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Name: Karl S.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: August 2002


Question:
Why does metal become so brittle when liquid nitrogen is poured on it?



Replies:
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.

Vince Calder


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 in 1948.

Andy Johnson



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