1. What is ductile-to-brittle transition?
At low temperatures metals having b.c.c. and h.c.p. structures become brittle and fail by cleavage, even though they may be tough at or above room temperature. In fact, only those metals with an f.c.c. structure (like copper, lead, aluminium) remain unaffected by temperature in this way.
In metals not having an f.c.c. structure, the motion of dislocations is assisted by the thermal agitation of the atoms. At lower temperatures the thermal agitation is less, and the dislocations cannot move as easily as they can at room temperature in response to a stress – the intrinsic lattice resistance increases. The result is that the yield strength rises, and the plastic zone at the crack tip shrinks until it becomes so small that the fracture mechanism changes from ductile tearing to cleavage. This effect is called the ductile-to-brittle transition. Thus steel structures like ships,
bridges and oil rigs are much more likely to fail in winter than in summer.
Ductility is a measure of the degree of plastic deformation that has been sustained at fracture.
A material that experiences very little or no plastic deformation upon fracture is termed brittle.
– Engineering Materials 1
