Phase Transformation Mcqs

Q:

Nucleation on dislocations may also be assisted by solute segregation which can raise the composition of the matrix to nearer that of the precipitate.

A) True B) False
 
Answer & Explanation Answer: A) True

Explanation: Nucleation on dislocations may also be assisted by solute segregation which can raise the composition of the matrix to nearer that of the precipitate. The dislocation can also assist in growth of an embryo beyond the critical size by providing a diffusion pipe with a lower ΔG(migration).

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8
Q:

In FCC crystals the a/2*[110] unit dislocations can dissociate to produce a ribbon of stacking fault. Which among the following is as an example of the same?

A) a/2*[110]->a/6*[121] + a/6[211] B) a/2*[110]->a/6*[101] + a/6[201]
C) a/2*[110]->a/6*[821] + a/6[911] D) a/2*[110]->a/6*[161] + a/6[221]
 
Answer & Explanation Answer: A) a/2*[110]->a/6*[121] + a/6[211]

Explanation: a/2*[110]->a/6*[121] + a/6*[211] is an example and this gives a stacking fault on (111) separated by two Shockley partials. Since the stacking fault is in effect four close-packed layers of hcp crystal, it can act as a very potent nucleation site for an hcp precipitate.

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8
Q:

A coherent nucleus with a negative misfit can reduce the critical volume free energy by forming a region of _________

A) Compressive strain B) Compressive stress
C) Tensile strain D) Tensile stress
 
Answer & Explanation Answer: A) Compressive strain

Explanation: This directly implies that the smaller volume than the matrix can reduce its ΔG* by forming a region of compressive strain above an edge dislocation, whereas if the misfit is positive it is energetically favourable for it to form below the dislocation.

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9
Q:

At very small driving forces, when activation energy barriers for nucleation are high, the highest nucleation rates will be produced by grain corner nucleation.

A) False B) True
 
Answer & Explanation Answer: B) True

Explanation: The driving force is the factor which determines whether the type of site gives the highest volume nucleation rate or not. At very small driving forces, when activation energy barriers for nucleation are high, the highest nucleation rates will be produced by grain corner nucleation.

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9
Q:

The relative magnitudes of the heterogeneous and homogeneous volume nucleation rate is given as 0.5. Calculate the factor C/C1 (the number of atoms on heterogeneous sites relative to the number within the matrix)? (Consider exp (ΔG*(hom) – ΔG*(het)/kT) to be 1)

A) 0.5 B) 1.2
C) 0.25 D) 1.25
 
Answer & Explanation Answer: A) 0.5

Explanation: The relative magnitudes of the heterogeneous and homogeneous volume nucleation rate is given by the equation (C/C1) *(exp (ΔG*(hom) – ΔG*(het)/kT). So in this case the nucleation rate is 0.5 which itself is the factor C/C1.

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14
Q:

What is special about the dark-field electron microscope micrograph?

A) Precipitates can be imaged bright and matrix dark B) 360 degree rotation is possible
C) High speed imaging is possible D) Anti roller scheme available
 
Answer & Explanation Answer: A) Precipitates can be imaged bright and matrix dark

Explanation: The so-called dark-field electron microscope micrograph in which the precipitates are imaged bright and the matrix dark. The precipitates lie in rows along dislocations. And this can be used in case of niobium carbonitride precipitates on dislocations in a ferritic iron matrix

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8
Q:

Excess vacancies are retained during the quench if the age hardening alloys are retained from______

A) Dark texture B) Room temperature
C) Low temperature D) High temperature
 
Answer & Explanation Answer: D) High temperature

Explanation: Excess vacancies are retained during the quench if the age hardening alloys are retained from high temperature. This can actually increase the diffusion rate or it can relieve the misfit strain energies hence it assist the nucleation.

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7
Q:

If the ΔG* heterogeneous is given as 5kJ/mol and ΔG* homogeneous is given as 10kJ/mol. Calculate the shape factor?

A) 0.5 B) 1
C) 2 D) 2.5
 
Answer & Explanation Answer: A) 0.5

Explanation: Here in this case the shape factor can be determined by the ratio of ΔG*heterogeneous/ΔG*homogeneous. So here for this particular case the shape factor value is 5/10 =0.5. Actually the grain boundary has the ability to reduce ΔG* heterogeneous, it’s the power it has as a nucleation site.

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