Metals
and Alloys
We study metals and alloys not to build things in space, but to improve things that are made on Earth.
Metals and alloys are everywhere around us: in our automobiles, in the engines of aircraft, in our power-plants, and elsewhere. Despite their presence in everyday life, there are many scientific aspects of metals that we do not understand.
Undercooling: The TEMPUS Facility
NASA and the German Aerospace Agency are flying a furnace on MSL-1 called TEMPUS. "The name of the game on TEMPUS is undercooling," describes Mission Scientist Dr. Mike Robinson.
Undercooling is a tremendously fascinating process. If done right, you can actually take a liquid and lower its temperature below the normal freezing point yet still keep the liquid from freezing!!! This is undercooling.
What's even more interesting is that when you do allow the undercooled liquid to freeze, it forms a kind of material that is very different from the "normally" frozen material!!
With TEMPUS, we will learn more about the properties of the undercooled liquid metals, and the interesting solid forms, called metastable states, that arise when you solidify an undercooled metal.
We're all familar with Undercooling
Whether you know it or not, you have probably seen an undercooled, then rapidly solidified liquid many times - sometimes it's several feet deep in your driveway: SNOW.
Snowflakes occur when undercooled water falls through the atmosphere, eventually striking another drop of water or piece of dust in the air that causes it to rapidly solidify into a beautiful snowflake structure.
We're also aware that snowflakes are very different from regular ice, although both are made of frozen water. This difference comes from the way in which the water was frozen, one from the normal state, one from the undercooled state. And as you know, if you leave snowflakes alone, they eventually turn into regular ice. The "snowflake state" is only partially stable, or metastable.
Undercooling metals and alloys in microgravity
If you heat a metal in microgravity, and allow it to cool without touching any container walls, it will continue to cool below its freezing point, but still remain as a liquid. The water in your ice-cube trays in the freezer at home first freeze to the sides of the tray, eventually freezing through to the center to make an ice-cube. But the liquid metals in the TEMPUS facility have nothing to "freeze" onto, and remain as a liquid right through the freezing temperature and down to lower temperatures. In some cases, we can have liquid metals that are at temperatures several hundred degrees below the normal freezing point of the metal.
When the metal eventually does freeze, it does so in just a fraction of a second emitting a pulse of light. You'll see this process of rapid freezing on the downlink video from the shuttle during the mission.
The kinds of metallic solids that we get out of this process are very different than one can obtain in any other way. One example is a superconducting phase in Niobium compounds, which cannot be formed by normal cooling.
Scientists will study properties of undercooled metals and alloys such as the surface tension, viscosity, electrical conductivity, and the dendritic structure of solidified metals. This last part is analogous to studying the structure of snowflakes, whose patterns and intricacies depend on how fast the snowflake formed. When you freeze a metal, its atoms are arranged in a tree-like, or dendritic, structure. The nature of this structure helps to determine, for example, how strong the metal is, and appears very different for metals that cool quickly compared to those that cool slowly.
Electromagnetic Positioning in TEMPUS
Within the TEMPUS facility, molten drops of metal are positioned through the use of electromagnetic forces that can be used to squeeze the drop or move it around while in the undercooled state. A solidified drop is pictured at top. Magnetic field lines which surround the molten drops are pictured below.
Did You Know That:
"TEMPUS" is an acronym for "Tiegelfreies Elektromagnetisches Prozessieren Unter Schwerelosigkeit"???
Diffusion: The Large Isothermal
Furnace
Japan's National Space Development Agency (NASDA) has designed a Large Isothermal Furnace for different experiments in diffusion and a process called sintering aboard MSL-1.
Diffusion is a familiar process to each of us. Its the process that helps carry the smell of baked bread from the oven throughout the house, or allows food coloring to disperse through a glass of water without stirring. This process is also very important in the study of metals and alloys, and on the ground in 1-g, it can be masked or altered in many ways by gravity-driven flows in molten metals.
On MSL-1, several experiments, using a variety of experiments will be performed to study the diffusion of liquid metals and alloys, to measure a property of diffusion called the diffusion coefficient. The diffusion coefficient is a number that describes the rate at which diffusion takes place. Its value is different for different physical systems, and depends on other physical properties of the system, such as temperature.
These LIF diffusion experiments include:
Did You Know That:
The Large Isothermal Furnace can heat samples to over 1,600 degrees C, nearly 25% of the temperature of the Sun's surface???
Coarsening: CSLM in the Glovebox
Coarsening is a process that can severely degrade the strength of an alloy-product such as a turbine blade in an aircraft engine. By learning about coarsening, we therefore gain insight into processes which can cause metal to weaken in strength or fail totally, with potentially serious consequences.
When you make an alloy, consisting of two or more different metals, as the alloy cools, the two metals generally have different freezing points. One metal can begin to solidify before the other. In the cooling process, the first metal to solidify can begin as very small particles, but coarsen, or grow into fewer larger particles before the second metal in the alloy cools. Alloys with a few larger particles tend to be weaker than alloys with many small particles.
The speed and mechanisms of coarsening are not well understood. Furthermore, in 1-g, the heavier metal can sink to the bottom, further confusing the ability to study the process. By studying this process in space, and learning more about how and why it occurs, we acquire important knowledge for input into our ability to design and control the processing of metals on earth.
Did You Know That:
The Middeck Glovebox (pictured at left), despite its name, will not be on the middeck during MSL-1, but instead in the Spacelab module???
