# Mechanical Behaviour Questions and Answers – Line Imperfections

This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Line Imperfections”.

1. Which one of the following, is a line imperfection?
a) Grain boundary
b) Tilt boundary
c) Dislocation
d) Stacking fault

Explanation: Dislocations look like lines in the lattice, which are one dimensional. So it falls under the category of line imperfections. While grain boundary, tilt boundary and stacking fault are surface imperfections.

2. What is the range of dislocation density in pure and unhardened metals?
a) 1010 – 1012 m-2
b) 1014 – 1016 m-2
c) 104 – 106 m-2
d) 108 – 1010 m-2

Explanation: Pure unhardened metals (real crystal) contain large number of dislocations of the order of1010 – 1012 m-2. So these can easily be deformed.

3. What is unit of dislocation density?
a) m
b) m-2
c) kg/m3
d) m-3

Explanation: Dislocation density is defined as number of lines in a unit volume. So the unit of dislocation density is m/m3 or m-2.

4. Dislocations are responsible for ________________
a) Increase in strength
b) Decrease in strength
c) Increase or decrease in strength depending on dislocation density
d) Don’t affect strength of metal

Explanation: If dislocation density is in range 1010 – 1012 m-2, it decreases the strength by easy plastic deformation. On the other hand, if it exceeds to 1016 m-2, these causes strengthening of metal by interaction to imperfections.

5. The number of dislocations increases drastically during ______
a) Solidification
b) Plastic deformation
c) Elastic deformation
d) Heat treatment

Explanation: Dislocations increase in number drastically during plastic deformation. It is due to the formation of a Frank-Read source.

6. Positive edge dislocation is denoted by ______
a) ↶
b) ↷
c) ꓕ
d) ꓔ

Explanation: Positive edge dislocation is represented as ‘ꓕ’ (inverted ꓔ) because the extra half plane lies above the shear plane. ‘ꓔ’ is used for negative edge dislocation. ‘↶’ and ‘↷’ are used to represent screw dislocations.

7. Edge dislocation introduces shear strain only.
a) True
b) False

Explanation: Edge dislocations cause compressive, tensile and shear lattice strains. While screw dislocations cause shear strain only.

8. What term is used for the defect, produced by an array of dislocations that produces a small difference in orientation between the adjoining lattices?
a) Tilt boundary
b) Twist boundary
c) Free surface
d) Low angle grain boundary

Explanation: The array of dislocations produces an angular mismatch between the lattices, which is referred to as low angle grain boundary. These have an angle less than 100.

9. Which statement is false?
a) Plastic deformation decreases dislocation density.
b) Strain hardening is the increase of dislocation density with plastic deformation.
c) Slip plane is the crystallographic plane of dislocation motion.
d) Dislocation can change its plane of motion by climb on high temperatures.

Explanation: Plastic deformation increases the dislocation density. Plastic deformation causes generation of dislocations due to the Frank-Read source. All other statements are correct.

10. Most crystalline materials have dislocations in their as formed state.
a) True
b) False

Explanation: Crystalline materials have dislocations due to stresses (mechanical, thermal …) associated with the forming processes. Thus forming processes are the source of dislocations.

11. Dislocation density can vary from _____ to _____ in metals.
a) 105 – 1012 cm-2
b) 105 – 1012 m-2
c) 108 – 1010 cm-2
d) 106 – 1010 cm-2

Explanation: Dislocation density varies in metals from 105 cm-2 (in carefully solidified metal) to 1012 cm-2 (in heavily deformed metal). It depends on various parameters as degree of deformation, temperature etc.

12. Large plastic deformation corresponds to _____ of grains along the direction of applied stress.
a) Growth
b) Rupture
c) Recrystallization
d) Elongation

Explanation: Larger plastic deformation leads to elongation of grains. This becomes possible due to the presence of dislocations.

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

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