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
View Answer

Answer: a
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
View Answer

Answer: d
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
View Answer

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

Answer: c
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
View Answer

Answer: b
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
View Answer

Answer: a
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
View Answer

Answer: a
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.
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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
View Answer

Answer: c
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
View Answer

Answer: c
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).
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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
View Answer

Answer: b
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]
View Answer

Answer: a
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
View Answer

Answer: c
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].

Sanfoundry Global Education & Learning Series – Mechanical Behaviour & Testing of Materials.

To practice all areas of Mechanical Behaviour & Testing of Materials, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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