ME Thermodynamics and Kinetics of Materials 3

Question 1

Spinodal decomposition differs from nucleation-and-growth decomposition in that spinodal decomposition:

Accepted Answer

Proceeds spontaneously without a nucleation barrier via uphill diffusion

Answer

Requires a nucleation energy barrier to initiate

Answer

Occurs only above the equilibrium solvus line

Answer

Produces a sharp compositional interface between product phases

Question 2

During parabolic oxidation of a metal, the oxide thickness x follows the relationship x² = kp·t. This parabolic behavior arises because:

Accepted Answer

Ion transport through the growing oxide layer becomes rate-limiting

Answer

The reaction rate is controlled by interfacial chemical reaction kinetics

Answer

Grain boundary diffusion dominates at all temperatures

Answer

The activation energy for oxidation decreases with time

Question 3

Precipitation hardening (age hardening) of Al-Cu alloys achieves maximum strength at peak aging because:

Accepted Answer

Fine, coherent GP zones and θ'' precipitates create maximum lattice strain fields

Answer

The CuAl2 equilibrium precipitates are fully coherent with the matrix

Answer

All copper atoms are in solid solution, maximizing solid solution strengthening

Answer

Grain growth is completely suppressed by the precipitates

Question 4

The driving force for grain growth in a polycrystalline material is the reduction of:

Accepted Answer

Total grain boundary area and associated energy

Answer

Elastic strain energy stored in dislocations

Answer

Chemical free energy difference between grain interiors

Answer

Vacancy supersaturation in the microstructure

Question 5

Chemical vapor deposition (CVD) at low pressures (LPCVD) is often run in a mass-transport-limited regime at high temperatures. Which process is rate-limiting in this regime?

Accepted Answer

Delivery of precursor species to the substrate surface

Answer

Surface reaction kinetics at the substrate

Answer

Desorption of by-product gases from the surface

Answer

Nucleation of the first monolayer on the substrate

Question 6

The solidification of a pure metal proceeds at a constant temperature because:

Accepted Answer

The latent heat released exactly compensates heat extraction, maintaining equilibrium

Answer

Diffusion in the liquid is infinitely fast, homogenizing temperature

Answer

Undercooling prevents any temperature change during solidification

Answer

The solid and liquid have identical heat capacities

Question 7

Which thermodynamic condition defines the onset of miscibility gap (two-phase region) in a binary phase diagram using the free energy-composition curve?

Accepted Answer

∂²G/∂c² = 0 (inflection points)

Answer

∂G/∂c = 0

Answer

∂³G/∂c³ = 0

Answer

G = minimum at all compositions