Hi Qi Han,
Yes. Cooling just about anything to liquid nitrogen temperatures makes it more brittle than at higher temperatures. At higher temperatures, defects in the crystal lattice of a material are more mobile. Bending a crystal will introduce slippage and cracking. At higher temperatures, bonds re-form around the dislocated surfaces, distorting the lattice nearby, shifting the stress around. At lower temperatures, the nearby atoms in the crystal lattice do not move and long cracks can form more easily.
A common process in the production of steel is "annealing", which raises the temperature of the metal so that accumulated slippages and other defects in the crystal lattice (usually caused by forceful mechanical shaping, such as rolling and stamping) may relax and a more ordered crystal can form. A piece of metal that has been repeatedly bent back and forth will become brittle due to an accumulation of defects in the crystal lattice.
Hitachi in Japan has an interesting materials recycling process in which everything is cooled down to liquid nitrogen temperatures and then crushed. Iron and steel components crumble more easily in a crusher at these temperatures, and then the remaining pieces can be separated from non-iron components with magnets later on.
Metals become brittle at temperatures much warmer than liquid nitrogen temperatures. On a cold winter night in Iowa, the door on my brother’s car was frozen shut. I didn’t think I was pulling on the handle too hard, but managed to snap the handle off anyway. I suspect the temperature had a lot to do with the brittleness of that metal door handle.
Tom
(published on 10/22/2007)
