# Phase Transformation Questions and Answers – Alloy Solidification

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This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Alloy Solidification”.

1. The solidification of pure metals is rarely encountered in practice. Which among the following is a reason for this?
a) Pure metals contain sufficient impurities
b) Pure metals have high value of Tm
c) Pure metals have low melting temperature
d) Pure metals can withstand high pressure

Explanation: There are a lot of reasons behind this but the major one is that even though we say something is pure or commercially pure, it contains an amount of impurities that we can consider sufficient to change the characteristics from pure to alloy behavior.

2. If we consider an idealized phase diagram by assuming that the solidus and liquidus are straight lines. Then the partition coefficient can be taken as _____ (Xs and Xl are the mole fractions of solid and liquid respectively)
a) K = (Xl-Xs)/Xs
b) K = (Xl-Xs)/Xl
c) K = Xl/(Xl-Xs)
d) K = Xs/Xl

Explanation: It is useful to take the partition coefficient as K = Xs/Xl, where Xs and Xl are the mole fractions of solid and liquid respectively. The way in which such alloys solidify in practice depends in rather a complex way on temperature gradients, cooling rates and growth rates.

3. “Non-equilibrium lever rule” is also known by the name _________
a) Matano analysis
b) Cape theory
c) Dobrienier theory
d) Scheil’s equation

Explanation: The equation XL =Xofl(j-1), (where the fl is the volume fraction not solidified) represents the Scheil’s equation. For j < 1 these equations predict that when there is no diffusion in the solid there will always be some eutectic in the last drop of liquid to solidify, no matter how little solute is present.
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4. When there will be a rapid build-up of solute ahead of the solid and a correspondingly rapid increase in the composition of the solid formed. This is known as ___
a) Final transient
b) Cover up
c) Random order
d) Initial transient

Explanation: Diffusion is the only mode of transportation for the solute rejected from the solid when there is no stirring or convection in the liquid phase. As an after effect there will be a rapid build-up of the solute much before the solid hence there will be a rapid increase in the composition of the solid formed. This is known as the initial transient.

5. Which among the following equation is related to diffusional mixing in liquid (during steady state)? (Where D, Cl, Cs, v represents the diffusivity in liquids, solute concentration of liquid and solid, constant rate and Cl’=dCl/dx respectively)
a) –DCl = v(Cl-Cs)
b) DCl = (Cl-Cs)/v
c) DCl = Cs-Cl/v
d) DCl = v (Cl-Cs)

Explanation: Steady state itself gives an idea that the particular state should be balanced. Here in this case the rate at which the solute is rejected from the solidifying liquid should be balanced by the rate at which solute diffuses down the concentration gradient away from the interface. That is –DCl = v(Cl-Cs).

6. Under the steady state growth of the critical gradient, if we consider T1 and T3 to be the liquidus and solidus temperatures for the bulk composition Xo. The equilibrium freezing range of the alloy solidus can be taken as _______
a) T3-T1
b) Xo*(T3-T1)
c) T3/T1
d) T1-T3

Explanation: T1 – T3, is known as the equilibrium freezing range of the alloy. Clearly planar front solidification is most difficult for alloys with a large solidification range and high rates of solidification. Except under well-controlled experimental conditions alloys rarely solidify with planar solid/liquid interfaces.

7. In practice alloy solidification will usually possess certain features. Actually there is some stirring caused by the turbulence of the liquid which can happen due to many reasons. Which among the following is not a reason for that?
a) Pouring
b) Convection currents
c) Gravity currents
d) Composition of alloy

Explanation: There are a lot of reasons for this stirring to take place and this can happen because of liquid turbulence which may be caused by pouring or it can be caused by the convection currents or the gravity effects may also play a role in this.

8. As the A-rich α phase solidifies excess B diffuses a short distance laterally where it is incorporated in the B-rich β phase. For an interlamellar spacing K, there is a total of X m2 of α/β interface per m3 of eutectic. Find the value of X?
a) 1/K
b) 3/K
c) 2/K
d) 2/K2

Explanation: For an interlamellar spacing K, there is a total of 2/K m2 of α/β interface per m3 of eutectic. However, there is a lower limit to K determined by the need to supply the α/β interfacial energy. Thus small interlamellar space should lead to rapid growth.

9. Unidirectional solidification has commercial importance.
a) False
b) True

Explanation: The production of creep resistant aligned microstructure for gas turbine blades is actually a commercial example of the unidirectional solidification which highlights its importance. Not only this but this is used during the zone refining for the production of extremely pure metals.

10. Which among the following can be categorized as a peritectic reaction?
a) Liquid + α(liquid phase)->β(liquid phase)
b) Liquid + α(solid phase)->β(liquid phase)
c) Liquid + α(liquid phase)->β(solid phase)
d) Liquid + α(solid phase)->β(solid phase)

Explanation: Liquid + α(solid phase)->β(solid phase), this kind of reactions are known as peritectic reaction. In peritectic reaction a second solid phase is created as a result of the reaction between a liquid phase and a solid phase.

11. In general the tendency to form dendrites increases as the solidification range______
a) Becomes constant
b) Increases
c) Decreases
d) Nothing can be predicted

Explanation: In general the tendency to form dendrites increases as the solidification range increases. Therefore the effectiveness of different solutes can vary widely. For solutes with a very small partition coefficient (k) cellular or dendritic growth can be caused by the addition of a very small fraction of a per cent solute.

12. In the case of the Al₆Fe-Al rod-like eutectic, the impurity causing the cellular structure is mainly___
a) Aluminum
b) Ferrous
c) Nickel
d) Copper

Explanation: A planar eutectic front is not always stable. If for example the binary eutectic alloy contains impurities, or if other alloying elements are present, the interface tends to break up to form a cellular morphology. In this case the impurity that leads to the cellular structure is copper.

13. Various different types of eutectic solidification are possible and these are usually classified as____
a) Normal and anomalous
b) Perfect and Imperfect
c) Proper and Dismantled
d) Even and uneven

Explanation: They are classified as normal and anomalous. Normal structures occur when both phases have low entropies of fusion. Anomalous structures, on the other hand, occur in systems when one of the solid phases is capable of faceting.

14. Under what condition is it possible to solidify an off-eutectic alloy without permitting the formation of the primary dendritic phase?
a) Controlled solidification
b) Rapid solidification
c) Multi-directional solidification
d) No such condition exist

Explanation: Under controlled solidification conditions, e.g. in unidirectional solidification experiments, it is possible to solidify an off-eutectic alloy without permitting the formation of the primary dendritic phase.

15. The total undercooling at the eutectic front (ΔTo) has two contributions.
a) True
b) False

Explanation: ΔTo =ΔTr+ ΔTd, ΔTr is the undercooling required to overcome the interfacial curvature effects and ΔTd is the undercooling required to give a sufficient composition difference to drive the diffusion.

Sanfoundry Global Education & Learning Series – Phase Transformation.

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