ME Thermodynamics and Kinetics of Materials 2

Question 1

The Arrhenius equation for a thermally activated process is k = A·exp(-Q/RT). What does Q represent?

Accepted Answer

The activation energy for the process

Answer

Q is the activation energy (in J/mol or eV/atom) representing the energy barrier that must be overcome for the process to occur.

Question 2

Fick's First Law of diffusion states that the diffusion flux J is related to the concentration gradient by:

Accepted Answer

J = -D (∂C/∂x)

Answer

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.

Question 3

A TTT (Time-Temperature-Transformation) diagram for a steel shows a 'nose' at intermediate temperatures. What does the nose represent?

Accepted Answer

The temperature at which transformation kinetics are fastest

Answer

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.

Question 4

Homogeneous nucleation requires greater undercooling than heterogeneous nucleation because:

Accepted Answer

Heterogeneous nucleation on surfaces lowers the energy barrier by reducing the contact angle contribution

Answer

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.

Question 5

The Kirkendall effect in diffusion couples demonstrates that:

Accepted Answer

Different atomic species can have different intrinsic diffusion coefficients

Answer

The Kirkendall effect (marker movement) proves that A and B atoms diffuse at different rates, requiring vacancy flux to balance the unequal atomic fluxes.

ME - Materials Engineer Practice Test

ME - Materials Engineer Practice Test

ME Thermodynamics and Kinetics of Materials 2