This set of Tricky Materials Science Questions and Answers focuses on “Recovery, Recrystallization and Grain Growth”.
1. In which of the following processes does recovery, recrystallization, grain growth take place?
a) Surface hardening
Explanation: In the process of annealing, the crystal goes through three stages which are called recovery, recrystallization and grain growth in order.
2. When a material is plastically deformed at low temperature(around 25 degrees) relative to the melting temperature, which of the following change isn’t observed?
a) Change in grain shape
b) Strain hardening
c) Decrease in dislocation density
d) Increase in dislocation density
Explanation: When a body is plastically deformed at low temperatures compared to its melting point, we observe micro structural changes like change in grain growth, strain hardening and increase in dislocation density.
3. At which of the following temperature can recovery occur? (where Tm is melting point)
Explanation: Recovery occurs at a lower temperature than recrystallization or grain growth. Recovery takes place at the temperature below 0.3Tm.
4. Strain-relief crystallization takes place when deformed metals are heated.
Explanation: Recovery is subdivided into two parts, strain-relief crystallization and polygonization. Strain-relief crystallization occurs when deformed metals are heated and the latter takes place under certain conditions.
5. Which of the following statements are false in strain-relief crystallization?
a) Dislocation climb
b) Decrease in the electric resistance
c) Increase in density
d) Interaction of like signed dislocation
Explanation: When strain-relief crystallization takes place dislocation climb, an increase in density, and a decrease in electrical conductivity. During strain-relief crystallization interaction of unlike signed dislocation is observed.
6. By what percentage can the hardness decrease after strain-relief crystallization?
Explanation: After strain-relief crystallization, the hardness and the strength of the material decrease by 10-15% from their original value. On the other hand, ductility increases by that percentage.
7. In which of the following metal polygonization rarely takes place?
Explanation: Polygonization is the process in which new grain boundaries are formed and isn’t common for all metals and observed after slight deformation. In copper and its alloy, this process rarely develops.
8. Which of the following processes can be used to refine the grain structure?
c) Grain growth
d) Such a process does not exist.
Explanation: In recrystallization new grains are formed with small nuclei and they grow until they cover the whole parent material. Therefore, the new grain structure is fine and hence the grain structure is refined.
9. Lowest temperature for recrystallization for iron is 270°C
Explanation: For commercially pure iron, the lowest temperature at which recrystallization can occur is equal to 450°C. For aluminium and copper, it is equal to 100°C and 270°C respectively.
10. Which of the following is false for grain growth?
a) The grains start growing
b) Grain growth accelerates with increase in temperature
c) Decrease in free energy
d) Increase of surface energy
Explanation: The surface energy decreases during grain growth because of which free energy decreases. The grain starts growing and grain growth accelerates with increase in temperature.
Sanfoundry Global Education & Learning Series – Materials Science.
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