This set of Phase Transformation Interview Questions and Answers for Experienced people focuses on “Interphase Interfaces in Solids – 2”.
1. Fully coherent precipitate is also known as GP zone, GP stands for _______
a) Gaff and Pearl
b) Gas and Pressure
c) Gatsby and Prince
d) Guinier and Preston
Explanation: GP for Guinier and Preston who first discovered their existence. This discovery was made independently by Preston in the USA and Guinier in France, both employing X-ray diffraction techniques.
2. Ag-rich zones in an AI-4 atomic % Ag alloy is an example of GP zone (Guinier and Preston).
Explanation: The above mentioned statement is true and these zones are silver rich FCC region within the aluminum-rich FCC matrix. And one more major thing to note is that the diameters of aluminum and silver differ only by 0.7% hence the coherency strains doesn’t make much contribution to the total free energy of the alloy.
3. From an interfacial energy standpoint it is favourable for a precipitate to be surrounded by____
a) High-energy coherent interfaces
b) High-energy incoherent interfaces
c) Low-energy incoherent interfaces
d) Low-energy coherent interfaces
Explanation: If we consider the interfacial energy standpoint it always favorable to be surrounded by low-energy coherent interfaces but when the crystal structures of the of the precipitates and matrix are different it is usually hard to find a plane in the lattice that is common to both planes.
4. Which among the following are most unlikely to arise when a second-phase particle is located on a grain boundary?
a) Precipitate can have incoherent interfaces with both grains
b) Precipitate can have coherent or semi coherent interface with one grain and an incoherent interface with the other
c) Precipitate can have a coherent or semi coherent interface with both grains.
d) Nothing is predictable
Explanation: The first two options are commonly encountered but the third possibility is unlikely since the very restrictive crystallographic conditions imposed by coherency with one grain are unlikely to yield a favourable orientation relationship towards the other grain.
5. If μ is the shear modulus of the matrix and V is the volume of the unconstrained hole in the matrix and the elastic energy does not depend on the shape of the precipitate, if so, calculate the elastic strain energy? (Assume the poissons ratio to be 1/3 and Misfit-Δ)
d) ΔG=4μΔ2 – V
Explanation: In general, the total elastic energy depends on the shape and elastic properties of both matrix and inclusion. However, if the matrix is elastically isotropic and both precipitate and matrix have equal elastic moduli, the total elastic strain energy ΔG is independent of the shape of the precipitate and is given as ΔG=4μΔ2 *V.
6. If the precipitate and inclusion have different elastic moduli the elastic strain energy is no longer shape independent.
Explanation: If the precipitate and inclusion have different elastic moduli the elastic strain energy is no longer shape independent but is a minimum for a sphere if the inclusion is hard and a disc if the inclusion is soft. The above statements are applicable to isotropic matrices.
7. What kind of misfit arises if the inclusion is the wrong size for the hole it is located?
a) Volume misfit
b) Lattice misfit
c) Vertical misfit
d) Lateral misfit
Explanation: When the inc1usion is incoherent with the matrix, there is no attempt at matching the two lattices and lattice sites are not conserved across the interface. Under these circumstances there are no coherency strains. Misfit strains can, however, still arise if the inclusion is the wrong size for the hole it is located in. In this case the lattice misfit has no significance and it is better to consider the volume misfit.
8. Calculate the punching stress P, if the constrained misfit is given as 1/3 and the shear modulus of the matrix is given as 5Pa?
Explanation: The punching stress P is independent of the precipitate size and depends only on the constrained misfit Ɛ. If the shear modulus of the matrix is μ, the punching stress is given as P=3μƐ. Substitute the respective value we get the required solution.
9. Under which circumstances does a glissile semi coherent interface gets formed?
a) If the dislocations do not have a Burgers vector that can glide on matching planes in the adjacent lattices
b) Depends on the extent of gliding
c) When the orientation of the plane is unmatching
d) If the dislocations have a Burgers vector that can glide on matching planes in the adjacent lattices
Explanation: It is however possible, under certain circumstances, to have glissile semi coherent interfaces which can advance by the coordinated glide of the interfacial dislocations. This is possible if the dislocations have a Burgers vector that can glide on matching planes in the adjacent lattices.
10. The formation of martensite in steel and other alloy systems occurs by the motion of_______
a) Incoherent-dislocation interfaces
b) Cropped-dislocation interfaces
c) Mixed-dislocation interfaces
d) Glissile-dislocation interfaces
Explanation: The formation of martensite in steel and other alloy systems occurs by the motion of glissile-dislocation interfaces. These transformations are characterized by a macroscopic shape change and no change in composition.
11. The point of intersection of the curves(X) in the graph represents?
a) Max radius
b) Cross radius
c) Critical radius
d) Inter-common point
Explanation: For a given lattice misfit, ΔG (coherent) and ΔG (non-coherent) vary with r as shown in above. When small, therefore, the coherent state gives the lowest total energy, while it is more favourable for large precipitates to be semi coherent or incoherent (depending on the magnitude of misfit). At the critical radius ΔG(coherent) = ΔG(non-coherent).
12. If the dislocation network (Burger and Interfacial) glides into the FCC crystal it results in a transformation of_____
Explanation: The glide planes of the interfacial dislocations are continuous from the FCC to the HCP lattice and the Burgers vectors of the dislocation’s, which necessarily lie in the glide plane, are at an angle to the macroscopic interfacial. If the dislocation network (Burger and Interfacial) glides into the FCC crystal it results in a transformation of FCC->HCP.
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