NCEES Materials Science 2 ā Questions and Answers
Question 1: Which crystal defect is formed when an atom is missing from its lattice site?
- Interstitial defect
- Vacancy defect (Correct answer)
- Frenkel defect
- Schottky defect
Correct answer: Vacancy defect
A vacancy defect (Schottky defect in ionic crystals) occurs when an atom is missing from its normal lattice position.
Question 2: What is the primary strengthening mechanism in cold-worked metals?
- Grain boundary strengthening
- Solid solution strengthening
- Dislocation density increase (Correct answer)
- Precipitation hardening
Correct answer: Dislocation density increase
Cold working increases dislocation density, causing dislocations to impede each other's motion and thereby strengthening the metal.
Question 3: The elastic modulus of a material is best described as:
- The stress at which permanent deformation begins
- The ratio of stress to strain in the elastic region (Correct answer)
- The energy absorbed before fracture
- The maximum stress a material can withstand
Correct answer: The ratio of stress to strain in the elastic region
Young's modulus (elastic modulus) is defined as the ratio of uniaxial stress to strain within the elastic (reversible) deformation range.
Question 4: Which type of polymer chain arrangement leads to the highest crystallinity?
- Atactic
- Syndiotactic
- Isotactic (Correct answer)
- Branched
Correct answer: Isotactic
Isotactic polymers, where all substituents are on the same side of the backbone, pack most efficiently and achieve the highest degree of crystallinity.
Question 5: In a eutectic binary alloy system, the eutectic reaction is best described as:
- A single liquid transforming into two solids simultaneously (Correct answer)
- A solid transforming into a liquid and another solid
- Two liquids combining to form one solid
- A solid decomposing into two different solid phases
Correct answer: A single liquid transforming into two solids simultaneously
At the eutectic point, a single liquid phase transforms simultaneously into two distinct solid phases at a fixed temperature.
Question 6: Which bonding type is primarily responsible for the high melting point of diamond?
- Metallic bonding
- Van der Waals forces
- Covalent bonding (Correct answer)
- Ionic bonding
Correct answer: Covalent bonding
Diamond's extremely high melting point results from its three-dimensional network of strong, directional covalent CāC bonds.
Question 7: A material that exhibits a linear stress-strain curve up to fracture with no plastic region is classified as:
- Ductile
- Viscoelastic
- Brittle (Correct answer)
- Elastomeric
Correct answer: Brittle
Brittle materials fracture within the elastic region without significant plastic deformation, showing no yield point on the stress-strain curve.
NCEES Materials Science 2
Which crystal defect is formed when an atom is missing from its lattice site?