Engineering Questions with Answers - Multiple Choice Questions
Manufacturing Processes – Annealing – 6
1 - Question
The rate at which a metal recrystallizes depends on the rate of ______
a) cooling
b) polygonization
c) nuclei formation
d) grain growth
View Answer
Explanation: The rate at which a metal recrystallizes depends on the rate at which nuclei form, and also on the rate at which they grow. These two rates also determine the final grain size of a recrystallized metal.
2 - Question
The grain size will be smaller when ______
a) nuclei form rapidly and grow slowly
b) nuclei form slowly and grow rapidly
c) recrystallization temperature is high
d) activation energy is low
View Answer
Explanation: If nuclei form rapidly and grow slowly, many crystals will form before their mutual impingement completes the recrystallization process. In this case, the final grain size is small. On the other hand, it will be larger if the rate of nucleation is small compared to the rate of growth.
3 - Question
The number of nuclei that form per second is called as _____
a) recrystallization frequency
b) nucleation frequency
c) nuclei growth frequency
d) grain growth frequency
View Answer
Explanation: It is customary to define the nucleation frequency, N, as the number of nuclei that form per second in a cubic centimetre of unrecrystallized matrix. The grain size is dependent on the rate of formation and growth of nuclei.
4 - Question
The time rate of change of the diameter of a recrystallized grain is known as_____
a) diameter rate
b) linear growth rate
c) growth rate
d) nucleation rate
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Explanation: The linear rate of growth, G, is defined as the time rate of change of the diameter of a recrystallized grain. In practice, G is measured by annealing for different lengths of time a number of identical specimens at a chosen isothermal temperature.
5 - Question
The variation of this diameter with isothermal annealing time gives the _____
a) nucleation rate
b) linear rate of growth
c) grain size change rate
d) diametrical growth rate
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Explanation: The variation of this diameter with isothermal annealing time gives the rate of growth, G. The diameter of the largest grain in each specimen is measured after the specimens are cooled to room temperature and prepared metallographically.
6 - Question
The rate of nucleation can be defined as______
a) number of grains per unit area
b) number of grains per unit volume
c) number of nuclei formed per unit area per unit time
d) number of grains formed per nuclei
View Answer
Explanation: The rate of nucleation can be determined from the same metallographic specimens by counting the number of grains per unit area on the surface of each.
7 - Question
For ______ nucleation will take place preferentially at highly energetic sites.
a) pure metals
b) alloys
c) polycrystalline
d) brittle
View Answer
Explanation: For polycrystalline materials, nucleation will take place preferentially at highly energetic sites, such as grain-boundary triple points, original grain boundaries, and boundaries between deformation bands.
8 - Question
The nuclei may form by ______
a) grain growth
b) sub-grain growth
c) nuclei boundary growth
d) combining multiple grains
View Answer
Explanation: The nuclei may form by subgrain growth and/or grain boundary migration. As a result of polygonization, it might be possible to produce a subgrain capable of growing out into the surrounding polygonized matrix.
9 - Question
In recrystallization, an entirely new set of grains is formed.
a) True
b) False
View Answer
Explanation: In recrystallization, an entirely new set of grains is formed. New crystals appear at points of high-lattice-strain energy, such as slip-line intersections, deformation twin intersections, and in areas close to grain boundaries. In each case, it appears at points of strong lattice curvature.
10 - Question
A region of a crystal can become a nucleus and grow only if its size exceeds some minimum value.
a) True
b) False
View Answer
Explanation: A number of models have been proposed, these models are in general agreement on 2 points. First, a region of a crystal can become a nucleus and grow only if its size exceeds some minimum value. The other condition for the formation of a nucleus is that it becomes surrounded by the equivalent of a high-angle grain boundary. This condition is required because the mobility of an arbitrary low-angle grain boundary is normally very low.