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Name: Gary
Status: Educator
Grade: 6-8
Location: CA
Country: N/A
Date: October 2005


Question:
Dear ask:

If you freeze iron to extremely low temperatures (approaching absolute zero?), does it become brittle and breakable, or stronger? Is this the same for steel?



Replies:
In general, as the temperature is reduced metals becomes stronger, but more brittle, at the same time. The temperature does not have to be very near absolute zero. Assuming that the iron is not quenched to form a glass, but is "regular old iron", the structure becomes more rigid (i.e. stronger). However, because the atoms cannot move past one another so readily at low temperatures, the metal becomes less ductile (i.e. more brittle). So when a crack is started it propagates rapidly through the metal causing it to fail.

Vince Calder


All metals will become brittle once they drop below what is known as the metal's transition temperature. Determining the transition temperature is not that easy, but at temperatures near absolute zero, they are both definitely brittle. For different metals with different compositions, the transition temperature can have a wide range. Since the transition temperature is not a sharp point in the testing, a range is given. That is to say, you cannot say that steel has a transition temperature of "x" degrees, but has a range where the failure is more likely to be brittle, say between "x" and "y" degrees. Anything below this range is definitely brittle failure. One of the classic examples of this transition temperature problem was the Liberty Ships during World War II. Ask your local librarian to get you some books or articles on the Liberty Ships. What happened with the Liberty ships is that the iron used for the ship had a transition temperature that was above the temperature of the North Atlantic where some of these ships were helping take supplies to Russia. They would hit a big wave and literally snap in two because the water temperature caused the metal to become brittle instead of forgiving of large, unexpected loads.

Thanks for using NEWTON.

Chris Murphy, P.E.
Air Force Research Laboratory



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