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QUESTION 1
The phase diagram of a system with A and B atoms is shown in Figure 3A. Answer the following questions with reference to this phase diagram:
Figure 1 Phase diagram A-B
a) Describe the different steps in a heat treatment which will yield a homogeneous distribution of fine ß phase in an a matrix for an alloy with 10 at% B knowing that the starting material has been solidified under rapid cooling conditions. For each of the steps in your procedure, explain what the changes are in the microstructure and why the temperature you use is appropriate to obtain such changes.[10 marks]
b) Is it possible to produce an alloy of A and B which consists of a ß matrix strengthened with fine a precipitates? Justify your answer.[3 marks]
c) Is a material with 50% B suited to precipitate strengthening? Justifiy your answer.[3 marks]
d) As shown in Figure 2, at 200 °C, a phase with 10 at% B has a primitive cubic crystal structure with lattice parameter, a, equal to 0.400 nm and ß phase with 90 at% B has a primitive tetragonal crystal structure with lattice parameters a = 0.400 nm, b = 0.395 nm, and c = 0.600 nm. Describe how the ß crystal will orient itself relative to the a crystal when a ß precipitate forms at this temperature, and explain why that orientation is favourable assuming you only need to consider the energy of the interphase interfaces that are forming. Using a similar argument explain why ß precipitates would form as plates and not as spheres.
Figure 2 Crystal structures
QUESTION 2
a) State the general equation for the free energy change upon formation of a nucleus of a solid phase inside another solid and define each term in the equation. Discuss how this equation can be used to explain why solid phases often form with an orientation relationship to the surrounding solid.
[5 marks]
b) An experimental study of a new alloy shows that precipitates form as small cylinders, see diagram. The height of these cylinders remains equal to the diameter at all…