Liquid Nitrogen and Brittle Metal
Name: Karl S.
Date: August 2002
Why does metal become so brittle when liquid nitrogen is poured on it?
Metals (and other solids for that matter) are brittle or ductile depending
upon how easily the atoms and/or molecules of solids can slide across one
another when a stress is applied. The higher the temperature the more the
atoms and molecules can glide past one another; the lower the temperature
the atoms and molecules are not "giggling" around very much, so the barrier
to moving is higher relative to the amplitude of the vibrational "giggling".
So at low temperatures, such as liquid N2, the material is very "stiff" and
"hard". Although it can then withstand higher applied forces i.e. the
material is harder, the material cannot deform readily, so it breaks.
This is a great question because many inquires to this NEWTON BBS service
ask what is the
"strongest" metal. There really is no way to answer this question because,
in fact, strength is just one factor in the suitability of a given metal for
an application. Actually, many metals can become brittle at temperatures
well above that of liquid nitrogen (-196 deg C or -321 deg F). This
tendency to be brittle (i.e., fracture under impact) is referred to as a
metal's "toughness" and this toughness is temperature sensitive.
When a metal is ductile, it can bend and stretch. This change in shape is
accompanied by actual translation or flow of the metal at the atomic level.
As the temperature decreases, it becomes more difficult to break these bonds
and, consequently, easier to develop stresses in the metal that can lead to
actual fracture, rather than flow.
The crystalline structure of the metal, along with many other factors,
influences the temperature at which that metal becomes brittle.
Metallurgists have learned to manipulate steel composition to achieve a
desired temperature sensitivity. For instance, steel alloys high in nickel
are used in cryogenic applications because they are more resistant to
becoming brittle at very low temperatures.
The effects of temperature on metal toughness can be critical in many
applications. For instance, it is hypothesized that the sinking of the
Titanic ocean liner might have been averted if the steel in the hull had had
greater low-temperature ductility. Tests on hull samples from the Titanic
retrieved in recent years indicate that the steel had a high sulfur content,
which caused it to become brittle at temperatures as high as -1 deg C, which
is substantially above the freezing point of salt water. As a result, when
the Titanic struck the iceberg, the steel in its hull fractured rather than
deformed, causing the fatal gash. If the metal had just buckled, it is
possible the ship would not have sunk. These properties were not
well-understood or appreciated until the 1940's.
As a matter of fact, this property has even co-starred in a 1951 movie with
Jimmy Stewart and Marlene Dietrich titled "No Highway in the Sky". It comes
on TV periodically and is the story of a metallurgist who learns that
low-temperature induced metal fatigue is causing the mysterious crashes of a
new airliner. It is based on the book "No Highway" by Nevil Shute, published
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Update: June 2012