# Mechanical Metallurgy Questions and Answers – Stress-Rupture Test – 2

This set of Mechanical Metallurgy Objective Questions & Answers focuses on “Stress-Rupture Test – 2”.

1. The material with low stacking fault energy is prone to sub-grain formation compared to the metal with high stacking fault energy.
a) True
b) False

Explanation: The material with high stacking fault energy form the subgrain structure quickly because low stacking fault energy material tends to recrystallize than forming the cell structure.

2. The grain boundary sliding is a shear process occurring along the ________
a) direction of the grain boundary
b) perpendicular direction of the grain boundary
c) outward plane of the grain boundary
d) inward plane of the grain boundary

Explanation: The grain boundary sliding is a shear process occurring along the direction of the grain boundary. This phenomenon is observed in polycrystalline materials.

3. The grain boundary sliding is promoted by increasing ____________ and decreasing ___________
a) temperature, strain rate
b) strain rate, temperature
c) temperature, stress
d) stress, temperature

Explanation: The grain boundary sliding is promoted by increasing temperature and decreasing strain rate. The sliding occurs due to the bulk movement of the two grains.

4. The total strain in the material is 47 %. Find the approximate percentage of strain which is caused by grain boundary sliding in total deformation?
a) 47%
b) 40%
c) 5%
d) 0.01%

Explanation: The elongation in the material is caused by the atomic movement of the crystal. It takes place by dislocation movement, shear band movement, grain boundary sliding, etc. The grain boundary sliding contribution in the total sum of strain is around 10% to 20 % for most of the material; hence, the contribution is around 5% for this case.

5. The dislocation glide is one of the mechanisms responsible for creep of the metals. Its condition for dislocation mechanism is _______________
a) high stress
b) low stress
c) high temperature
d) low temperature

Explanation: The dislocation glide involves dislocation moving along the slip plane. The mechanism occurs at high stress as per the given formula:
-> σ/G > 10-2.
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6. The dislocation creep mechanism is active in the region ___________, where σ is applied stress and G is shear modulus.
a) σ/G > 10-2
b) 10 > σ/G > 10-2
c) 10-4 < σ/G < 10-2
d) σ/G < 10-4

Explanation: The mechanism involves the movement of dislocation, which overcomes barriers by a thermally assisted mechanism involving diffusion of vacancy or interstitial atoms.
-> 10-4 < σ/G < 10-2.

7. The diffusional creep is the chief mechanism activated in the stress range of ___________
a) σ/G > 10-2
b) 10 > σ/G > 10-2
c) 10-4 < σ/G < 10-2
d) σ/G < 10-4

Explanation: This mechanism involves the flow of vacancies and interstitial atoms through crystal under the influence of applied stress.

8. The chief mechanism responsible for Nabarro-Herring creep is _____________
a) dislocation glide
b) dislocation creep
c) diffusional creep
d) grain boundary sliding

Explanation: This mechanism involves the flow of vacancies and interstitial atoms through crystal under the influence of applied stress. The Nabarro-Herring creep comes under this category only.

9. The chief mechanism responsible for Coble creep is _____________
a) dislocation glide
b) diffusional Creep
c) dislocation creep
d) grain boundary sliding

Explanation: This mechanism involves the flow of vacancies and interstitial atoms through crystal under the influence of applied stress. The Coble creep comes under this category only.

10. The metal undergoing creep operates under only one creep mechanism at a time.
a) True
b) False

Explanation: In the real world, there is no Sharpe boundary between the creep mechanisms, so there are multiple mechanisms involved for the creep of a metal at a given time.

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