BE/B Eng Bachelor of Engineering Materials Engineering 2 β Questions and Answers
Question 1: What is the mechanism of corrosion in metals involving an electrochemical reaction?
- An oxidation reaction at the anode and a reduction reaction at the cathode in the presence of an electrolyte (Correct answer)
- A direct chemical reaction between the metal and oxygen only
- A physical erosion of the metal surface by fluid flow
- A thermal decomposition of the metal's crystal lattice
Correct answer: An oxidation reaction at the anode and a reduction reaction at the cathode in the presence of an electrolyte
Electrochemical corrosion occurs when a galvanic cell forms: the anodic metal oxidizes (loses electrons) while cathodic reduction occurs at another site, with ions transported through the electrolyte.
Question 2: What is the purpose of age hardening (precipitation hardening) in aluminum alloys?
- To increase strength by forming fine precipitates that impede dislocation movement (Correct answer)
- To reduce brittleness at low temperatures
- To improve electrical conductivity
- To eliminate residual stresses after machining
Correct answer: To increase strength by forming fine precipitates that impede dislocation movement
Precipitation hardening involves solution heat treatment, quenching, and controlled aging to form fine dispersed precipitates within the matrix, which block dislocation motion and increase yield strength.
Question 3: What is a composite material?
- A material made from two or more constituent materials with different properties that produce a material with superior characteristics (Correct answer)
- A material produced by alloying two metals
- A polymer with a high degree of crosslinking
- A ceramic material fired at extremely high temperature
Correct answer: A material made from two or more constituent materials with different properties that produce a material with superior characteristics
Composites combine a reinforcement (e.g., carbon fiber, glass fiber) within a matrix (e.g., polymer resin, metal) to achieve properties neither component possesses alone, such as high strength-to-weight ratio.
Question 4: In ceramics, what causes brittleness compared to metals?
- The dominance of ionic and covalent bonds that prevent dislocation movement (Correct answer)
- The high density of grain boundaries
- The large grain size in crystalline ceramics
- The presence of glassy phases at grain boundaries
Correct answer: The dominance of ionic and covalent bonds that prevent dislocation movement
Ceramic brittleness arises because ionic and covalent bonds are directional and strong, preventing the dislocation slip that allows metals to deform plastically before fracturing.
Question 5: What is Fick's First Law of diffusion in materials science?
- The diffusion flux is proportional to the concentration gradient (J = -D dC/dx) (Correct answer)
- The diffusion rate doubles with every 10Β°C temperature increase
- Atoms diffuse only along grain boundaries
- Diffusivity is independent of temperature
Correct answer: The diffusion flux is proportional to the concentration gradient (J = -D dC/dx)
Fick's First Law states J = -D(dC/dx), where J is diffusion flux, D is diffusivity, and dC/dx is the concentration gradient; flux is proportional to and opposite in direction to the gradient.
Question 6: What is the Hall-Petch relationship in materials science?
- The yield strength of a metal increases as grain size decreases (Correct answer)
- Hardness decreases with increasing grain boundary area
- Ductility improves with coarser grain size
- Fatigue life is inversely proportional to grain size
Correct answer: The yield strength of a metal increases as grain size decreases
The Hall-Petch equation (Οy = Οβ + k/βd) shows yield strength increases as grain size (d) decreases, because more grain boundaries impede dislocation motion.
BE/B Eng Bachelor of Engineering Materials Engineering 2
What is the mechanism of corrosion in metals involving an electrochemical reaction?