BME Materials Science 4 — Questions and Answers
Question 1: In the iron-carbon phase diagram, cementite (Fe3C) contains what weight percentage of carbon?
- 2.14 wt%
- 4.30 wt%
- 6.70 wt% (Correct answer)
- 0.77 wt%
Correct answer: 6.70 wt%
Cementite (iron carbide, Fe3C) has a fixed stoichiometric composition of 6.70 wt% carbon.
Question 2: Which type of bonding is responsible for the unique properties of graphite, including its lubricity and electrical conductivity?
- Ionic bonding between carbon layers
- Covalent bonding within layers and van der Waals forces between layers (Correct answer)
- Metallic bonding throughout
- Hydrogen bonding between carbon sheets
Correct answer: Covalent bonding within layers and van der Waals forces between layers
Graphite has sp2 covalent bonding within hexagonal layers and delocalized π electrons that provide conductivity, with weak van der Waals forces between layers enabling easy shear.
Question 3: A material tested in tension shows a distinct upper and lower yield point followed by Lüders band propagation. This behavior is characteristic of:
- Face-centered cubic metals
- Low-carbon steels with interstitial solute pinning (Correct answer)
- Amorphous polymers above Tg
- Ceramic materials at room temperature
Correct answer: Low-carbon steels with interstitial solute pinning
In low-carbon steel, interstitial carbon and nitrogen atoms pin dislocations; the upper yield point corresponds to unpinning, after which Lüders bands propagate.
Question 4: The glass transition temperature (Tg) of an amorphous polymer marks the transition from:
- Crystalline to liquid state
- Glassy (rigid) to rubbery (viscoelastic) behavior (Correct answer)
- Brittle to ductile fracture mode only in composites
- Ionic to covalent bonding character
Correct answer: Glassy (rigid) to rubbery (viscoelastic) behavior
Below Tg, polymer chain segments are frozen; above Tg, segmental motion becomes possible, giving rubbery, viscoelastic behavior.
Question 5: Which non-destructive testing method uses high-frequency sound waves to detect internal flaws in materials?
- Magnetic particle inspection
- Liquid penetrant testing
- Ultrasonic testing (Correct answer)
- Eddy current testing
Correct answer: Ultrasonic testing
Ultrasonic testing sends high-frequency acoustic waves into a material and detects reflections from internal discontinuities like cracks and voids.
Question 6: In precipitation hardening (age hardening), the maximum strength is achieved at the stage called:
- Solution treatment
- Underaging
- Peak aging (Correct answer)
- Overaging
Correct answer: Peak aging
Peak aging produces the optimal precipitate size and distribution that maximally impedes dislocation motion, giving maximum hardness and strength.
Question 7: The Peierls-Nabarro stress is the theoretical stress required to:
- Initiate crack propagation in a brittle material
- Move a dislocation through a perfect crystal lattice (Correct answer)
- Cause grain boundary sliding during creep
- Nucleate a new phase from a supersaturated solid solution
Correct answer: Move a dislocation through a perfect crystal lattice
The Peierls-Nabarro stress represents the lattice friction stress that must be overcome to move a dislocation one Burgers vector in a perfect crystal.
BME Materials Science 4
In the iron-carbon phase diagram, cementite (Fe3C) contains what weight percentage of carbon?