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ISS Experiments: Hardening of Liquid Metals

By Erin Biba

Metal changes from liquid to solids differently in micro-gravity, which may have an effect on the strength of hardened metal.

You know how ice crystals form on your windshield and look sort of like Christmas Trees? Well, it turns out that form, called a dendrite, happens inside metal when it hardens from a liquid into a solid. Scientists don’t fully understand why, but the way these dendrites are created inside the metal corresponds to how strong it will become when it has fully hardened.

On earth, these dendrites float to the surface of a material as it solidifies (because the hard dendrites have a lower density than the liquid metal). Sometimes the arms of a dendrite will also break off and merge with other formations. But in space, they behave quite differently. They don’t float -- they become suspended evenly throughout the liquid as it hardens. If an arm breaks off it stays put. Microgravity is the ideal environment to study how these states of solid matter ultimately affect and change the strength of a hardened metal. Researchers can account for time and temperature without having to worry about how gravity is changing things too.

Photo credit: NASA

Astronauts on the space station will be working mostly with tin and tin-lead mixtures. They’ll heat the samples to 185 degrees Celsius inside a specially made chamber and then cool them quickly using pressurized water. Once the dendrites are formed, the samples will be stored and returned to earth, where scientists at Northwestern University will examine their properties.

You can read more about this experiment here.