14. Compare between the plasticity of edge and screw dislocations in BCC metals 15. Why does a screw dislocation in a BCC metal needs high thermal activation to move?

14) The edge dislocation is more plastic than the screw dislocation

15) So as to form kinks that are fast moving

Explanation:

14) Edge and screw dislocations are the two main types of mobile dislocations

The three dimensional core of the screw dislocation prevents the slipping of the layers (one over the other) in a BCC metal such that kinks are required to be formed first  by thermal activation (heating) in order. The kinks are edge dislocation that move such that the screw dislocation moves forward

Hence, the edge dislocation is more plastic than the screw dislocation

15) The three dimensional structure of a screw dislocation acts like a wedge which resists the slipping of the layers in the BCC structure such that the screw dislocation needs to be highly thermally activated forming kinks before the surrounding layers can move.