the scanning stage.

Materials-Science-Specific Image-Analysis Software:

Image-analysis software dedicated to materials-science microscopes often offers optional add-on modules enabling users to analyze grains 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 metallurgical microscope, 10X objective lens, and a high-resolution microscope camera

Typical equipment configuration: inverted metallurgical microscope, 10X objective lens, and a high-resolution microscope camera

Typical equipment configuration: inverted metallurgical microscope, 10X objective lens, and a high-resolution microscope camera


Typical equipment configuration: inverted metallurgical microscope, 10X objective lens, and a high-resolution microscope camera

10× Metallurgical Objective Lens:

This is the required objective magnification for grain analysis.

High-Resolution CCD or CMOS Digital Microscope Camera:

When considering a digital camera for grain analysis, you should prioritize digital resolution over the pixel size, or resulting pixel density. To ensure that there are enough pixels to sample and digitally reconstruct the smallest detail, many microscopists follow the “Nyquist Theorem”, which states that 2 to 3 pixels are required to sample the smallest detail, or optical resolution. Considering that grain analysis is performed with a 10× objective lens (coupled with 10× eyepieces = equals a total magnification of 100×), the optical resolution of a typical mid-grade objective lens would be approximately 1.1 μm. This means that the actual, calibrated pixel size must be smaller than 366 nm (providing the required 3 pixels per smallest distinguishable feature). For example, a five-megapixel camera with 3.45 μm pixel size yields a calibrated pixel size of 345 nm (dividing the actual pixel size by the 10× objective lens, using a 1× camera adaptor). Dividing the lens resolution (1.1 μm) by the calibrated pixel size (345 nm) equals 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 materials-science microscopy-specific microscope cameras with a 3 megapixel or greater rating (considering the pixel size of most common CCD and CMOS sensors) are recommended for grain analysis.

Because grain-size analysis can be performed reliably in grayscale mode (where setting the threshold parameters is simpler than when in color mode), the chosen camera should have the grayscale mode option. Also, choosing a camera that can achieve a fast refresh-rate in live mode will prove to be advantageous when you are 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 automatic recognition helps eliminate the risk of manually entering the incorrect lens magnification in the software.

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

The PC that you choose should meet the minimum system requirements of the camera and image-analysis software. A highresolution monitor is also required.

Procedure

  1. Select the 10x objective lens and then, under reflected-light, brightfield conditions, maneuver the sample on the XY stage to view the area of interest.
  2. Capture the digital image via the image-analysis software. Note: If the software platform you are using offers the ability to analyze a live image, you can observe the live image instead.
  3. Within the grain-analysis software, apply the required filters to ensure that the 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 software analyzes the image in compliance with the chosen standard. The resulting data is written directly into a spreadsheet within the image-analysis software.
  5. It is not uncommon for grain analysis to be performed over 5 random fields. If so, repeat steps 1 through 4 five consecutive times.
  6. Based on a user's predefined template, a report is automatically generated incorporating the analysis results, supporting grain images, and relevant data.

Results of an ASTM E112 analysis

Summary

Unlike manual techniques where operators perform a visual estimate of the grain size, or G number, by eye, modern materialsscience microscope-specific image-analysis software enables 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 they can be implemented with minimal effort. Going beyond the scope of the analysis, many software programs 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 grain analysis, working directly with an experienced materials-science-specific microscope manufacturer is of utmost importance, as they can assist you in every step of this process, from equipment selection to deployment.

References
Carmo Pelliciari, Dr. Eng., Metallurgical Consultant
American Society for Testing and Materials (ASTM) E112-13 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
ProductsUsedApplications

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.
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.
OLYMPUS Stream image analysis software offers fast, efficient inspection workflows for all process steps for image acquisition, quantitative measurements and image analysis, reporting, and advanced materials science inspections tasks.
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