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(a) Describe any two of the crystal defects shown below - Leaving Cert Engineering - Question 4 - 2008
Question 4
(a) Describe any two of the crystal defects shown below. (i) (ii) (iii) (b) The table shows the temperatures at which solidification starts and ends as the alloy... show full transcript
Worked Solution & Example Answer:(a) Describe any two of the crystal defects shown below - Leaving Cert Engineering - Question 4 - 2008
Step 1
Describe any two of the crystal defects shown below.
Answer
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Vacancy or vacant site defect: This defect occurs when an atom is missing from its lattice site in the crystal structure. The absence creates a distortion that causes nearby atoms to adjust their positions, which can alter properties such as density and diffusion.
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Interstitial defect: This type occurs when a smaller atom occupies a space between the larger atoms of the lattice. This can lead to an increase in distortion, affecting the overall structure and properties of the material.
Step 2
Using the graph paper supplied, draw the thermal equilibrium diagram.
Answer
Construct a graph with temperature (°C) on the y-axis and % Zinc on the x-axis. Plot the solidification start and end points based on the provided data. Draw the liquids line and solidus line accordingly, indicating the eutectic point where the phase transition occurs.
Step 3
Label the diagram and describe the main features.
Answer
- Liquids Line: This line represents the boundary where the alloy transitions from liquid to pasty.
- Solidus Line: Indicates the temperature below which the alloy is completely solid.
- Eutectic Point: Marks a unique composition where both liquid and solid phases exist simultaneously. This point is critical for understanding the solidification process in alloys.
Step 4
Explain any two of the following:
Answer
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Solvus line: This line indicates the limit of solubility for a solid solution; above it, the alloy will separate into different phases.
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Solid solution: This is a homogenous mixture where one metal is dissolved in another; an example is copper-nickel, where the two metals form a single-phase structure.
Step 5
The stages of metal solidification from the liquid phase;
Answer
Metal solidification typically follows these stages:
- Nucleation: Formation of small solid clusters as the liquid cools.
- Growth: These clusters grow into larger dendrites as the cooling continues.
- Completion: Final solid structures form, characterized by grain boundaries.
Step 6
The difference between BCC and FCC crystal structures.
Answer
In a BCC (Body-Centered Cubic) structure, atoms are arranged at the corners and one atom is positioned in the center of the cube, leading to a lower packing density and more open structure. In contrast, an FCC (Face-Centered Cubic) structure has atoms located at the corners and the centers of each face of the cube, resulting in a higher packing density, which allows for better ductility.