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.
by Patrick L Barry and Dr Tony
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.
(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.
Solid tin particles coarsen within a liquid mixture of tin
and lead over a 24-hour period. Snapshots of three different
samples were combined to create this time series.
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.
Image courtesy NASA
Glenn Research Center
Gravity causes the tin particles to quickly sediment to the
top of the chamber during ground experiments (right).
For the same experiment run in orbit, the particles remain
evenly dispersed (left).
"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 185°C, or 365°F. (Tin melts at 232°C, or 449°F.)
Scientists use lead and tin because the basic physical properties
of this mixture are well understood, making the analysis of the
results more fruitful.
applications employing alloys will benefit from
improved theories for coarsening.
As 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 aluminium car chassis, or a suspension
bridge near you. CSLM-2 will teach us even more....