Dislocation Theory - Mechanical Behaviour of Materials - Metallurgical Engineering Previous Year Questions

1

2007 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2007

Which of the following statements are true about edge dislocations?

A

Edge dislocations do not have an extra half plane associated with them

B

The Burgers vector is perpendicular to the line direction

C

Edge dislocations can avoid obstacles by cross-slip

D

Depending on geometry, parallel edge dislocations of opposite sign can attract or repel one another

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2

2007 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2007

A pure low-angle tilt boundary may be equivalently represented by

A

an array of jogs on an edge dislocation

B

a cross grid of screw dislocations

C

a dislocation pileup consisting of both edge and screw dislocations

D

an array of edge dislocations perpendicular to the slip plane

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3

2008 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2008

The correct statements among the following are (P) screw dislocations cannot climb (Q) screw dislocations cannot cross-slip (R) edge dislocations cannot climb (S) edge dislocations cannot cross-slip

A

P, R

B

P, S

C

Q, R

D

Q, S

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4

2009 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2009

A unit dislocation with a Burgers vector \(\vec{b_1}\) will dissociate into two partial dislocations with Burgers vectors \(\vec{b_2}\) and \(\vec{b_3}\), if and only if
P. \(b_1^2 > b_2^2 + b_3^2\)
Q. \(b_1^2 < b_2^2 + b_3^2\)
R. \(\vec{b_1} = \vec{b_2} + \vec{b_3}\)
S. \(\vec{b_1} \neq \vec{b_2} + \vec{b_3}\)

A

P, R

B

P, S

C

Q, R

D

Q, S

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5

2009 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2009

Stacking fault energy (SFE) plays an important role in determining the work hardening ability of a metal. In this context, the correct logical sequence is

A

High SFE → easy cross-slip → low work hardening

B

High SFE → difficult cross-slip → high work hardening

C

Low SFE → easy cross-slip → low work hardening

D

Low SFE → difficult cross-slip → low work hardening

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6

2011 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2011

The angle between the line vector and the burgers vector of an edge dislocation is

A

0 degree

B

90 degrees

C

120 degrees

D

180 degrees

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7

2011 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2011

The largest size of immobilized segment of dislocation in a Frank Read (FR) source contained in a polycrystalline material is of the order of grain size. In a metal of 10 \( \mu \)m grain size, the shear stress required to operate such a FR source is 100 MPa. If the grain size in the same metal is reduced to 10 nm, the shear stress required to operate such FR source would be

A

\(10^2 \) MPa

B

\(10^3 \) MPa

C

\(10^5 \) MPa

D

\(10^6 \) MPa

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8

2011 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2011

The elastic strain energy per unit length of dislocation line in copper is

A

34.8 × 10^-10 N

B

28.8 × 10^-10 N

C

24.8 × 10^-10 N

D

14.5 × 10^-10 N

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9

2013 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2013

Energy (in J/m) of the dislocation is

A

\(0.5 \times 10^{-9}\)

B

\(1.1 \times 10^{-9}\)

C

\(2.2 \times 10^{-9}\)

D

\(4.4 \times 10^{-9}\)

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10

2016 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2016

A plastically deformed metal crystal at low temperature exhibits wavy slip line pattern due to

A

Dislocation pile-up

B

Large number of slip systems

C

Low stacking fault energy

D

Dislocation climb

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11

2017 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2017

Stress required to operate a Frank-Read source of length L is approximately given by:

A

\(\frac{Gb}{L}\)

B

\(\frac{Gb^2}{L}\)

C

\(\frac{Gb^2}{L^2}\)

D

\(\frac{Gb^2}{2L^2}\)

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12

2019 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2019

Cold working of iron leads to increase in dislocation density from 10¹⁰ to 10¹⁵ m^-2. The associated stored energy (in MJ. m^-3, rounded off to one decimal place) is ____________.

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13

2020 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2020

In the edge dislocation configuration given in the figure, dislocations X and Y are fixed and separated by a distance 2h on the same slip plane. Dislocation Z is free to glide on a parallel slip plane. The two slip planes are separated by a distance h. Which one of the following statements is TRUE regarding the stability of dislocation Z at positions 1, 2 and 3? Assume all dislocations have identical Burgers vector.

