Grain boundary diffusion makes a significant contribution to the total flux when___(Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickneβ and grain size).

> 1b) (Db*ȣ/D1*d) < 1c) (Db*ȣ/D1*d) = 1d) (Db*ȣ/D1*d) < 0

.a) (Db*ȣ/D1*d) > 1b) (Db*ȣ/D1*d) < 1c) (Db*ȣ/D1*d) = 1d) (Db*ȣ/D1*d) < 0

The dislocations effectively act as pipes along which atoms can diffuse with a diffusion coefficient 2.5mm2/sec. Calculate the apparent diffusivity if the cross-sectional area of pipe per unit area of matrix is given as 10-1? (If D1 (diffusivity through defect free lattice) is given as 5mm2/sec)

5.45 mm2/secb) 5.25 mm2/secc) 6.35 mm2/sec

is given as 5mm2/sec)a) 5.45 mm2/secb) 5.25 mm2/secc) 6.35 mm2/secd) 5.75 mm2/sec

The relative magnitudes of Db*ȣ and D1*d are most sensitive to_______(Where Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickness and grain size)

Pressureb) Temperaturec) Volumed) Composition

The diffusion through the lattice can be ignored under certain circumstances. Which among the following condition should be satisfied for the same if the diffusivity through a defect free lattice is given as 2.5 mm2/sec and the grain size is given as 5mm? (Consider the diffusivity through the grain boundary as 2mm2/sec)

When the effective thickness of grain boundary is 6.25mmb) When the effective thickness of grain boundary is 1000mmc) When the effective thickness of grain boundary is 0.001mm

When the effective thickness of grain boundary is 6.25mm

When the effective thickness of grain boundary is 1000mm

When the effective thickness of grain boundary is 6.25mmb) When the effective thickness of grain boundary is 1000mmc) When the effective thickness of grain boundary is 0.001mm

When the effective thickness of grain boundary is 10mm

In an average metallic specimen the total grain boundary area is_______

Relatively less than surface area

13 + Mcqs in Diffusion High Diffusivity Path Page 1 McqOptions

1.	Grain boundary diffusion makes a significant contribution to the total flux when___(Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickneβ and grain size).
A.	(Dbȣ/D1d) > 1
B.	(Dbȣ/D1d) < 1
C.	(Dbȣ/D1d) = 1d) (Dbȣ/D1
D.	> 1b) (Dbȣ/D1d) < 1c) (Dbȣ/D1d) = 1d) (Dbȣ/D1d) < 0
E.	.a) (Dbȣ/D1d) > 1b) (Dbȣ/D1d) < 1c) (Dbȣ/D1d) = 1d) (Dbȣ/D1d) < 0
Answer» B. (Dbȣ/D1d) < 1

Discussion

2.	At high temperatures diffusion through the lattice is rapid and g*Dp / D1 is very small so that the dislocation contribution to the total flux of atoms is_______ (Dp, D1 are diffusivity through pipe and defect free lattice and g is the cross-sectional area of ‘pipe’ per unit area of matrix).
A.	Extremely high
B.	Negligible
C.	Low
D.	High
Answer» C. Low

Discussion

3.	Find the frequency factor of the following grain boundary diffusion, if the activation energy is given as 90KJ/mol and the diffusivity at the grain boundary at 1071K is given as 49*10-6m2/sec?
A.	1.2mm2/sec
B.	1.2m2/sec
C.	2.2mm2/sec
D.	4.2mm2/sec
Answer» B. 1.2m2/sec

Discussion

4.	The dislocations effectively act as pipes along which atoms can diffuse with a diffusion coefficient 2.5mm2/sec. Calculate the apparent diffusivity if the cross-sectional area of pipe per unit area of matrix is given as 10-1? (If D1 (diffusivity through defect free lattice) is given as 5mm2/sec)
A.	5.45 mm2/sec
B.	5.25 mm2/sec
C.	5.45 mm2/secb) 5.25 mm2/secc) 6.35 mm2/sec
D.	5.75 mm2/sec
E.	is given as 5mm2/sec)a) 5.45 mm2/secb) 5.25 mm2/secc) 6.35 mm2/secd) 5.75 mm2/sec
Answer» C. 5.45 mm2/secb) 5.25 mm2/secc) 6.35 mm2/sec

Discussion

5.	The rate at which atoms diffuse along different boundaries is_________
A.	Same
B.	Different
C.	Constant
D.	Cannot be predicted
Answer» C. Constant

