Metals with Memories
Accidentally sitting on a pair of glasses can be a disastrous experience. Even though the lenses might survive, the frame will often be bent out of shape, and attempting to bend them back into their original form may completely break them. Wouldn’t it be great if the metal used in the frames could remember how it was supposed to be shaped, even after it had been bent? Well, even though it doesn’t have a “memory” in the same way as you or I, metal alloys that return to their original shapes have found widespread use throughout a variety of modern technologies, including eyeglasses.
Amazing But True!
- When two or more metals are mixed together, the result is an alloy, which has physical and chemical properties that are often quite different than any of the individual elements from which they are composed.
- A simple alloy that has been used since ancient times is bronze, which combines tin and copper to provide a stronger material than either of these pure metals. For example, bronze is still used in the production of musical bells.
Bronze was also used to cast cannons. This is the Dardanelles gun cast in 1464. It was last used in 1807 by the Ottoman Empire against the British
- Some alloys can exist in multiple crystal forms, and the preferred form might vary according to changes in temperature and pressure. An object made of such an alloy might be relatively rigid at one temperature but easy to deform at another. The result is a metal that appears to remember its original shape, as seen in the following video: http://www.youtube.com/watch?v=QYp9rIJRM8s
With the links below, learn more about alloys and metallic structure. Then answer the following questions.
- Steel is an iron-based alloy that contains small amounts of carbon and often other elements as well. One common variety of steel is called “stainless steel”, because it does not corrode (rust) as easily as normal steel. What additional metal is always present in stainless steel?
- Why is stainless steel resistant to corrosion?
- One of the physical properties of metals that can be drastically modified by forming mixtures is its melting point. For example, consider the alloy shown here: http://www.rotometals.com/product-p/lowmeltingpoint144.htm. Compare the melting points of pure bismuth, indium, and tin to that of their alloy.
- If two conductive metals are mixed together, is the alloy likely to be a conductor as well? (Hint: Think about the atomic structure of the mixture.)