There is a Screw Loose

Dr. René de Kloe, Applications Specialist, EDAX

I dare say that in everyday life, most people do not think about crystallography very often. Equally, when we think of grains, a familiar image that comes to mind is children playing on the beach, building sandcastles (or in good Dutch tradition, perhaps a dam to keep the sea out).

Children already know about building things. They know you must use moist sand to make nice figurines. They also know that when you dig too deep on the beach, that water may come in and wreck your castle. You have to know your stuff when you start building things. Parents stimulate these construction experiments by supplying the building materials for some serious out-of-the-box thinking. The children start small, developing new, and intriguing concept cars (Figure 1), and then move on to bigger ideas and perhaps they build robots (Figure 2).

Figure1. A concept car made of Duplo blocks.

Figure 2. Robots made from carton boxes.

Why should this stop when you grow up? Some people might say there is a screw loose inside if you occupy yourself with carton robots (I designed the robots for a children’s vacation camp 😊). Still, the fascination with building beautiful things remains at all ages. A while ago, my neighbor asked me to take a look at this impressive tower built of Anker stones without using any glue (Figure 3,

Figure 3. An Anker stone model of the Grunewaldturm in Berlin.

Engineers have never outgrown the desire to put bits together to build things, and with the knowledge they gained during their education and experience, amazing things have been created. But as with the Anker tower, to have a stable structure, you need to keep paying attention to detail. If you have ever built anything yourself, you know how important it is to use the right components and ensure that all the parts fit together.

During my work at EDAX, I often work with engineers who are creating and testing new materials. Such materials are typically being deliberately developed for certain purposes by mixing components and then treating them just so, but sometimes also found by accident. And of course, it is not only the composition of a material that defines its properties, it is also the microstructure that makes a material suitable for specific applications. When you take care to pick the proper starting material for your product, you can successfully build something. However, sometimes corners are cut, and things go wrong.

For example, take a look at the two montage EBSD maps of iron screws in Figure 4.

Figure 4. An EBSD IPF (Y) on image quality maps of a) a coarse grained screw and b) a fine-grained screw. All the green grains are aligned with one of the edges of the unit-cell cube facing towards the tip of the screw.

These are two multi-million-point EBSD maps showing the microstructure in two screws. The greenish color indicates that in both screws the crystallographic [011] direction lies along the length of the screw. This is indicative of the production process of the metal rods from which the screws are cut. The different purple colors in the head are caused by the stamp that shapes it and pushes the cross into the top of the screw. But that is where the similarities end.

The top screw shows a very coarse grain structure, while the bottom screw has a much finer interconnected grain structure. This difference in grain structure has consequences. When we zoom in on the shaft of the coarse-grained screw (Figure 5a), the large grains appear flattened in between the threads, and there is a strong change in grain size from the center to the edge of the shaft. In between the threads, some of these larger grains have even been forced apart to form cracks. This combination is bad news for the strength of the screw. When you tighten this screw, the force gets “focused” on the weak areas between the threads, and the screw breaks easily. In the fine-grained screw (Figure 5b), a minor grain size reduction is visible right at the edge of the shaft, but the internal structure is constant over the entire screw. This homogeneous structure distributes the force evenly over the screw, and it does not break easily.

Figure 5. Grain maps of the two screws shown at the same scale illustrating the difference in grain size. a) Shows a coarse-grained microstructure and b) depicts a fine-grained microstructure.

A final detail scan of the grain structure shows an additional difference (Figure 6). In the coarse-grained screw, long trails of carbide particles can be observed in between the grains, which effectively separate the grains and facilitate cracking. In the fine-grained screw, the grains show a lamellar martensitic microstructure with very few carbides. These microstructures exacerbate the difference in strength between the screws.

Figure 6. Detail maps of the grain structure in a) the weak and b) the strong screw.

The investigation example shown above was born out of frustration when I tried to build something, and the screws just kept breaking while I thought I was doing nothing wrong. So, I decided to cut up one of the failing screws and compare it with a screw from another box that had never given me trouble.

This was just about a screw used in a DIY project to put a wooden panel to a wall. Nothing crucial, you would think. But just imagine when this screw would have been used to hold up something a bit more impressive, like that big, heavy chandelier 10 meters above your head in the lobby of a hotel? Then suddenly, the microstructure of a humble construction component, such as a screw, becomes crucial, and thinking about the crystallography and grain structure of everyday items turns out to be really important.

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