# Phase Transformation Questions and Answers – Diffusion in Ternary Alloys

This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Diffusion in Ternary Alloys”.

1. The addition of a third diffusing species to a solid solution produces some effects. Fe-Si-C alloys are particularly instructive for some reasons. Which among the following is a reason for the above-mentioned statement?
a) Carbon raises the chemical potential of silicon
b) Silicon raises the chemical potential of carbon
c) Ferrous becomes inactive
d) Mobility of carbon and silicon are almost same

Explanation: Firstly silicon raises the chemical potential (or activity) of carbon in solution, carbon will not only diffuse from regions of high carbon concentration but also from regions rich in silicon. Secondly the mobility’s of carbon and silicon are widely different.

2. In Fe-Si-C alloys, why does carbon diffuse far more rapidly than the substitutionally dissolved silicon?
a) Because it is large in size
b) Because it has more mass
c) Because it is an interstitial solute
d) Because it is more volatile

Explanation: If we consider the mobilities of carbon and silicon, they are actually widely different. The difference comes because silicon is a substitutionally dissolved solute and carbon is an interstitial solute hence carbon has more advantage in the diffusion process in terms of rate.

3. The inter-diffusion flux for a multicomponent system can be calculated using____
a) Matano-Boltzmann analysis
b) Henry’s laws
c) Raoult’s Equation
d) Perkins equation

Explanation: Using the Matano-Boltzmann analysis for the multicomponent system, the inter-diffusion fluxes can be calculated from the experimental diffusion profile. They actually covert the complex differential equation into more easily solvable ODE’s.

4. Less striking effects can arise in ternary systems where all three components diffuse substitutionally if their diffusivities (or mobilities) are unequal.
a) True
b) False

Explanation: The redistribution of an atom in the ternary system is particularly interesting when the mobilities of the two different atoms are so different. Something which is almost identical but not so common effect can arise in ternary systems. Here in this case all the 3 components can diffuse substitutionally but under the condition that their diffusivities (or mobilities) are unequal.

5. In ternary alloy systems, because of the additional degree of freedom in the composition we may see composition profile which do not change monotonically through solid solution ranges and stable non- planar phase interfaces.
a) True
b) False

Explanation: Yes the statement is true. Even though two phase regions may lie between the terminal compositions of a diffusion sample on the constitutional diagram, they will not necessarily appear on the diffusion profile.

6. Which of the following relate the flux of one component to the composition gradient of the other independent constituent in ternary system?
a) Off-diagonal
b) Side-diagonal
c) Triangle rule
d) Fick’s law

Explanation: Actually, by using linear extension of Fick’s law we introduce the cross or off-diagonal diffusion coefficient which relate the flux of one component to the composition gradient of the other independent constituent in ternary system.

7. Who made the extension of Fick’s law to multicomponent system in the year 1945?
a) Onsager
b) Kirkland
c) Brown
d) Raoult

Explanation: He proposed that, most generally if there are n diffusing species then we may consider the motion of each to be linearly dependent on all gradients.

8. In order to describe the transport process in a ternary diffusion system, we need 4 inter-diffusion coefficients. Why?
a) Doppler principle
b) Catherine cross diagonal
c) Linear extension of ficks law
d) Shear pressure

Explanation: By using linear extension of Fick’s law we introduce the cross or off-diagonal diffusion coefficient which relate the flux of one component to the composition gradient of the other independent constituent. Hence we require 4 diffusion coefficient to describe the transport process.

9. What was the new feature introduced in the study of diffusion by Onsager?
a) Barrel effect
b) Cross effect
c) Multi phase effect
d) Side effect

Explanation: The generalization of Fick’s law by Onsager introduces a significant new feature into the study of diffusion, the presence of “Cross Effects”. In a ternary system, for example the flux of element 1 will depend not only on its own gradient but also on the gradient of element 2.

10. If a system has 3 components, the number of independent coefficients present if we utilize the ORR is given by____________
a) 3
b) 6
c) 2
d) 4

Explanation: The number of independent coefficients present for the above system can be calculated using the formula (n)*(n-1)/2, where n denotes the number of components present in the system. Substituting the value of n as 3 we get the required solution.

11. According to the model based on atomic mobility (as suggested by Darken in the year 1950), if a system has 3 mobilities then the number of independent coefficients is given by_____
a) 6
b) 5
c) 3
d) 2

Explanation: For the model based on atomic mobility (as suggested by Darken in the year 1950), if a system has n mobilities then the number of independent coefficients is given by n.

Sanfoundry Global Education & Learning Series – Phase Transformation.

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