Manufacturing Processes Multiple Choice Questions on “Annealing – 9”.
1. Grain cells having a number of sides _____ have grain walls which are concave inwards.
a) >3
b) <6
c) <8
d) >5
Answer: b
Clarification: The cells with less than 6 sides have walls that are primarily concave toward their centres. Those cells with more than 6 sides have walls convex toward their centres, this effect is more pronounced the larger the number of sides above 6.
2. Grain cells with _____ sides are unstable.
a) <3
b) >8
c) <6
d) >10
Answer: c
Clarification: All grain cells with less than 6 sides are basically unstable and tend to shrink in size and these grains will therefore shrink and eventually disappear during annealing; while grain cells with more than 6 sides tend to grow in size.
3. If the grain has 6 boundaries, the structure will be _____
a) stable
b) metastable
c) quasi-stable
d) non-planner
Answer: b
Clarification: If the grain has 6 boundaries, they will be planar and the structure metastable. However, if the total number of boundaries around grain is less than 6, each boundary must be concave inwards.
4. Grain boundary migration occurs to _____
a) increase the strain energy
b) increase the surface hardness
c) reduce grain boundary energy
d) reduce recrystallization temperature
Answer: c
Clarification: Large grains will have more than 6 boundaries and will grow. The overall result of boundary migration is to reduce the number of grains, thereby increasing the mean grain size and reducing the total grain boundary energy. This phenomenon is known as grain growth or grain coarsening.
5. The phenomenon of formation of grain boundary of much lower surface energy than that of the average boundary is known as _____
a) grain boundary coalescence
b) atomic coalescence
c) geometrical coalescence
d) grain boundary migration
Answer: c
Clarification: Geometrical coalescence can be simply described as an encounter of 2 grains whose relative orientations are such that the boundary formed between the 2 grains is one of much lower surface energy than that of the average boundary. In a polycrystalline metal, such a boundary would be equivalent to a sub-grain boundary.
6. The boundary atoms in the crystal on the _____ of the boundary are more tightly bound than the boundary atoms in the crystal in the _____
a) concave side, convex side
b) convex side, concave side
c) surface, inner side
d) surface, concave side
Answer: a
Clarification: The boundary atoms in the crystal on the concave side of the boundary are more tightly bound than the boundary atoms in the crystal in the convex side because they are more nearly surrounded by neighbouring atoms of the same crystal.
7. The rate at which atoms jumps from ______ is greater than that in the opposite direction.
a) convex side to concave side of the grain
b) surface high strain energy to surface of low strain energy
c) grain at higher temperature to grain at lower temperature
d) concave side to convex side of the grain
Answer: a
Clarification: Tighter binding of the atoms on the concave side of the boundary should make the rate at which atoms jump across the boundary from the convex to the concave crystal greater than that in the opposite direction. The greater the curvature of the boundary, then the greater should be this effect, and faster the movement of the crystal boundary.
8. If _________ then there will be an elastic stress field introduced into the lattice.
a) number of foreign atoms is more than that of parent crystal atoms
b) number of foreign atoms is less than that of parent crystal atoms
c) size of foreign atoms is different than that of the parent crystal atoms
d) temperature of foreign atoms is different than that of parent crystal atoms
Answer: c
Clarification: If the size of a foreign atom and that of the parent crystal are different, then there will be an elastic stress field introduced into the lattice by each foreign atom.
9. The strain energy of the grain boundary can be reduced by migration of the foreign atoms into the pure crystal grain.
a) True
b) False
Answer: a
Clarification: Since grain boundaries are regions of lattice misfit, the strain energy of the boundary, as well as that of the lattice surrounding a foreign atom, can be reduced by the migration of the foreign atom to the neighbourhood of the grain boundary.
10. Higher the temperature faster will be the grain growth.
a) True
b) False
Answer: a
Clarification: As the metal approaches 100% purity, the grain-growth exponent increases toward the theoretical value ½. Also, note that at the higher temperature (around 900 K) the rate of approach is faster.