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Consider two perfect dislocations a/2 [101] and a/2 [110] lying in different {111} slip planes and both parallel to the line of intersection of the {111} planes. The reaction a/2 [101]+ a/2 [110] →a/2 [011] is feasible as the new dislocation has a lower energy according to Frank’s rule.

The two dislocations attract each other and combine to form a new dislocation. This new dislocation lies parallel to the line of intersection of the two slip planes. The direction of the line vector of this new dislocation, which can be found using the Weiss Zone law, is [011]. Since the line vector is perpendicular to the new Burgers vector it is a pure edge dislocation.

The edge dislocation can only glide on the plane which contains both the line and Burgers vector, which is the (100) plane bisecting the slip planes. Since (100) is not a closed-packed slip plane in the fcc lattice, the dislocation will not glide freely and is therefore sessile.