Thermodynamics and Kinetics of Materials Flashcards
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Read the first 7 Thermodynamics and Kinetics of Materials flashcards as text
Spinodal decomposition differs from nucleation-and-growth decomposition in that spinodal decomposition:
Answer: Proceeds spontaneously without a nucleation barrier via uphill diffusion
Inside the spinodal region, the second derivative of free energy with composition is negative, making any composition fluctuation spontaneous and eliminating the nucleation barrier.
During parabolic oxidation of a metal, the oxide thickness x follows the relationship x² = kp·t. This parabolic behavior arises because:
Answer: Ion transport through the growing oxide layer becomes rate-limiting
As the oxide layer thickens, ions must diffuse longer distances through it, causing the rate to decrease with time and giving parabolic kinetics.
Precipitation hardening (age hardening) of Al-Cu alloys achieves maximum strength at peak aging because:
Answer: Fine, coherent GP zones and θ'' precipitates create maximum lattice strain fields
Peak hardness corresponds to fine coherent or semi-coherent precipitates (GP zones and θ'') that maximally strain the surrounding lattice, impeding dislocation motion.
The driving force for grain growth in a polycrystalline material is the reduction of:
Answer: Total grain boundary area and associated energy
Grain boundaries have excess energy per unit area; grain growth reduces total boundary area, lowering the overall free energy of the system.
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?
Answer: Delivery of precursor species to the substrate surface
In the mass-transport-limited regime, the surface reaction is fast but precursor delivery through the boundary layer to the substrate limits the overall deposition rate.
The solidification of a pure metal proceeds at a constant temperature because:
Answer: The latent heat released exactly compensates heat extraction, maintaining equilibrium
At the melting point, the latent heat released by solidification balances the heat being extracted, holding the temperature constant until solidification is complete.
Which thermodynamic condition defines the onset of miscibility gap (two-phase region) in a binary phase diagram using the free energy-composition curve?
Answer: ∂²G/∂c² = 0 (inflection points)
The spinodal boundary (limit of the miscibility gap) is defined by the inflection points where ∂²G/∂c² = 0; inside these points the alloy is unstable to composition fluctuations.