BME Materials Science 2 — Questions and Answers
Question 1: Which crystal defect involves an extra half-plane of atoms inserted into the crystal lattice?
- Vacancy
- Edge dislocation (Correct answer)
- Schottky defect
- Interstitial defect
Correct answer: Edge dislocation
An edge dislocation is characterized by an extra half-plane of atoms, creating a line defect that enables plastic deformation by slip.
Question 2: The Hall-Petch relationship describes how yield strength varies with grain size. As grain size decreases, yield strength:
- Decreases linearly
- Increases (Correct answer)
- Remains constant
- Decreases exponentially
Correct answer: Increases
The Hall-Petch equation shows yield strength increases as grain size decreases because grain boundaries impede dislocation motion.
Question 3: Which phase transformation occurs when austenite is rapidly quenched to form a body-centered tetragonal structure in steel?
- Pearlite formation
- Bainite formation
- Martensite formation (Correct answer)
- Ferrite formation
Correct answer: Martensite formation
Rapid quenching of austenite traps carbon in the BCC iron lattice, distorting it into a BCT martensitic structure.
Question 4: A material has a fracture toughness KIc = 50 MPa√m and a yield strength of 500 MPa. What is the critical crack length for plane strain fracture?
- ~3.2 mm (Correct answer)
- ~10 mm
- ~6.4 mm
- ~1.0 mm
Correct answer: ~3.2 mm
Using a = (1/π)(KIc/σ)² = (1/π)(50/500)² ≈ 0.0032 m = 3.2 mm for a through crack with Y=1.
Question 5: Which polymer chain configuration results in the highest degree of crystallinity?
- Atactic
- Isotactic (Correct answer)
- Random copolymer
- Branched chains
Correct answer: Isotactic
Isotactic polymers have all substituents on the same side of the backbone, allowing chains to pack regularly into crystalline regions.
Question 6: In a binary eutectic system, the eutectic point represents:
- Maximum melting temperature
- The composition and temperature where two solid phases and liquid coexist (Correct answer)
- A solid-to-solid phase transformation
- The solubility limit at room temperature
Correct answer: The composition and temperature where two solid phases and liquid coexist
At the eutectic point, the liquid phase transforms simultaneously into two distinct solid phases at a fixed composition and temperature.
Question 7: Which strengthening mechanism involves introducing solute atoms that distort the crystal lattice and impede dislocation movement?
- Precipitation hardening
- Work hardening
- Solid solution strengthening (Correct answer)
- Grain boundary strengthening
Correct answer: Solid solution strengthening
Solid solution strengthening occurs when solute atoms create local stress fields that interact with and pin dislocations.
BME Materials Science 2
Which crystal defect involves an extra half-plane of atoms inserted into the crystal lattice?