Brad Mclvor, of King's High School, asks :-
When things become cold the particles slow down. Is this why materials become brittle when cold?
Michael Cowling, a metallurgist at the University of Canterbury, responded.
You can think of the way solids behave when a force is applied to them as a competition between two possible types of behaviour, ductile and brittle. Ductile behaviour involves a significant permanent change in shape (called plastic deformation) before the item breaks, whereas brittle fracture results in it breaking with little or no plastic deformation. Whichever type of behaviour occurs most easily (needs the least force) will be the way the material behaves.
Many materials (eg metals) undergo plastic deformation by the movement of tiny defects in the crystal structure called dislocations. At high temperatures the dislocations can move easily so the metals are ductile, they stretch a long way before they break. As it gets colder the force needed to move the dislocations goes up, but the force needed to cause brittle fracture hardly changes. Eventually the force needed for brittle fracture becomes less than that needed to deform the material so the material becomes brittle.
At room temperature glass and most ceramics are brittle while most metals are ductile. Some metals remain ductile to very low temperatures, for example stainless steel can be use for vessels which store liquid nitrogen at minus 196 Celsius (196 degrees of frost) whereas similar vessels made of ordinary steel would shatter.