Thermodynamics and Kinetics of Materials Flashcards
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Read the first 7 Thermodynamics and Kinetics of Materials flashcards as text
The Arrhenius equation for a thermally activated process is k = A·exp(-Q/RT). What does Q represent?
Answer: The activation energy for the process
Q is the activation energy (in J/mol or eV/atom) representing the energy barrier that must be overcome for the process to occur.
Fick's First Law of diffusion states that the diffusion flux J is related to the concentration gradient by:
Answer: J = -D (∂C/∂x)
Fick's First Law is J = -D(∂C/∂x), where D is the diffusivity and the negative sign indicates flux from high to low concentration.
A TTT (Time-Temperature-Transformation) diagram for a steel shows a 'nose' at intermediate temperatures. What does the nose represent?
Answer: The temperature at which transformation kinetics are fastest
The nose of the TTT curve marks the temperature where the combined effect of undercooling (driving force) and atomic mobility produces the shortest transformation time.
Homogeneous nucleation requires greater undercooling than heterogeneous nucleation because:
Answer: Heterogeneous nucleation on surfaces lowers the energy barrier by reducing the contact angle contribution
Nucleation on pre-existing surfaces or particles reduces the surface energy term via the contact angle factor, lowering ΔG* compared to nucleation in a homogeneous melt.
The Kirkendall effect in diffusion couples demonstrates that:
Answer: Different atomic species can have different intrinsic diffusion coefficients
The Kirkendall effect (marker movement) proves that A and B atoms diffuse at different rates, requiring vacancy flux to balance the unequal atomic fluxes.
The Avrami equation, f = 1 - exp(-kt^n), is used to describe isothermal phase transformation kinetics. The exponent n provides information about:
Answer: The nucleation and growth mechanism
The Avrami exponent n reflects the dimensionality of growth and whether nucleation is continuous or site-saturated, revealing the transformation mechanism.
For diffusion of carbon in austenite (FCC iron), the diffusivity is higher than carbon in ferrite (BCC iron) at the same temperature primarily because:
Answer: Austenite has larger octahedral interstitial sites relative to atom size
Although BCC has a more open structure overall, the octahedral interstitial site in FCC (austenite) is larger relative to the iron atom, accommodating carbon more easily and giving higher diffusivity.