Mechanical Behaviour Questions and Answers – Unit Dislocations and Partial Dislocations

This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Unit Dislocations and Partial Dislocations”.

1. A dislocation with its Burgers vector equals one lattice spacing is called ______ dislocation.
a) Unit
b) Partial
c) Imperfect
d) Frank

Explanation: A dislocation of unit Burgers vector is known as unit dislocation. It is also known as the perfect dislocation. While partial dislocation or imperfect dislocation has burgers vector less than 1.

2. What is not a type of partial dislocation?
a) Frank
b) Shockley
c) Stair rod
d) Unit

Explanation: Unit dislocation is a perfect dislocation. Partial dislocations are imperfect dislocations. These can be Frank, Shockley or stair rod type.

3. The energy released on dissociation of perfect dislocation into partial dislocations is _____
a) Stacking fault energy
b) Dislocation energy
c) Elastic energy
d) Core energy

Explanation: The energy released by dissociation of perfect dislocation is supplied to create a stacking fault in crystal. This energy is hence known as stacking fault energy. It gives stacking fault defect in the crystal.

4. The pair of partial dislocations along with stacking fault region is called ______ dislocation.
a) Frank partial
b) Lomer-Cottrell
c) Extended
d) Unit

Explanation: The pair of partial dislocations along with fault region is called extended dislocation. The stacking fault region separates fully slipped and unslipped regions.

5. SFE of copper increases with the amount of zinc.
a) True
b) False

Explanation: SFE of metal is lowered by solid solution alloying. So SFE of copper decreases with increasing amount of zinc. So ductility of copper also decreases.
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6. Extended screw dislocations can move only on the plane with stacking fault.
a) True
b) False

Explanation: Extended screw dislocations only can move on a plane containing stacking fault. It is due to their edge character. Thus cross slip is difficult for extended screw dislocations.

7. What is the strength of the Burgers vector for dislocations in FCC structure?
a) a/√2
b) a√3/2
c) 3a/√2
d) a/2

Explanation: In FCC structure, close packed direction is <1 1 0>. The Burgers vector is a/2<1 1 0>. The strength of vector is a/√2.

8. What is the strength of the Burgers vector for dislocations in BCC structure?
a) a/√2
b) a/3
c) a√3/2
d) 3a/√2

Explanation: In BCC structure, close packed direction is <1 1 1>. The Burgers vector is a/2<1 1 1>. The strength of the vector is a√3/2.

9. The reaction a/2[1 1 0] → a/6[1 2 1] + a/6[2 1 1] is _______
a) Favourable energetically
b) Favourable vectorially
c) Favourable vectorially and energetically
d) Not favourable vectorially and energetically

Explanation: The reaction is vectorially favourable which can be seen by vector addition. The reaction is energetically favourable too. It can be proved using Frank’s criterion (b12 > b22 + b32).

10. The reaction a/6[1 2 1] + a/6[2 1 1] → a/2[1 1 0] is ________
a) Favourable energetically
b) Favourable vectorially
c) Favourable vectorially and energetically
d) Not favourable vectorially and energetically

Explanation: The reaction is vectorially favourable through vector addition. Reaction is not energetically favourable. It doesn’t follow Frank’s criterion (b12 + b22 > b32).

11. a/6[1 2 1] + a/6[1 2 1] → _______
a) a/3[1 0 1]
b) a/6[1 0 1]
c) a/3[1 0 1]
d) a/6[1 0 1]

Explanation: Burgers vector of dislocations follow addition rules. Hence a/6[1 2 1] + a/6[1 2 1] → a/6[2 0 2]. It can be further simplified as a/3[1 0 1].

12. What will be strength, if Burgers vector of dislocation is a [x y z]?
a) a2[x2 + y2 + z2]
b) a[x2 + y2 + z2]
c) a[x2 + y2 + z2]1/2
d) a[x2 + y2 + z2]2

Explanation: The strength of Burgers vectors a [x y z] is given as a[x2 + y2 + z2]1/2. It is also known as magnitude of vector. For a[1 0 0] it is a [unit].

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