This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Interface Migration”.
1. In a way we can say that an interface is created during the____
Explanation: In a way we can say that an interface creation occurs during the nucleation stage and then the next step is that it migrates into the surrounding parent phase during the growth stage. Essentially this type of transformation is heterogeneous in nature.
2. Most of the transformation products are formed during the growth stage by the_____
a) Transfer of atoms across the moving parent/product interface
b) Movement of atoms in the formed nuclei
d) Random oscillation of parent particle
Explanation: Even though the nucleation stage plays the key role in determining many of the features of transformation but most of the transformation products are formed or created during the growth stage and this takes place by the transfer of atoms across the moving parent interface.
3. Athermal migration is related to which of the following interface?
Explanation: There are basically two different types of interface: glissile and non-glissile. Glissile interfaces migrate by dislocation glide that results in the shearing of the parent lattice into the product. The motion of glissile interfaces is relatively insensitive to temperature and is therefore known as athermal migration.
4. The migration of which among the following interfaces is extremely sensitive to temperature?
Explanation: In many cases it is found that the interfaces are non-glissile in nature and migrate by the more or less random jumps of individual atoms across the interface. The thermal activation is the process by which we get the extra energy that the atom needs to break free of one phase and attach itself to the other. The migration of non-glissile interfaces is therefore extremely sensitive to temperature.
5. Civilian transformation is a phenomenon that occurs when the uncoordinated transfer of atoms occurs across a____________
a) Glissile interface
b) Mixed interface
c) Partially coherent interface
d) Non-Glissile interface
Explanation: Transformations that are created by the migration of a glissile interface are referred to as military transformations. This gives a clarity and of the analogy between the coordinated motion of atoms crossing the interface and that of soldiers moving in ranks on the parade ground. In contrast the uncoordinated transfer of atoms across a non-glissile interface results in what is known as a civilian transformation.
6. During civilian transformations the parent and product will have _____ (Composition).
a) Same composition
b) Different composition
c) Time dependent composition
d) Nothing can be predicted
Explanation: Here in the case of a civilian transformation nothing can predicted about its composition, it can be same or may be different and if there is no change in composition, the new phase growth will be almost at the speed at which the atoms cross the interface.
7. Martensitic transformations belongs to____
a) Base transformation
b) Civilian transformation
c) Military transformation
d) Mixed transformation
Explanation: During a military transformation the nearest neighbours of any atom are essentially unchanged. Therefore, the parent and product phases must have the same composition and no diffusion is involved in the transformation. Martensitic transformations belong to this group
8. Just for a simplicity consider a pure substance B. By analogy with the migration of a high-angle grain boundary the net flux of B across the interface will produce an interface velocity v given by____
(Interface mobility is given as M, Molar volume V, and the driving force is given as Δμ)
b) MΔμ +V
Explanation: The interface velocity is the product of the interface mobility and the driving force per unit molar volume, so in this case the interface mobility is given as M, Molar volume V, and the driving force is given as Δμ so the required velocity is MΔμ/V.
9. If we consider a β precipitate of an almost pure metal B with Interface mobility given as M, Molar volume V of the β phase, and the driving force is given as Δμ. Calculate the flux across the interface?
a) J = MΔμ/V²
b) J = -MΔμ/V
c) J = -MΔμ/V²
d) J = MΔμ/V
Explanation: Actually, the interface velocity per molar volume is the flux across the surface. The interface energy is given as the product of the interface mobility and the driving force per unit molar volume hence the flux is given as J = -MΔμ/V², here the negative sign indicates that flux is in the negative direction along the oriented axis.
10. In the limit of a very low mobility it is possible that ____
a) (∂C/∂x) interface = 0
b) (∂C/∂x) interface > 0
c) (∂C/∂x) interface < 0
d) (∂C/∂x) interface = 1
Explanation: In the limit of very low mobility the value of approach 0. Under this situation the growth is described as interface controlled and here the chance of having a maximum possible driving force is very high.
11. What happens to the driving force Δμ, when Xi (Concentration adjacent to the interface) approaches Xe (equilibrium concentration)? (For a dilute or ideal solution)
d) Δμ cannot be predicted
Explanation: The driving force Δμ approaches 0 when Xe=Xi, this happens because the driving force is directly proportional to (Xi-Xe) and it can be calculated using the given formula Δμ= (RT/Xe)*(Xi-Xe), this is only applicable for dilute or ideal solution.
12. The accommodation factor (A) at the incoherent interfaces and diffuse solid/liquid interfaces can be close to________
Explanation: The accommodation factor (A), i.e. the probability that an atom crossing the boundary will be accommodated on arrival at the new phase. It is likely that incoherent interfaces and diffuse solid/liquid interfaces, as high angle grain boundaries, will have values of (A) close to unity.
13. If a single atom attaches itself to a flat close-packed interface it will raise the interfacial free energy.
Explanation: If a single atom attaches itself to a flat close-packed interface it will raise the interfacial free energy and will therefore tend to detach itself again. It can thus be seen that continuous growth at the above type of interfaces will be very difficult which means the mobility and the accommodation factor will be very low.
14. Growth ledges are by no means restricted to solid/solid systems.
Explanation: Growth ledges are by no means restricted to solid/solid systems. The first evidence for the existence of growth ledges came from studies of solid/vapour interfaces. They are also found on faceted solid/liquid interfaces.
15. Calculate the driving force at temperature 300K if the value of Xi (Concentration adjacent to the interface) and Xe (equilibrium concentration) are given as 4mols and 2mols respectively?
Explanation: Here the value of driving force can be calculate using the equation Δμ = RTln (Xi/Xe). So at a temperature of 300K the value of the driving force is give as 300Rln2 but in the case of an ideal solution the equation can be re-written as Δμ= (RT/Xe)*(Xi-Xe).
Sanfoundry Global Education & Learning Series – Phase Transformation.
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