Excerpt of the Day: The Incredible Power of Crystalline Fractures in Metal

As part of my Excerpt of the Day series, here’s an interesting quote from Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World by Mark Miodownik.

Stuff Matters Book Cover“Metals are made from crystals. The average razor blade contains billions of them, and in each of these crystals the atoms are arranged in a very particular way, a near-perfect three-dimensional pattern. The bonds between the atoms hold them in place and also give the crystals their strength. A razor gets blunt because the many collisions with hairs that it encounters force bits of these crystals to rearrange themselves into a different shape, making and breaking bonds and creating tiny dents in the smooth razor edge…

Viewed through an 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 they 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 the 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 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 crystals to behave like a super-strong plastic rather than a brittle rock…

Gold is a relatively soft metal, so much so that rings are very rarely made from pure gold metal because they quickly scratch. But if you alloy gold, by adding a small percentage of other metals such as silver or copper, you not only change the color of the gold—silver making the gold whiter, and copper making the gold redder—you make the gold harder, much harder. This changing of the properties of metals by very small additions of other ingredients is what makes the study of metals so fascinating. In the case of gold alloys, you might wonder where the silver atoms go. The answer is that they sit inside the gold crystal structure, taking the place of a gold atom, and it is this atom substitution inside the crystal lattice of the gold that makes it stronger…

Alloys tend to be stronger than pure metals for one very simple reason: the alloy atoms have a different size and chemistry from the host metal’s atoms, so when they sit inside the host crystal they cause all sorts of mechanical and electrical disturbances that add up to one crucial thing: they make it more difficult for dislocations to move. And if dislocations find it difficult to move, then the metal is stronger, since it’s harder for the metal crystals to change shape. Alloy design is thus the art of preventing the movement of dislocations…

Atom substitutions happen naturally inside other crystals too. A crystal of aluminum oxide is colorless if pure but becomes blue when it contains impurities of iron atoms: it is the gemstone called sapphire. Exactly the same aluminum oxide crystal containing impurities of chromium is the gem called ruby…

Lead has a low melting point and so dislocations move with consummate ease, making it a very soft metal. Copper has a higher melting point and is stronger. Heating metals allows dislocations to move about and reorganize themselves, with one of the outcomes being that it makes metals softer.”

You can find the full book here on Amazon.

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