BME Materials Science 5 — Questions and Answers
Question 1: Which corrosion type occurs when two dissimilar metals are in electrical contact in an electrolyte, causing the more active metal to corrode preferentially?
- Crevice corrosion
- Galvanic corrosion (Correct answer)
- Intergranular corrosion
- Pitting corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs because the less noble (more active) metal becomes the anode and dissolves preferentially when coupled with a more noble metal in an electrolyte.
Question 2: Superelastic behavior in shape memory alloys (e.g., Nitinol) is driven by:
- Dislocation glide and permanent plastic strain
- Stress-induced martensitic phase transformation that is fully reversible (Correct answer)
- Grain boundary sliding at elevated temperature
- Twinning deformation with no recovery
Correct answer: Stress-induced martensitic phase transformation that is fully reversible
In superelastic alloys, applied stress induces a martensitic transformation; upon unloading, the martensite reverts to austenite, recovering all strain.
Question 3: In Fick's second law of diffusion (∂C/∂t = D ∂²C/∂x²), what assumption is made about the diffusion coefficient D?
- D varies linearly with concentration
- D is independent of concentration (Correct answer)
- D decreases with increasing temperature
- D depends on the flux direction
Correct answer: D is independent of concentration
Fick's second law in its standard form assumes D is constant (independent of concentration), simplifying the partial differential equation.
Question 4: Which microstructural feature of cast iron gives gray iron its characteristic fracture appearance and machinability?
- Spheroidal graphite nodules
- Flake graphite in a pearlitic matrix (Correct answer)
- Temper carbon in a ferritic matrix
- Massive carbide network at grain boundaries
Correct answer: Flake graphite in a pearlitic matrix
Gray iron contains graphite flakes that act as stress concentrators, causing fracture along the flakes and producing the characteristic gray fracture surface.
Question 5: The Weibull modulus (m) is used in ceramics to characterize:
- Average creep strain rate
- The variability (scatter) in fracture strength (Correct answer)
- Thermal expansion coefficient
- Crack propagation velocity
Correct answer: The variability (scatter) in fracture strength
A high Weibull modulus indicates low scatter in fracture strength (reliable material), while a low modulus indicates high variability due to flaw distribution.
Question 6: Which heat treatment process for steel involves austenitizing followed by quenching to a temperature above Ms and holding isothermally to form bainite?
- Full annealing
- Austempering (Correct answer)
- Martempering
- Normalizing
Correct answer: Austempering
Austempering holds steel in the bainite transformation range isothermally, producing bainite with good toughness without the distortion risk of full quenching.
Question 7: In a stress-strain curve for a ductile metal, the engineering stress decreases after the ultimate tensile strength (UTS) because:
- The material work hardens and becomes stiffer
- Necking occurs and the cross-sectional area decreases faster than the load (Correct answer)
- The material undergoes a phase transformation to a stronger phase
- The test machine reduces applied load automatically
Correct answer: Necking occurs and the cross-sectional area decreases faster than the load
After UTS, localized necking causes the cross-sectional area to decrease rapidly; although true stress continues rising, engineering stress (load/original area) drops.
BME Materials Science 5
Which corrosion type occurs when two dissimilar metals are in electrical contact in an electrolyte, causing the more active metal to corrode preferentially?