A

Position 1: unstable equilibrium; Position 2: unstable; Position 3: unstable

B

Position 1: stable equilibrium; Position 2: unstable; Position 3: unstable

C

Position 1: unstable equilibrium; Position 2: stable; Position 3: unstable

D

Position 1: stable equilibrium; Position 2: unstable; Position 3: stable

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14

2020 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2020

Which one of the following dislocation reactions is NOT feasible in a FCC crystal?

A

1/2[0̅11] → 1/6[1̅2̅1] + 1/6[1̅12̅]

B

1/2[1̅01] + 1/2[01̅0] → [10̅0]

C

1/6[1̅12] + 1/3[11̅1̅] → 1/2[110]

D

1/2[011] → 1/6[21̅1̅] + 1/6[112̅]

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15

2021 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2021

In the absence of any external stress, which one of the following statements related to the interaction of point defect and a dislocation is FALSE?

A

An oversized solute atom would preferentially migrate below the slip plane of an edge dislocation.

B

A spherically symmetric point defect can interact with both the hydrostatic and shear stress fields of a dislocation.

C

A point defect can locally modify the elastic modulus and thereby can change the interaction energy.

D

Vacancies are attracted towards the compressive region of dislocation.

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16

2021 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2021

In a material, a shear stress of 100 MPa is required to bow a dislocation line between precipitates with a spacing of 0.2 µm. If the spacing between the precipitates is increased to 0.5 µm, the shear stress (in MPa) to bow the dislocation would be: ______ (round off to nearest integer).

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17

2022 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2022

Given the strain rate (\(\dot{\varepsilon}\)), dislocation density (\(\rho\)), dislocation velocity (\(v\)), which of the following relationship(s) is(are) correct? Assume that Orowan equation for plastic flow due to the dislocation movement is obeyed.

A

\(\dot{\varepsilon} \propto v^2\)

B

\(\dot{\varepsilon} \propto v\)

C

\(\dot{\varepsilon} \propto \rho^2\)

D

\(\dot{\varepsilon} \propto \rho\)

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18

2022 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2022

Find the correct match between dislocation reactions (Column A) to the descriptions (Column B)

Column A
(P) \( \frac{a_o}{2}[\overline{1} \overline{1} 1] + \frac{a_o}{2}[111] = a_o[001] \)
(Q) \( \frac{a_o}{6}[\overline{2} \overline{2} 1] + \frac{a_o}{6}[1 \overline{1} \overline{2}] = \frac{a_o}{6}[011] \)
(R) \( \frac{a_o}{6}[1 \overline{2} 1] + \frac{a_o}{6}[\overline{1} \overline{1} 2] = \frac{a_o}{6}[0 \overline{1} 1] \)

Column B
(1) Leading partials merging to form a Lomer-Cottrell lock in an FCC metal
(2) Energetically unfavorable dislocation reaction in an FCC metal
(3) Typical dislocation reaction in a BCC metal

A

P – 3, Q – 2, R – 1

B

P – 3, Q – 1, R – 2

C

P – 2, Q – 3, R – 1

D

P – 2, Q – 1, R – 3

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19

2023 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2023

A dislocation loop PQRSTU is on the (111) plane of a cubic single crystal with Burgers vector \( \frac{1}{2}[\bar{1}2\bar{1}] \). The dislocation segments \( \overline{\mathbf{P} \mathbf{U}} \) and \( \overline{\mathbf{P} \mathbf{Q}} \) are parallel to \( [0\bar{1}1] \) and \( [1\bar{1}0] \) directions, respectively.

The correct statement(s) is/are

A

Dislocation segment PQ is mixed in character.

B

Dislocation segment UT is screw in character.

C

Dislocation segment PU is mixed in character.

D

Dislocation segment QR is edge in character.

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20

2024 · Metallurgical Engineering · Mechanical Behaviour of Materials · Dislocation Theory

Metallurgical Engineering (MT) 2024

The figure shows a dislocation loop (shown by the solid circle), whose Burgers vector is b (shown by the horizontal arrow inside the dislocation loop). Identify the nature of the dislocation segment at locations p, q and r.
The dash-dot lines show the horizontal and vertical diameters of the loop, and the arrow along the dislocation loop indicates the line vector.

A

p: pure edge, q: mixed, r: pure screw

B

p: pure edge, q: pure screw, r: pure edge

C

p: pure screw, q: mixed, r: pure screw

D

p: pure screw, q: pure edge, r: pure screw

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