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

The pre-exponential frequency factor

Answer

The universal gas constant times temperature

Answer

The equilibrium constant

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

J = D (∂C/∂t)

Answer

J = D (∂²C/∂x²)

Answer

J = -D (∂²C/∂x²)

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 fastest cooling rate to avoid martensite

Answer

The temperature of maximum hardness

Answer

The eutectoid temperature of the alloy

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

Homogeneous nucleation releases more latent heat

Answer

Homogeneous nucleation occurs at grain boundaries where energy is higher

Answer

Heterogeneous nucleation requires more activation energy

Question 5

The Kirkendall effect in diffusion couples demonstrates that:

Accepted Answer

Different atomic species can have different intrinsic diffusion coefficients

Answer

Both species diffuse at the same rate in alloys

Answer

Vacancies play no role in substitutional diffusion

Answer

Diffusion flux is always equal in both directions

Question 6

The Avrami equation, f = 1 - exp(-kt^n), is used to describe isothermal phase transformation kinetics. The exponent n provides information about:

Accepted Answer

The nucleation and growth mechanism

Answer

The activation energy of the transformation

Answer

The equilibrium volume fraction of the product phase

Answer

The diffusion coefficient of the slowest species

Question 7

For diffusion of carbon in austenite (FCC iron), the diffusivity is higher than carbon in ferrite (BCC iron) at the same temperature primarily because:

Accepted Answer

Austenite has larger octahedral interstitial sites relative to atom size

Answer

BCC iron has a lower melting point than FCC iron

Answer

Carbon has a lower activation energy in BCC structures

Answer

Ferrite has fewer grain boundaries for short-circuit diffusion