# Mechanical Behaviour Questions and Answers – Movement of Dislocations

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

1. Match the following?

 i) Conservative motion p) Slip plane {111} ii) Non-conservative motion q) Slip plane {110} iii) BCC crystals r) Climb iv) FCC crystals s) Glide

a) i-r, ii-s, iii-p, iv-q
b) i-r, ii-s, iii-q, iv-p
c) i-s, ii-r, iii-q, iv-p
d) i-s, ii-r, iii-p, iv-q

Explanation: The motion of glide is a conservative motion whereas the motion of climb is non-conservative. The slip of BCC and FCC crystals are {110} and {111} respectively.

2. What is the slip plane in HCP crystals?
a) (0001)
b) (0110)
c) (1011)
d) (1012)

Explanation: In HCP crystals slip plane is (0001). It is also known as slip plane. It is close packed plane.

3. What is the slip direction in BCC crystal structure?
a) <110>
b) <1120>
c) <111>
d) <101>

Explanation: The family of slip direction of BCC, FCC and HCP crystals is <111>, <110> and <1120> respectively. And the family of slip plane for BCC is {110}.

4. Resolved shear stress is given as τ = ______
a) σ sinφ sinλ
b) σ sinφ cosλ
c) σ cosφ sinλ
d) σ cosφ cosλ

Explanation: The resolved shear stress is given by τ = σ cosφ cosλ. Here, σ is normal tensile stress, φ is the angle between the normal of slip plane and the direction of applied force and λ is the angle between the slip direction and direction of applied force.

5. Lattice frictional stress is known as ____ stress.
a) Shear
b) Normal
c) Peierls and Nabbaro
d) Isotropic

Explanation: Lattice frictional stress is commonly known as Peierls and Nabbaro stress. It is after the names of originators. Dislocation can move only when the applied force is greater than lattice frictional stress.

6. What is experimental CRSS value of silver crystal?
a) 0.37 x 103 MPa
b) 0.49 x 103 MPa
c) 0.78 x 103 MPa
d) 0.26 x 103 MPa

Explanation: CRRS is abbreviated as Critical Resolved Shear Stress.

Silver 0.37 x 103
Copper 0.49 x 103
Aluminum 0.78 x 103

7. What is the theoretical CRSS value for Iron crystal?
a) 9.2 x 103 MPa
b) 10.3 x 103 MPa
c) 11.0 x 103 MPa
d) 21.0 x 103 MPa

Explanation: Iron has high value of CRSS 21.0 x 103 MPa. It is 9.2 x 103 MPa for silver. It is 10.3 x 103 MPa and 11.0 x 103 MPa for copper and titanium crystal respectively.

8. ____ is important in ductile metals for plastic deformation.
a) Climb
b) Glide
c) Cross slip
d) Twinning

Explanation: Cross slip is important in ductile metals. Cross slip allows easy dislocation motion. It makes plastic deformation possible.

9. The positive climb involves diffusion of _____ while negative climb involves diffusion of _____
a) Vacancies, vacancies
b) Vacancies, interstitials
c) Interstitials, vacancies
d) Interstitials, interstitials

Explanation: The dislocation line moving up by one atomic spacing from the original slip plane is a positive climb. It involves diffusion of vacancies. While negative climb involves interstitial diffusion.

10. Pure screw dislocation can climb.
a) True
b) False

Explanation: A pure screw dislocation has no extra half plane of atoms. Therefore, it cannot climb. It can change a slip plane by cross slip.

11. Jogs are sinks and sources of vacancies.
a) True
b) False

Explanation: Jogs are produced by diffusion of vacancies to the dislocation. Jogs require diffusion of vacancies to climb. Jogs are also sources of vacancies.

12. ____ is a thermal activated process.
a) Cross slip
b) Climb
c) Glide
d) Shear

Explanation: Climb is a thermal activated process. Climb requires diffusion of point defects. On low temperature diffusion is negligible due to very low concentration.

13. Movement of dislocation from one position to next involves the movement of atoms to ____
a) Fraction of an interatomic distance
b) An interatomic distance
c) Few interatomic distances
d) Large number of interatomic distance

Explanation: Movement of dislocation involves small rearrangement of atomic bonds. This involves a movement of atoms to fraction of an interatomic distance.

14. Which statement is false regarding dislocation?
a) Dislocation can move if applied shear stress is greater than lattice frictional stress
b) Pure screw dislocations cannot climb
c) Critical normal stress is much greater than the theoretical
d) Peierls and Nabbaro stress is much greater than the theoretical shear stress of a perfect lattice

Explanation: Lattice frictional stress is smaller than the theoretical shear stress of a perfect lattice. It is due to the presence of dislocations. Lattice frictional stress is known as Peierls and Nabbaro stress.

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

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