The ductile-brittle transition temperature is the minimum temperature in which a given material has the ability to absorb a specific amount of energy without fracturing.
At low temperatures some metals that would be ductile at room temperature become brittle. This is known as a ductile to brittle transition.
The ductile to brittle transition temperature is strongly dependent on the composition of the metal. Steel is the most commonly used metal that shows this behavior.
For some steels, the transition temperature can be around 0°C, and in winter the temperature in some parts of the world can be below this. As a result, some steel structures are very likely to fail in winter.
This occurs mainly due to the atoms present in the material. As the temperature increases, the frequency and amplitude with which the atoms vibrate increases. This causes the atoms under stress to break bonds and form new ones with other atoms(slip). This slipping of atoms is seen as plastic deformation on the external surface of the material, a common feature of ductile materials. However, below the transition temperature, the frequency and amplitude of the vibrations decreases and the atoms just break the bonds but does not slip to a new place. As there is no slip, there is very little plastic deformation before fracture, commonly known as a brittle fracture.