ME Metals and Alloys 1 — Questions and Answers
Question 1: Which crystal structure do most face-centered cubic (FCC) metals exhibit at room temperature?
- BCC
- HCP
- FCC (Correct answer)
- Simple cubic
Correct answer: FCC
Metals such as aluminum, copper, gold, silver, and nickel have an FCC crystal structure with atoms at cube corners and face centers.
Question 2: The iron-carbon phase diagram shows that the maximum solubility of carbon in austenite (FCC iron) is approximately:
- 0.02 wt%
- 0.77 wt%
- 2.14 wt% (Correct answer)
- 6.67 wt%
Correct answer: 2.14 wt%
Austenite can dissolve up to 2.14 wt% carbon at 1147 degrees C; beyond this, cementite (Fe3C) forms, and compositions above 2.14 wt% C are classified as cast irons.
Question 3: The eutectoid composition in the iron-carbon system is approximately:
- 0.02 wt% C
- 0.77 wt% C (Correct answer)
- 2.14 wt% C
- 4.30 wt% C
Correct answer: 0.77 wt% C
At 0.77 wt% C and 727 degrees C, austenite transforms to pearlite (alternating layers of ferrite and cementite) in the eutectoid reaction.
Question 4: Age hardening (precipitation hardening) in aluminum alloys involves which sequence of steps?
- Annealing, quenching, cold work
- Solution treatment, quenching, aging (Correct answer)
- Normalizing, tempering, aging
- Cold work, annealing, quenching
Correct answer: Solution treatment, quenching, aging
Precipitation hardening requires solution treatment to dissolve alloying elements, rapid quenching to create a supersaturated solid solution, then aging to precipitate fine strengthening particles.
Question 5: Martensite in steel is formed by:
- Slow cooling through the eutectoid temperature
- Rapid quenching that suppresses diffusional transformation (Correct answer)
- Extended annealing at low temperatures
- Hot rolling followed by air cooling
Correct answer: Rapid quenching that suppresses diffusional transformation
Martensite forms by a diffusionless, shear-type transformation when austenite is rapidly quenched, trapping carbon in a body-centered tetragonal structure.
Question 6: Which alloying element is primarily added to steel to improve hardenability?
- Silicon
- Chromium (Correct answer)
- Sulfur
- Lead
Correct answer: Chromium
Chromium significantly increases hardenability by shifting the TTT (time-temperature-transformation) curves to longer times, allowing martensite to form at slower cooling rates.
ME Metals and Alloys 1
Which crystal structure do most face-centered cubic (FCC) metals exhibit at room temperature?