Great answer found [here][1]:

> Ductile and Brittle Failure of Materials
>
> When a stress is applied to any object, it deforms, i.e., changes shape and/or size. This deformation is called _elastic_ if the object returns to its original shape after the applied stress has been removed. Deformation that is permanent is called _plastic_ deformation.
>
> All materials can undergo only a limited amount of elastic deformation, after which either plastic deformation sets in or the material fractures. 
>
> Materials that fracture without any plastic deformation are called brittle materials. Examples include glass and most other ceramic materials.
>
> Ductile materials undergo plastic deformation before fracture. Examples include aluminum, copper, steel and many metals, as well as polyethylene, nylon and many other polymers.
>
> A number of factors determine whether a material is going to behave in a ductile or brittle manner. Among these factors are
>
>• The structure and composition of the material, i.e., what are the atoms that make up the material, how are they bonded to each other, are there impurities, etc.
>
>• The rate at which the material is deformed
>
>• The temperature at which the material is deformed
>
>Generally high rates of deformation and low temperatures promote brittle fracture.
Brittle failure usually occurs very rapidly and can be catastrophic. Many materials which are ductile at high temperatures become brittle when cooled below a critical temperature. **This temperature is called the ductile to brittle transition temperature (DBTT) for metals and the glass transition temperature (Tg) for polymers.**

  [1]: http://cecs.wright.edu/~rsrin/Courses/Egr190-191/Brittle-EGR191.pdf