Question

Cross-slip is possible only for

• A. Edge dislocations
• B. Grain boundaries
• C. Mixed dislocations
• D. Screw dislocations

Hint:

Cross-slip requires the dislocation's Burgers vector to lie along the intersection line of two active slip planes. Since a screw dislocation's line is parallel to its Burgers vector, it can move freely onto any intersecting plane that contains that line.

Answer 1

The Correct Option is D

Concept: Cross-slip is a process in plastic deformation where a moving dislocation changes tracks, leaving its original slip plane to glide onto an intersecting slip plane that shares the same Burgers vector.

Step 1: Geometric constraints of Edge vs. Screw dislocations.
Let us examine the geometric relationship between the dislocation line vector (\(\vec{t}\)) and its Burgers vector (\(\vec{b}\)) for different types of dislocations:
• Edge Dislocation: For an edge defect, the Burgers vector is strictly perpendicular to the dislocation line (\(\vec{b} \perp \vec{t}\)). Because these vectors are perpendicular, they define a single, unique spatial plane. The edge dislocation is geometrically locked into this plane and can only glide within it.
• Screw Dislocation: For a screw defect, the Burgers vector lies completely parallel to the dislocation line (\(\vec{b} \parallel \vec{t}\)). Because they are parallel, they do not define a single unique plane. Instead, an infinite number of intersecting planes can pass through that same parallel vector pair.

Step 2: Mechanism of Cross-Slip.
If a moving screw dislocation encounters a local structural obstacle (such as an impurity or a precipitate) within its primary slip plane, it can easily shift onto a secondary, intersecting slip plane that contains the same Burgers vector without changing its internal structure. This ability to switch planes allows screw dislocations to bypass obstacles cleanly.

Step 3: Verification of options.
Because only screw dislocations have a parallel configuration (\(\vec{b} \parallel \vec{t}\)) that allows them to glide on multiple intersecting planes, cross-slip is uniquely restricted to them. This matches option (D).