# Phase Transformation Questions and Answers – Diffusion – Atomic Mobility

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This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Diffusion – Atomic Mobility”.

1. The potential energy of the atom will then be relatively high due to the strain in the surrounding matrix. However, this strain energy can be reduced if the atom is located in a position where it better matches the space available.
a) False
b) True

Explanation: This strain energy can be reduced if the atom is located in a position where it better matches the space available, e.g. near dislocations and in boundaries, where the matrix is already distorted.

2. The diffusion of carbon or nitrogen to dislocations in mild steel is responsible for strain ageing and blue brittleness.
a) True
b) False

Explanation: The technological importance in separating and dividing the atoms to grain boundaries, interfaces and dislocations is really immense.In mild steel, the main reason for the strain ageing and blue brittleness is caused by the diffusion of carbon or nitrogen to dislocations.

3. The segregation of impurities such as Sb, Sn, P and As to grain boundaries in low-alloy steels produces____________
a) Ageing
b) Strain hardening
c) Temper embrittlement
d) Red brittleness

Explanation: Temper embrittlement is the result of above phenomenon. Segregation to grain boundaries affects the mobility of the boundary and has pronounced effects on recrystallization, texture and grain growth.
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4. The value of Atomic mobility is given as Mв and the potential gradient is given as (∂u/∂x) .Calculate the value of drift velocity?
a) Vв = -Mв*(∂u/∂x)
b) Vв = Mв/(∂u/∂x)
c) Vв = Mв+(∂u/∂x)
d) Vв = Mв*(∂u/∂x)

Explanation: Since atoms always migrate so as to remove differences in chemical potential it is reasonable to suppose that the drift velocity is proportional to the local chemical potential gradient, and the drift velocity is given by Vв = -Mв*(∂u/∂x), where Mв is a constant of proportionality known as the atomic mobility.

5. The atoms diffusing towards regions of high concentration can be found when diffusion occurs in the presence of an electric field or a temperature gradient. These are known as_____
a) Electro-migration and Thermo-migration
b) Electric-migration and Thermal-migration
c) Electro-migration and Thermal-migration
d) Electric-migration and Thermo-migration

Explanation: These are known as electro-migration and thermos-migration, thus it can be seen that in addition to the effect of the concentration gradient the diffusive flux is also affected by the gradient of strain energy.

6. What can be done to the strain energy if the atom is located near the dislocations?
a) Can be reduced
b) Can be increased
c) Can be maintained at a constant value
d) Cannot say

Explanation: If we consider the regions near dislocations or near boundaries, the matrix there is really distorted and the strain energy there can be reduced. This happens because it has a better match available at its new position.

7. Diffusion in the vicinity of the defects is affected by______
b) Velocity
c) Interaction energy and Speed of diffusion
d) Gradient of potential energy and Composition

Explanation: Actually the atoms can reduce their free energies if they shift or migrate to the some defects but at the equilibrium, it is found that their concentration compared to the matrix is higher at the vicinity of the defects and one of the reason is that both the concentration gradient and the gradient of the interaction energy plays an important role during the time when the diffusion takes place in the vicinity of these defects.

8. If the atomic mobility is given as 5(sec/Kg) and the potential gradient is given as 4(Kg*m/sec²), Then calculate the drift velocity from the given data? (In m/sec)
a) 1.25
b) 9
c) -20
d) 20

Explanation: The drift velocity is the negative of the product of the atomic mobility and the driving force F. The force F can be either an elastic energy gradient, interfacial energy gradient or chemical potential gradient, it depend on the situation, here in this case we consider the local potential gradients as the drift force, hence the product of this and the drift velocity gives the required solution that is V=-4*5=-20 and they are negative.

9. Mobility in terms of drift velocity and driving force can be described as __________
a) V/F
b) F*V
c) F+V
d) F/V

Explanation: The simple and basic definition of mobility is, M =V/F, where V is the velocity of the entity and F the force or the driving force. The force F can be either an elastic energy gradient, interfacial energy gradient or chemical potential gradient, it depend on the situation.

10. For an ideal solution the diffusive coefficient in terms of atomic mobility is given as_______
a) D = MRT
b) D = M/RT
c) D = M/T
d) D = M+RT

Explanation: For an ideal solution the diffusion coefficient is given by the equation MRT. But in the case of non-ideal solution a thermodynamic factor is also included along with this equation so there will be a deviation in the diffusion coefficient of the real solution.

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