48 new materials - Material ConneXion

to certain structures of metals. At this temperature,
which is often the lowest temperature at
which a structural engineering material can be
considered useful, a metal will fail in a brittle
manner rather than in a ductile manner. That
is, it will shatter rather than bend before giving
way. The DBTT is very sensitive to alloy composition
and processing. This makes it a useful
criterion for quality control.
The notorious catastrophe that befell the World
War II Liberty ships illustrates the importance
of considering DBTT. When launched in cold,
northern waters, the steel hulls of the Liberty
ships split in half. Evidently a hapless engineer
had forgotten that the DBTT of some steel
roughly coincides with the freezing point of water.
Of course, one person’s catastrophe is another
person’s good fortune. Aristotle Onassis
purchased these ships for a pittance and, with
them, created a shipping empire. He knew that
in warmer, equatorial waters, they were completely
safe.
Materials at elevated temperatures
It is important to remember that the description
“solid” is a relatively general term that
Brittle fracture in steel. This micrograph of the fracture
surface shows the cleavage along crystal planes that
absorbs little energy.
refers to a range of material behaviors. On
their way to becoming liquids, some solid
materials transition through a viscous,
gooey state (e.g. molten magma). However,
long before this solid to liquid transition
happens, the material loses its mechanical
performance. Aluminum melts at around
1220ºF but is of little use in any structural
application above 360ºF. Polymers behave
similarly, with the softening point typically
falling at 70% of their melting point. Exceptionally,
the upper limits of ceramics’
operating temperatures are much closer to
their melting points.
Materials at low temperatures
Absolute zero (0 Kelvin [0K], -273.15ºC,
-459.7ºF) is as cold as anything can get. At this
temperature the molecules in an object cease
to move. It is actually impossible to reach this
temperature, though we’ve got within a few
millionths of a degree. The coldest naturally occurring
place we know of is deep space, a very
chilly 3K. In fact, this slight temperature elevation
above 0K is the best evidence we have that
the Big Bang actually occurred.
The most significant change in material properties
at below-ambient (or below-room) temperatures
is the increasing brittleness of the
material, but there are also other properties
that manifest very close to 0K.
These include superconductivity (which you
can read about in our last issue of matter) and
superfluidity, a semiconductor-like property
that results in the complete loss of viscosity.
This creates impressive and somewhat mindboggling
effects such as: When a superfluid is
placed in an open container it will rise up the
sides and flow over the top. And: If the fluid’s
container is rotated from stationary, the fluid
Ductile fracture in the same steel. Breakage at a higher
temperature is ductile. This takes a lot more energy to break.
The fracture surface shows plastic deformation of the steel.
inside will not move relative to the container.
(In the last case, the viscosity of the liquid is
zero, so any part of the liquid or its container
can move at any speed without affecting any of
the surrounding fluid.)
The best part about most materials is that they
tend to perform in ways we like them to at the
temperatures that humans like. Few materials
are helpful to humanity at 3000ºF, but then,
how many situations do you need (or want to be
around) that require that kind of temperature?
That said, if we decide that Mercury is a viable
place to colonize, then all bets are off.
MATERIAL PROFILE
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