NCEES Materials Science 5 — Questions and Answers
Question 1: Creep in metals is most significant at temperatures above approximately what fraction of the absolute melting temperature (Tm)?
- 0.1 Tm
- 0.3 Tm
- 0.5 Tm (Correct answer)
- 0.7 Tm
Correct answer: 0.5 Tm
Significant creep deformation typically occurs above 0.4–0.5 Tm (absolute), where thermally activated diffusion and dislocation climb mechanisms become active.
Question 2: Which heat treatment process is used to increase the surface hardness of steel by diffusing carbon into the surface layer?
- Nitriding
- Carburizing (Correct answer)
- Induction hardening
- Shot peening
Correct answer: Carburizing
Carburizing exposes steel to a carbon-rich atmosphere at elevated temperature, allowing carbon to diffuse into the surface to create a hard, wear-resistant case.
Question 3: In the iron-carbon phase diagram, cementite (Fe₃C) contains approximately what weight percent carbon?
- 2.14 wt%
- 4.30 wt%
- 6.67 wt% (Correct answer)
- 0.76 wt%
Correct answer: 6.67 wt%
Cementite (iron carbide, Fe₃C) has a fixed stoichiometric composition of 6.67 wt% carbon, corresponding to 25 at% carbon.
Question 4: Which semiconductor doping type introduces extra electrons into the conduction band?
- p-type doping
- n-type doping (Correct answer)
- Intrinsic doping
- Compensated doping
Correct answer: n-type doping
n-type doping introduces donor atoms (e.g., phosphorus in silicon) with extra valence electrons that become conduction electrons, increasing electron carrier concentration.
Question 5: The phenomenon where a material becomes magnetic below a specific temperature (Curie temperature) is called:
- Paramagnetism
- Diamagnetism
- Ferromagnetism (Correct answer)
- Antiferromagnetism
Correct answer: Ferromagnetism
Below the Curie temperature, magnetic domains in ferromagnetic materials align spontaneously due to quantum exchange interactions, producing strong net magnetization.
Question 6: Which non-destructive testing (NDT) method uses high-frequency sound waves to detect internal flaws in materials?
- Magnetic particle inspection
- Dye penetrant testing
- Ultrasonic testing (Correct answer)
- Eddy current testing
Correct answer: Ultrasonic testing
Ultrasonic testing transmits high-frequency sound waves into a material and analyzes reflections to locate internal discontinuities, cracks, and voids.
Question 7: Which phenomenon explains why ceramics are generally much weaker in tension than in compression?
- They have low Young's modulus
- Surface and internal microcracks propagate easily under tensile stress but close under compression (Correct answer)
- Ceramics lack slip systems for plastic deformation only in tension
- Grain boundaries dissolve under tensile loading
Correct answer: Surface and internal microcracks propagate easily under tensile stress but close under compression
Pre-existing microcracks in ceramics act as stress concentrators under tension, promoting brittle fracture, while compressive loading closes these cracks rather than opening them.
NCEES Materials Science 5
Creep in metals is most significant at temperatures above approximately what fraction of the absolute melting temperature (Tm)?