Which is more ductile BCC or FCC

Why do FCC metals not have a temperature transition from brittle to ductile?

For what it's worth (http://materialiaindica.ning.com/forum/topics/ductile-brittle-transition):

"Let's compare BCC and FCC here. At high temperatures, both have movable dislocations and can therefore withstand large plastic deformations without suffering breakage.

At low temperatures, dislocations in BCC can no longer be mobile, but dislocations in FCC can still move very quickly. This lack of dislocation movement makes BCC brittle while FCC remains ductile ...

The crucial question now is why dislocations in the FCC remain mobile at low temperatures, while dislocations in the BCC make it increasingly difficult to move when the temperature is lowered.

This is easy to understand: in the FCC, there are tightly packed layers that belong to every hatch system, and hatch means that a corner atom is moved to the center of the face.

Although there are as many slip systems in BCC as in FCC (12), the movement of dislocations occurs only when an atomic line jumps from one potential energy valley to another (called Peierls valleys) - a process that could do so through the application of stress or improved by thermal activation. The planes are not as tightly packed and any slip means that a corner atom is moved to the center of the cube. Thus, one would see kink nucleation and propagation in the BCC dislocation motion because the cost of moving a number of atoms from the corner to the center at one time is too high.

In summary, the movement of BCC dislocations is activated thermally, while the movement of (relative) FCC dislocations requires much less activation. This causes BCC materials to become brittle at low temperatures, while FCC remains ductile regardless of temperature - but probably not at 0 Kelvin as an atom has yet to be moved. However, I think there is an activation energy for the FCC slip, only it is much less than kbT. "