Explain why is crossslip of screw dislocations more difficul

Explain why is cross-slip of screw dislocations more difficult in FCC metals with low stacking fault energy than those with high stacking fault energy?

Solution

In FCC crystal the lines of intersection between slip planes are [110] which happen to be the Burgers vector of perfect dislocations in FCC crystal.  Therefore it may have screw dislocations which could cross slip from one close packed plane to another.

However cross slip could not have taken place if were an edge dislocation. In FCC crystal perfect dislocations often split into two partial dislocations. The tendency to split depends on the stacking fault energy of the crystal. If it is low the partials are far apart whereas if it is high these are nearby. The shear stress needed to join these to form a perfect dislocation will be high in the case of the former and low for the latter. Therefore cross slip will be more difficult in crystals having low stacking fault energy.