A Troublesome Theory in Materials Science

A physics theory used to create cutting-edge "designer materials" doesn't work as scientists expect. A new experiment on the ISS could reveal why.

A quiet revolution is happening in the science of designing materials.

In times past, finding a material with just the right strength, elasticity, or other desirable traits involved a process of trial and error. People would "discover" a new material like steel or rubber, not "invent" it. Only after the fact would scientists figure out why that certain mixture of chemicals behaved a certain way.

But the burgeoning field of materials science is turning all of that on its head. Scientists can now start with a list of desired traits and design a custom material to suit - specifying the atomic structure, grain structure, and even heat treatments needed--without needing to resort to the old cycle of make, test, refine.

The secret behind this radical new ability is a combination of two modern trends: the availability of powerful, affordable computers; and advances over the last 50 years in the fundamental physics of solids. By plugging the equations of physics into a fast enough computer, you can see how a certain material will behave before it's ever made.

But experiments flown on the space shuttle in 1997 showed that one of the classic physics theories used to design materials doesn't work as scientists expected.

The theory in question, known as the Lifshitz-Slyozov-Wagner theory, is important to designers of metal alloys--that is, mixtures of two or more metals. Stainless steel is an alloy (it's a mixture of iron, nickel, and chromium) as is most gold jewelry (gold and nickel). Why make alloys? Because a mixture of metals can be, for example, tougher or lighter-weight than any one metal by itself.

Alloys are formed by heating the ingredients until they liquefy, mixing them together, and letting the batch cool. As the mixture cools and solidifies, tiny crystalline grains form. With the passage of time, these grains do something odd: larger grains tend to grow while smaller ones vanish - a process called "coarsening." Surprisingly, this coarsening continues to happen long after the alloy has fully solidified, often weakening the alloy. This could be a catastrophic problem if, say, the material was used to make the fast-spinning blade of a jet turbine.