This set of Tricky Manufacturing Processes Questions and Answers focuses on “Annealing – 4”.
1. The yield point can be completely recovered in a zinc crystal at _____
a) room temperature
Explanation: Recovery of the yield point thus begins very rapidly. The yield point can be completely recovered in a zinc crystal at room temperature in a period of a day. The stress-strain diagrams indicate a well-known fact: the rate at which a property recovers isothermally is a decreasing function of the time.
2. Polygonization is associated with_____
a) grain size
b) plastically bent crystals
d) strength of the material
Explanation: Another recovery process is called polygonization. In its simplest form it is associated with crystals that have been plastically bent. Many workers have shown that the Laue spots of bent crystals assume a fine structure after a recovery anneal (anneal that does not recrystallize the specimen).
3. Polygonization is a process of _____
a) annealing a bent crystal
b) annealing a pure metal
c) heating the metal above recrystallization temperature
Explanation: When a bent crystal is annealed, the curved crystal breaks up into a number of closely related small perfect crystal segments. This process has been given the name polygonization.
4. The regrouping of edge dislocations into low-angle boundaries helps in _____
a) lowering the strain energy
b) increasing the stored energy
c) lowering the recrystallization temperature
d) increasing the strength of the metal
Explanation: In addition to lowering the strain energy, the regrouping of edge dislocations into low-angle boundaries has a second important effect. This is the removal of general lattice curvature. As a result of polygonization, crystal segments lying between a pair of low-angle boundaries approach the state of strain-free crystals with flat uncurved planes.
5. An edge dislocation is capable of moving by a climb in a direction _____ to the slip plane.
Explanation: An edge dislocation is capable of moving either by slip on its slip plane or by a climb in a direction perpendicular to its slip plane. The driving force for these movements comes from the strain energy of the dislocations, which decreases as a result of polygonization.
6. The rate of polygonization increases rapidly with _____
d) grain size
Explanation: The strain field of dislocations grouped on slip planes produces an effect force that makes them move into sub-boundaries. This force exists at all temperatures, but at low temperatures edge dislocations cannot climb. Since dislocation climb depends on the movement of vacancies (an activated process), the rate of polygonization increases rapidly with temperature.
7. As the polygon walls become more widely separated, the rate of ______becomes a decreasing function of time and temperature.
d) grain growth
Explanation: As the polygon walls become more widely separated, the rate of coalescence becomes a decreasing function of time and temperature so that the polygonization process approaches a more or less stable state with widely spaced, approximately parallel (in a single crystal deformed by simple bending) sub-boundaries.
8. The rate at which a recovery process occurs always decreases with _____
c) yield strength
Explanation: In an isothermal anneal, the rate at which a recovery process occurs always decreases with time; that is, it starts rapidly and proceeds at a slower and slower rate as the driving force for the reaction is expended.
9. In deformed polycrystalline metals, high-temperature recovery is considered to be essentially a matter of polygonization.
Explanation: In deformed polycrystalline metals, high-temperature recovery is considered to be essentially a matter of polygonization and annihilation of dislocations. At lower temperatures other processes such as occur in dynamic recovery are of greater importance, current theories picture the recovery process as primarily a matter of reducing the number of point defects to their equilibrium value.
10. Recrystallization during an isothermal anneal begins very slowly.
Explanation: Recrystallization during an isothermal anneal begins very slowly, and builds up to a maximum reaction rate, after which it finishes slowly. Recovery processes start at the beginning of the annealing cycle and account for the initial energy release, while recrystallization starts later and accounts for the second (larger) energy release.
Sanfoundry Global Education & Learning Series – Manufacturing Processes.
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