This set of Engineering Materials and Metallurgy Questions and Answers for Campus interviews focuses on “Nucleation, Growth and Segregation, Recovery, Recrystallisation and Grain Growth”.
1. The beginning of a new phase transformation is known as ________
Explanation: Nucleation is the first step that occurs in the formation of a new phase. In this process, molten metal appears in regions which then become crystals. It involves assembly of atoms by diffusion, changes in structure, and formation of nuclei.
2. In nucleation, particles having radius less than rc are known as _______
Explanation: In the homogeneous or self-nucleation process, there is the formation of particles in the melt. When their radius is less than rc(embryo), they tend to redissolve which lowers the energy. Particles having a radius more than rc tend to grow and are called nuclei.
3. How is the critical radius of particles calculated?
Explanation: In nucleation or self-nucleation process, there is the formation of particles in the melt. The critical particle radius is defined as Here, rc is the critical radius and ∆F is the free energy.
4. In the homogeneous nucleation, nucleation rate is maximum __________
a) At freezing point
b) Above boiling point
c) Below melting point
d) At room temperature
Explanation: From the free energy change diagram for homogeneous nucleation, it is seen that the rate of nucleation is affected by a change in temperature. It is maximum at a point below the melting point temperature.
5. Heterogeneous nucleation differs from others due to ________
a) Faster rate of nucleation
Explanation: In heterogeneous nucleation, the liquid metal must be wetted with the foreign particles. Wetting is defined as the spontaneous spreading of one phase over the surface of another. Very little or no supercooling occurs in this type of nucleation.
6. In supercooled metals, the grain growth occurs in ____________ manner.
Explanation: Growth is defined as an increase in the size of the nucleus, which occurs after nucleation. Grain growth may occur dendritically if the liquid metal is supercooled.
7. Where does the solidification in a phase diagram start?
a) Liquidus line
b) Solidus line
c) At equilibrium
d) At freezing point
Explanation: In a phase diagram, two phases are known as liquidus and solidus exist. Liquidus line is that line above which the alloy is in a liquid state and where solidification starts. At solidus line, the alloy is in solid state and solidification is complete.
8. Which of the following does not apply to non-equilibrium solidification?
Explanation: Non-equilibrium solidification occurs under non-equilibrium conditions and involves a fast cooling rate. This results in the formation of porous, dendritic, and cored (segregated) material with inhomogeneous composition.
9. Micro-segregation pattern can be identified using ________ technique.
a) Positron emission radiography
b) Single photon emission computed tomography
c) Magnetic resonance imaging
Explanation: In microsegregation, compositional differences occur on a microscopic level. These samples can be studied by metallographic and autoradiographic techniques.
10. Which of the following does not affects the pattern of segregation?
a) Freezing rate
b) Motion of crystals
c) Rate of solidification
d) Model of development of grain structure
Explanation: Segregation is a common process in cast alloys which alters the microstructure of castings. Freezing rate, the motion of crystals, a model of development of grain structure, and residual liquid under various forces are the factors affecting the degree and pattern of segregation.
11. What is the glass transition temperature of Nylon 6,6?
Explanation: Glass transition temperature of a material is defined as the temperature at which glass transition occurs. It is denoted by Tg. The value of Tg for Nylon 6,6 is 50oC, and is 265oC when crystallized.
12. Plastic working in the temperature range of 0.3-0.5 Tm is called _________
a) Hot work
b) Cold work
d) Sheet metal
Explanation: The density of point imperfections and dislocations increases with increasing plastic deformation in the temperature range below 0.3-0.5 Tm. Plastic working below this range is called as cold work. Here, Tm is the melting point in Kelvin.
13. There is _________ change in crystal structure during recrystallization.
Explanation: Recrystallization is defined as the process of nucleation and growth of new strain-free crystals. It starts with heating to temperatures in the range of 0.3-0.5 Tm. In this stage, there is no change in crystal structure.
14. On which factor does the recrystallization temperature depend?
c) Melting point
d) Grain size
Explanation: The recrystallization temperature depends on the purity of the material. Very pure materials crystallize around 0.3 Tm, whereas impure materials crystallize around 0.5-0.6 Tm.
15. During grain growth, the number of grains _________ in number.
a) Increase exponentially
b) Increase constantly
d) Do not change
Explanation: During cold work, the grains become elongated in the direction of working. During crystallization, new grains are formed. During grain growth, the new crystals increase in size but decrease in number.
Sanfoundry Global Education & Learning Series – Engineering Materials & Metallurgy.
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