Discussion

6.	In FCC metals it is generally found that ___
A.	Qb >> 0.5Q1
B.	Qb << 0.5Q1
C.	Qb = Q1
D.	Qb ̴̴ Q1
Answer» E.

Discussion

7.	The jump frequency for atoms migrating along the defects is higher than that for diffusion in the lattice.
A.	True
B.	False
Answer» B. False

Discussion

8.	The relative magnitudes of Dbȣ and D1d are most sensitive to_______(Where Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickness and grain size)
A.	Pressure
B.	Temperature
C.	Volume
D.	Composition
E.	Pressureb) Temperaturec) Volumed) Composition
Answer» C. Volume

Discussion

9.	The diffusion through the lattice can be ignored under certain circumstances. Which among the following condition should be satisfied for the same if the diffusivity through a defect free lattice is given as 2.5 mm2/sec and the grain size is given as 5mm? (Consider the diffusivity through the grain boundary as 2mm2/sec)
A.	When the effective thickness of grain boundary is 6.25mm
B.	When the effective thickness of grain boundary is 1000mm
C.	When the effective thickness of grain boundary is 6.25mmb) When the effective thickness of grain boundary is 1000mmc) When the effective thickness of grain boundary is 0.001mm
D.	When the effective thickness of grain boundary is 10mm
Answer» C. When the effective thickness of grain boundary is 6.25mmb) When the effective thickness of grain boundary is 1000mmc) When the effective thickness of grain boundary is 0.001mm

Discussion

10.	In a diffusion couple made by welding together two metals, A and B.A-atoms diffusing along the boundary will be able to penetrate much deeper than atoms which only diffuse through the lattice.
A.	True
B.	False
Answer» B. False

Discussion

11.	The fluxes of solute through the lattice J1 and along the boundary Jb are 3 and 4 mol/mm2sec. If the grain boundary has an effective thickness 2mm and the grain size is 2.5mm. Calculate the total flux?(In mol/mm2sec)
A.	6.2
B.	4.2
C.	6.2b) 4.2c) 3.2
D.	2.2
Answer» B. 4.2

Discussion

12.	In an average metallic specimen the total grain boundary area is_______
A.	Much greater than surface area
B.	Same as the surface area
C.	Relatively less than surface area
D.	Much less than surface area
Answer» D. Much less than surface area

Discussion

13.	In general, at any temperature the magnitudes of Db and Ds relative to the diffusivity through defect-free lattice D1 are such that_________ (Db and Ds are grain boundary and surface diffusivity)
A.	Db>Ds>D1
B.	Db<Ds<D1
C.	Db>D1>Ds
D.	Ds>Db>D1
Answer» E.

Discussion

Explore topic-wise MCQs in Phase Transformation.

Grain boundary diffusion makes a significant contribution to the total flux when___(Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickneβ and grain size).

At high temperatures diffusion through the lattice is rapid and g*Dp / D1 is very small so that the dislocation contribution to the total flux of atoms is_______ (Dp, D1 are diffusivity through pipe and defect free lattice and g is the cross-sectional area of ‘pipe’ per unit area of matrix).

Find the frequency factor of the following grain boundary diffusion, if the activation energy is given as 90KJ/mol and the diffusivity at the grain boundary at 1071K is given as 49*10-6m2/sec?

The dislocations effectively act as pipes along which atoms can diffuse with a diffusion coefficient 2.5mm2/sec. Calculate the apparent diffusivity if the cross-sectional area of pipe per unit area of matrix is given as 10-1? (If D1 (diffusivity through defect free lattice) is given as 5mm2/sec)

The rate at which atoms diffuse along different boundaries is_________

In FCC metals it is generally found that ___

The jump frequency for atoms migrating along the defects is higher than that for diffusion in the lattice.

The relative magnitudes of Db*ȣ and D1*d are most sensitive to_______(Where Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickness and grain size)

In a diffusion couple made by welding together two metals, A and B.A-atoms diffusing along the boundary will be able to penetrate much deeper than atoms which only diffuse through the lattice.

The fluxes of solute through the lattice J1 and along the boundary Jb are 3 and 4 mol/mm2sec. If the grain boundary has an effective thickness 2mm and the grain size is 2.5mm. Calculate the total flux?(In mol/mm2sec)

In an average metallic specimen the total grain boundary area is_______

In general, at any temperature the magnitudes of Db and Ds relative to the diffusivity through defect-free lattice D1 are such that_________ (Db and Ds are grain boundary and surface diffusivity)

The relative magnitudes of Dbȣ and D1d are most sensitive to_______(Where Db and D1 are the diffusivities through the grain boundary and defect free lattice and ȣ, d are the effective thickness and grain size)