This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Interphase Interfaces in Solids – 1”.
1. HCP silicon-rich k phase and the FCC copper-rich α-matrix in Cu-Si alloys forms ___
a) A coherent interface
b) An incoherent interface
c) Mixed interface
d) A semi coherent interface
Explanation: Here it forms a fully coherent interface. This requires the two crystals to be oriented relative to each other in a special way such that the interfacial plane has the same atomic configuration in both phases, disregarding chemical species.
2. Within the bulk of each phase every atom has an optimum arrangement of nearest neighbours that produces a low energy. At the interface, however, there is usually a change in composition so that each atom is partly bonded to wrong neighbours across the interface.
Explanation: At the interface there is usually a change in composition so that each atom is partly bonded to wrong neighbours across the interface. This increases the energy of the interfacial atoms and leads to a chemical contribution to the interfacial energy.
3. The resultant lattice distortion to maintain the coherency is known as__
a) Strain rupture
b) Coherency strain
c) Rupture plane
d) Maintenance plane
Explanation: It is possible to maintain the coherency even when the distance between the atoms in the interface is not identical and this can be done by straining one or both of the two lattices and the lattice distortions which results from this is known as coherency strain.
4. In which among the following case there is only one plane that can form a coherent interface?
a) Simple cubic
c) Edge centered lattice
Explanation: Only a single plane can form a coherent interface in case of a HCP/FCC interface, no other plane is identical in both crystal lattices. However all the lattice planes are identical apart from the differences in the composition if the two adjoining phases have the same crystal structure and lattice parameter.
5. In a semi coherent interface, the disregistry is periodically taken up by _________
a) Coarsened structure
b) Proper fit
c) Misfit dislocations
d) Cross dislocation
Explanation: The misfit dislocations periodically take up the disregistry and it becomes energetically more favorable to replace the coherent interface with a semi coherent interface.The strains associated with a coherent interface raise the total energy of the system.
6. Assume that 5mm and 5mm are the unstressed interplanar spacing’s of matching planes in the α and ß phases respectively, the disregistry, or misfit between the two lattices (∆) is defined by__
Explanation: If dα and dβ are the unstressed interplanar spacing’s of matching planes in the α and β phases respectively, the disregistry, or misfit between the two lattices (Δ) is defined by Δ = (dα –dβ)/dα, here since the interplanar spacing’s are same, the misfit will be 0.
7. A spacing ‘’D’’ can be used to completely accommodate the lattice misfit in the one direction without any long-range strain field by a set of edge location. Calculate the value of D if the interplanar spacing’s of matching planes are given as 20mm, 19mm respectively? (Assume the misfit to be very small)
Explanation: The value of D is given as b/Δ. Where the value of b = (da+db)/2, in this case 20mm and 19mm and this is actually the Burgers vector for dislocation. And the value of Δ is given as (20-19)/20, that is 0.05 and hence substituting this we get the required solution.
8. The interfacial energy of a semi coherent interface can be approximately considered as the sum of two parts. What are they?
a) Chemical and structural contribution
b) Chemical and bulk contribution
c) Magnetic contribution and structural
d) Physical and bulk contribution
Explanation: The interfacial energy of a semi coherent interface can be approximately considered as the sum of two parts: (a) a chemical contribution, same as for a fully coherent interface, and (b) a structural term, which is the extra energy due to the structural distortions caused by the misfit dislocations.
9. For small values of misfit ∆, the structural contribution to the interfacial energy is approximately proportional to the____ (Semi coherent interface).
a) Size and shape of dislocation
b) Density of dislocation
d) Position of dislocation
Explanation: As the misfit Δ increases the dislocation spacing diminishes. For small values of Δ the structural contribution to the interfacial energy is approximately proportional to the density of dislocations in the interface. However, the interfacial energy increases less rapidly as Δ becomes larger and it levels out at a particular value of Δ.
10. When the value of (Δ) misfit is greater than 0.25, the kind of interface is known as _________
a) Coherent interface
b) Mixed interface
c) Semi coherent interface
d) Incoherent interface
Explanation: When Δ > 0.25, that is one dislocation every four interplanar spacing’s, the regions of poor fit around the dislocation cores overlap and the interface cannot be considered as coherent, and it is known as the incoherent interface.
11. Incoherent interface can also exist between crystals with an orientation relationship if the interface has a different structure in the two crystals.
Explanation: In general, incoherent interfaces result when two randomly oriented crystals are joined across any interfacial plane. They may, however, also exist between crystals with an orientation relationship if the interface has a different structure in the two crystals and they have many features in common with high angle grain boundaries.
12. The degree of coherency can be greatly increased if_______
a) Macroscopically irrational interface is formed
b) Microscopically rational interface is formed
c) Macroscopically rational interface is formed
d) Microscopically irrational interface is formed
Explanation: The degree of coherency can, however, be greatly increased if a macroscopically irrational interface is formed, that means the indices of the interfacial plane in either crystal structure are not small integers and the detailed structure is very complex in nature.
13. The interfacial energies of semi coherent interfaces are generally in the range of_______ (approximately)
a) 0-200 mJm-2
b) 200-500 mJm-2
c) 500-1000 mJm-2
d) 10000 mJm-2
Explanation: The energies of semi coherent interfaces are generally in the range 200-500 mJm-2. In general, coherent interfacial energies range up to about 200 mJm-2 and the incoherent interfaces are characterized by a high energy (500-1000 mJm-2).
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