Q & A: Brittleness of metals at low temperature

Can liquid nitrogen make strong metals brittle?
- Qi Han

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)

Do structures made of wood also become brittle at low temperature like being exposed to liquid oxygen and nitrogen?
- Dibyendu (age 49)
India

Yes, I think so.  In liquid nitrogen, rubber gets hard and brittle.  Flower petals get brittle and can shatter.  Bananas can be broken at low temperatures where at high temeperatures they just squish.  Even steel gets brittle and breaks easily at low temperatures.

Wood is quite complicated, though.  It is made of long, strong fibers, and even at normal temperatures, wood is much stronger in some directions than in others.  You can crack a piece of wood along the grain much more easily than across it.  And wood that has a lot of water in it (say, a green twig, or a waterlogged branch) will undergo damage to its fibers when the water freezes.  The freezing process itself may crack the wood as different parts cool down at different times.

Now about the liquid oxygen: adding this to wood makes for a highly combustible, even explosive mixture.  I remember that the speed record for igniting a grill full of charcoal briquettes was to douse it first not in lighter fluid but rather with liquid oxygen.  Proper safety precautions have to be taken in case flaming coals flew off.  I'd keep the liquid oxygen away from flammable substances.

Tom

(published on 10/22/2007)