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NASA Science News for July 7, 2003

Trouble with Lifshitz, Slyozov and Wagner

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. 

July 7, 2003:  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, e.g., 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. 

The Lifshitz-Slyozov-Wagner (LSW) theory predicts the rate of
coarsening in alloys. What's wrong with the theory? Strictly
speaking, nothing. It's the way engineers have been using it that's
wrong. The equations of LSW describe how fast materials will coarsen
if you let them sit for an infinite amount of time. Forever. Most
engineers can't wait that long, so they've assumed that the theory
also works for shorter times--like hours and days. 

Testing this assumption was one of the goals of the Coarsening in
Solid-Liquid Mixtures (CSLM) experiment, which flew onboard the space
shuttle in 1997. 

"The first shuttle experiments worked just as we'd hoped," recalls
principal investigator Peter Voorhees, professor of materials science
at Northwestern University near Chicago, Illinois. "But when we
looked at the sizes of the grains, they were larger on average than
the theory would predict." 

Something was amiss.

Scientists had never been able to fully test the predictions of LSW
in a liquid mixture because gravity always interfered with the most
ideal experiments. To mirror the assumptions of the theory, an
experiment would need to have solid, microscopic grains scattered
evenly within a liquid. If you try this on the ground, the solid
particles will quickly settle out of the liquid and accumulate at the
top or bottom of the container, ruining the experiment. 

"In space, the solid particles stay evenly dispersed for hours or
even days, so we can compare the results directly with the theory,"
Voorhees says. 

The shuttle experiments, however, ran for only 10 hours. And perhaps
that's the problem. Computer simulations suggest that when coarsening
is allowed to continue somewhat longer, the theory redeems itself. 

With longer trials in mind, Voorhees and his colleagues designed
CSLM-2, a 2nd-generation coarsening experiment for the International
Space Station. The device will heat a mixture of lead and tin until
it melts. Because pure tin has a higher melting temperature than the
lead-tin mixture, tiny embedded crystals of tin will remain solid at
the experiment's temperature: about 185øC, or 365øF. (Tin melts at
232øC, or 449øF.) Scientists use lead and tin because the basic
physical properties of this mixture are well understood, making the
analysis of the results more fruitful. 

The furnaces keep the samples melted, the tiny tin crystals will
coarsen for times ranging from 1.5 to 48 hours. After the larger
crystals have grown and the smaller ones shrunk, the samples will be
cooled and solidified to preserve them, then returned to Earth where
Voorhees and his team of scientists will slice them open and examine
them to see if the theory held true for the longer experiment runs. 

Although there's still much to learn about coarsening, some of the
results from the first CSLM experiment are already being used by
industry. For example, Voorhees helped an Evanston, Illinois, company
called QuesTek to integrate the findings of the first experiment into
the computer software they use to make material design
recommendations. QuesTek's clients--which include major manufacturing
companies--then use those materials to build a wide range of
products. 

This means the physics revealed by CSLM may already be finding its
way to a jet engine, or an aluminum car chassis, or a suspension
bridge near you. CSLM-2 will teach us even more.... 

Credits & Contacts
Authors: Patrick L. Barry, Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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