Dislocations and heavy metal concerts

26 October 2019

ROCK CONCERT

Audience at a rock concert

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Research findings in metallurgy hardly ever make it on the front page of a newspaper... 

It neither produces “quirky” outcomes such as “Pressures produced when penguins poo – calculations on avian defaecation”, nor does it add to understanding illnesses or the black box of human body. But this hidden discipline, combining physics, chemistry and engineering, has a huge impact on our daily lives.

Metallurgists develop and improve metals and alloys so you don’t need to cycle around with a 30kg solid steel-frame bike anymore! Instead, you have the option of getting a frame made of lightweight aluminium. It was also the work of a metallurgist who found that stainless steel can be used for cutlery. Thanks to this breakthrough, we taste only the food we're eating without the unappetising taint brought by traces of bronze, copper or silver.

Metallurgists make sure that even in a car crash the bonnet deforms in a way it protects the passengers. How a metal deforms or breaks is still subject of intense research. Components will fail eventually but it is important to know when and how this will happen. These deformation mechanisms vary between alloys. Bending an aluminium wire is easily possible, but the same wire made of steel will be harder to bend but it will break more readily. Taking the same steel wire and heating it up, though, changes its characteristics and improves its ability to bend more easily without breaking.

So what makes a material able to bend or break? It all comes down to atoms. If this word triggers nightmares from your sciences classes at school, don’t panic. The only thing to know at this stage is that everything is built of tiny billiard balls. In fact, for metals, these atoms are arranged as close to each other as possible, just like the balls in the triangle you use for setting up a game of billiards. To use a livelier comparison, one can imagine a pop-concert where the crowd in front of the stage is aligned in perfect rows. People stand shoulder to shoulder and, to maximise their views of the musicians, the rows are slightly offset so the people in the line behind can peer between the shoulders of those ahead.

But imagine that this ideal configuration is suddenly disturbed by a group of latecomers. They squeeze in between the rows and form a line ending in the middle of the concert hall. As such, they disturb the perfect alignment of the crowd. Even worse, these inconsiderate gate crashers start queue (or row)-jumping and make their way closer to the stage. In metallurgy, we call the bunch of naughty atoms that behave like this and disturb our even configuration a “dislocation”.

Dislocations are atomic party-crashers. But they are also the reason why a component can deform without breaking. Returning to the audience at the concert,  if the audience were moved to a U-shaped concert hall, gaps would open up between the rows at the outer area of the rounded part of the hall. Similarly, at the inner side, the concert goers will be squeezed together so they will try to leave their spots. The obvious solution is to form a couple of “half-rows”, or dislocations, to fill in the gaps as well as creating more space in the bended region.

This is what can be observed in real materials. Dislocations can form and move through an alloy to fill in the gaps which otherwise would get bigger and bigger and eventually create a crack. Dislocations (or party-crashers) can not only help by making bending easier, they can also make the material stronger. If a dislocation has successfully squeezed between the rows of perfectly aligned atoms, it creates tension for the atoms which were there before. Atoms, likewise concert-goers, enjoy wiggling around with their peers at a well-defined distance: “This is my dance space. This is your dance space. I don't go into yours, you don't go into mine!”

A dislocation disturbs the ideal space and squeezes some areas while others need stretch. As another dislocation comes by to enable more bending and softening, it will have difficulties passing the strained atoms. Hence, the material becomes stronger when it gets deformed. Applying temperature to the mix makes the atoms wiggle faster so they need even more space. As such, the space between them can get bigger without creating a crack. It also makes it easier for the dislocations to move through the structure. This explains why metals can be deformed more easily at higher temperatures.

Dislocations are the core element for metallurgists studying the deformation of materials. Even if they study certain defects with funny or long names, such as “twins” or “anti-phase boundaries”, they are all formed by our naughty fellas, the dislocations. Scientists are still working on understanding certain behaviours of dislocations. The dislocation itself couldn't care less: it still glides, climbs and slips through a material, following the laws of nature we're yet to define...

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