the ductile-to-brittle transition
Some metals have a Jekyll-and-Hyde character with temperature. Warm, they are tough and bend before they break; cold, the very same metal snaps like a biscuit with no warning. The temperature range where this personality flips is the ductile-to-brittle transition, and the temperature at its middle is the transition temperature (DBTT).
The transition happens because two competing processes have very different temperature sensitivities. Yielding (dislocations gliding) gets harder as the metal cools, while the stress to cleave crystals barely changes. Below the transition, cleavage wins and the metal breaks in a brittle way before it can yield; above it, the metal yields first and fails in a ductile, energy-absorbing way. Which metals do this? It is mainly a crystal-structure story: body-centered-cubic metals (plain-carbon and low-alloy steels) and many ceramics show a sharp transition; face-centered-cubic metals (aluminum, copper, austenitic stainless steel) stay tough all the way down and barely have one.
This is one of the deadliest facts in engineering, because a structure signed off as tough in a warm lab can be brittle on a freezing night. The remedy is to pick a steel whose transition temperature sits safely below the coldest service temperature — checked with impact tests across a range of temperatures. History paid in blood to learn this: the Liberty ships of World War II used steel with a transition temperature near 0 degrees C and cracked catastrophically in cold North Atlantic water.
A Charpy impact test run on the same steel from -100 to +100 degrees C gives high absorbed energy when warm and low energy when cold, with a steep drop in between. An engineer picks the alloy whose drop-off sits, say, 30 degrees C below the coldest expected service temperature, with margin to spare.
Warm and tough, cold and brittle: BCC steels flip character over a narrow temperature band.
Not every metal has a transition. FCC metals (aluminum, copper, austenitic stainless) stay ductile even in liquid-nitrogen cold, which is exactly why cryogenic tanks are built from them, not from ordinary BCC carbon steel.