Metallography

Jun 5, 2024

4 min read

Metallography studies the microstructure of a metallic material to find the relationship with its physicochemical and mechanical properties. This scientific discipline studies how a metallic material will behave, such as whether it can withstand significant mechanical stress, conduct electricity efficiently, or be used in applications involving extreme temperatures.

To illustrate the importance of this science in multiple applications, some examples are presented below:

Monitoring heat treatment effects on the microstructure

Will the heat treatment we are designing for this material be effective? Are we truly altering the microstructure as intended? Have we successfully eliminated the defects we aimed to remove, or, on the contrary, have we introduced new ones? With metallographic studies, we can answer all of these questions and detect key microstructural features that will likely influence the final performance of the metallic part.

With metallography we can monitor the changes produced by heat treatment in the microstructure

The image on the right shows the microstructure of a copper-based alloy. Metallographic analysis allows us to draw some preliminary conclusions from this image. Experts can make early assumptions about the material’s main properties or quality by observing key characteristics such as the shape and size of the grains (clear regions in the image), the distribution of particles (dark dots in the image), and any potential cracks and pores.

With heat treatments, the microstructure of the metal changes, as can be observed in the microstructure on the left. By comparing microstructural images, we can track the changes produced by the heat treatment. The differences observed in their microstructures can explain changes observed in the final performance of the material with heat treatment.

Metallography not only connects microstructural changes to material behaviour but also can be used to assess whether the heat treatment was effective or if adjustments, such as varying the application time, temperature, or cooling method, could improve the effects of the heat treatment on the final microstructure.

Monitoring experiments in the laboratory

Metallography offers many possibilities before and after testing the metallic samples.

Before testing:

While the micrographs can offer some initial insights, further testing is necessary to fully understand how the alloy will behave in industrial applications. First observations from the microstructure imaging are crucial in guiding the design of the tests needed to thoroughly study the alloy’s properties.

After testing:

How do we know if we are performing a test correctly in our laboratory? Sometimes it is enough to put our sample under the lens of a microscope to find out.

With metallography, we can verify the effectiveness of experiments performed on metallic materials

Sample after corrosion test — Results of a liking during a corrosion test

For example, when you apply a corrosion test to a delimited area of a specimen, you could check if you are properly isolating the surface to be tested. In the image on the right, you can see an attack on the metal's microstructure outside the area that has been exposed to the electrolyte, indicating that the test was not performed correctly and some electrolyte leaked outside the delimited area.

Monitoring manufacturing processes

Each manufacturing process is unique, and optimising it to achieve maximum potential requires many studies, tests, prototypes, etc. One way to improve the design of manufacturing processes or detect possible failures is to subject the materials to microstructural tests.

With metallography we can monitor the proper functioning of each metal manufacturing step and detect defective parts

For example, the material in the image shown below was obtained by powder metallurgy. The presence of porosity is inherent to this branch of manufacturing processes and reducing the porosity completely can be a real challenge. For this reason, metallic materials processed by powder metallurgy methods generally do not have strict porosity requirements (if minimal porosity were required, another processing method should be sought). However, a pore of such size as the one in the image could be concerning even in powder metallurgy, and it is important to have detected this processing defect, as it could lead to unexpected responses of the material in service.

Porosity in a metallic alloy — Big unexpected pore in a metallic alloy

Forensic Engineering

Metallography is not restricted to evaluating metallic materials for their design and manufacturing. A crucial application of metallography is in forensic engineering. This discipline focuses on examining the material after failure and uses metallography to evaluate the damage and find hints of the origin of failure.

With metallography we can investigate the causes of damage and solve study cases like a detective does

Let's say that a piece of metal that failed spectacularly arrived at the laboratory and was still under warranty! Investigations begin just as we see in CSI episodes: Was the material behaving strangely before the unfortunate event? Has it been in contact with any suspicious material in the days leading up to the accident? Are there any precedents? Do you have any idea what might have caused it? Factor, let's not point fingers at anyone. Could it have been avoided? Was there any material that didn't get along well with it...? Oh, our poor material! It didn't deserve this!

Thankfully, metallography allows us to investigate the failure, identify its cause, and ensure it won’t happen again. By observing the damaged microstructure, we can find clues to determine the cause of the material failure. For example, not only the presence of cracks but also their type gives us a lot of information. Cracks can form between grains (intergranular), or they can trespass the grains (transgranular), they can be small (microcracks) or grow and cross millimetres of material. Other clues are the porosity, cavities, and machining marks.

The images below reveal failure clues in ZAMAK, a zinc alloy made from aluminium, magnesium, and copper. In this case, many of the clues pointed to corrosion. But let's save the details of corrosion for another post.