S&M Question: Change in Tempurates Making Materials more Brittle/Ductile?

[ahh.liz]

So I've been doing a spot of physics homework and am a tad bit confused as to how to go about answering the question as follows:

At which temperature is the material most brittle and why? What are the advantages and disadvantages of the material at this temperature?

My physics teacher provided a stress/strain graph with some lovely lines on it. It seems to be at its most brittle at -40 Degrees Celsius and becomes more ductile with the increasing temperatures (up to 200 degrees Celsius), but I can't seem to decipher why this is. Any suggestions friendly physicists?

[Jordan.Carroll]

Sort of more a chemistry question, its due to the lattice like arrangement of atoms in metals.

Bending, or plastically deforming a Crystal lattice introduces slipping and cracking, like when you stretch chewing gum.
At higher temperatures the bonds around these deformities form faster, and distribute the stress around the deformity, at lower temperatures this doesn't happen as quick, meaning cracks appear quicker.