Approximately what is the vacancy concentration in a metal at a temperature just below its melting point? Approximately what is the dislocation density (in cm/cc) in a heavily deformed metal? When dislocations start to slip in a crystalline solid, what will happen to the solid? Solution a) Approximate vacancy concentraion in a metal at temperature just below its melting point = 10-5 - 10-4 b) Dislocation density in heavily deformed metal = 1011- 1012 cm/cc C) Plastic deformation starts when dislocatons start to slip in a crystalline solid.