The GATE Grind

GATE 2026 MT – Question 16

Mechanical Behaviour of Materials · Dislocations and strengthening · 1 mark · Multiple choice

Which one of the following options is correct? In a face-centered cubic metal, Shockley partial is:

  1. Perfect and mobile dislocation
  2. Perfect and immobile dislocation
  3. Imperfect and immobile dislocation
  4. Imperfect and mobile dislocation

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Show answer and explanation

Correct answer: (D) Imperfect and mobile dislocation

Explanation

A Shockley partial in FCC has Burgers vector $a\langle112\rangle/6$, which is not a full lattice translation. It is therefore an imperfect dislocation and bounds a stacking fault.

Its Burgers vector lies in a suitable $\{111\}$ slip plane, allowing glide. Thus it is **imperfect and mobile (D)**; an immobile stair-rod partial is a different type.