Saturday, September 6, 2014

The joy of how stuff works

I’ve been enjoying reading Professor Mark Miodownik’s new book Stuff Matters, which is subtitled Exploring the Marvelous Materials that Shape Our Man-Made World. He does an excellent job of telling personal stories about ten very different materials, and then ties them together with a synthesis. His chapters are titled:

Indomitable (steel)
Trusted (paper)
Fundamental (concrete)
Delicious (chocolate)
Marvelous (aerogel)
Imaginative (cellulose nitrate and plastics)
Invisible (glass)
Unbreakable (diamond and carbon)
Refined (china and ceramics)
Immortal (biomedical materials)

On page 6 of the first chapter about steel and other metals he says that:

“It may be odd to think that metals are made of crystals, because our typical image of a crystal is of a transparent and highly faceted gemstone such as a diamond or emerald. The crystalline nature of metals is hidden from us because metal crystals are opaque, and in most cases microscopically small. Viewed through a electron microscope, the crystals in a piece of metal look like crazy paving, and inside those crystals are squiggly lines - these are dislocations. They are defects in the metal crystals, and represent deviations in the otherwise perfect crystalline arrangement of the atoms - they are atomic disruptions that shouldn’t be there. They sound bad, but turn out to be very useful. Dislocations are what make metals so special as materials for tools, cutting edges, and ultimately the razor blade, because they allow the metal crystals to change shape.

You don’t need to use a hammer to experience the power of dislocations. When you bend a paper clip, it is in fact the metal crystals that are bending. If they didn’t bend, the paper clip would be brittle and snap like a stick. This plastic behavior is achieved by the dislocations moving within the crystal. As they move they transfer small bits of material from one side of the crystal to the other. They do this at the speed of sound. As you bend a paper clip, you are causing approximately 100,000.000,000,000 dislocations to move at a speed of thousands of hundreds of meters per second. Although each one only moves a tiny piece of the crystal (one atomic plane in fact) there are enough of them to allow the crystal to behave like a super-strong plastic rather than a brittle rock.”

Professor Miodownik illustrates those two paragraphs with a simple sketch. (There are obviously severe economic limits on how you can illustrate a book). Some readers may be surprised to find that metals are crystals.

One place where you can see large metal crystals is on a hot-dip galvanized (zinc coated) steel product, like the corrugated beam guard rail shown above by the north side of Crescent Rim Drive in Boise.

To understand how dislocations move a little bit of a crystal at a time, think about moving a rug on your living room floor a few feet to the left. As shown above, one way is to grab the left end and pull all of it at once. Another is to kick the right end to create a bump, and then push the bump along.

The bump on a rug is a local disturbance or defect, like a dislocation. Gliding a dislocation from right to left as shown above (by the downward pointing red arrow), eventually moves one atomic plane across a crystal.  

In the last chapter Miodownik has a drawing that describes six scales ranging down from human to atomic. I’ve summarized it above in a table. When I described how stainless steels work, I used a similar set of scales with explicit powers of ten.

The schematic dislocation image came from Wikimedia Commons.

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