MAT2320 - SPRING 2019-2020 EXAM PAPER
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MAT2320
SECTION A
1. Figure 1 is a phase diagram of the system composed of substances A and B. Answer the following questions (a) ~ (e) regarding this diagram.
Figure 1 - Phase diagram of A-B system.
(a) What phase(s) are forming in Region 1 and Region 2 shown in Figure 1 when
the composition of the system is X2? Name the phase(s) and their fractions for
both Region 1 and Region 2. [4 marks]
(b) Provide correct information for (B1)-(B4) in the following sentences to
complete the description for the Gibbs free energy of regular solutions.
Regular solutions are a type of ( B1 ) solution. Differing from a mechanical mixture, they consider the ( B2 ) in their Gibbs free energies. They also consider ( B3 ) to be zero, and this is the main difference from ( B4 ) solutions,
making their Gibbs free energy expression simpler. [4 marks]
(c) Equation 1 is the quasi-chemical expression for the excess Gibbs free energy of the regular solution of substances A and B. Write down the expected mathematical relations among νAB, νAA and νBB when the temperature is T1 and T2 and show how they were obtained from Equation 1.
编GXS = ZNX A XB (|vAB -vAA +vBB )|
\ 2 ) -------- (1)
Z: number of atoms surrounding A or B
N: Avogadro number
XA: Composition of A
XB: Composition of B
νAB : Attraction between A and B
νAA : Attraction between A and A
νBB : Attraction between B and B
[4 marks]
(d) Figures 2 (D1), (D2) and (D3) show the configurational entropy of mixing and the excess Gibbs free energy for the solid solution of substances A and B. Identify the figures representing the temperatures T1 and T2 . Also, identify the matching Gibbs energy curves from Figures 2 (d1), (d2) and (d3).
Figure 2 - Configurational entropy of mixing, excess Gibbs free energy and Gibbs energy curves for different mixtures of A and B.
[4 marks]
(e) Compare the chemical potential of substance A for the compositions X1, X2
and X3 at temperature T1 and complete the following two sentences.
- The chemical potential of A becomes largest when the composition is…
- The chemical potential of B becomes largest when the composition is… [4 marks]
2. Figure 3 indicates the binary phase diagram for a system composed of substances C and D. Answer the following questions (a) ~ (f) based on this figure.
Figure 3 - Phase diagram of C-D system
(a) Provide correct information for (b1)-(b4) in the following sentences to complete
the description for the Gibbs free energy of ideal and non-ideal solutions.
In the expression for the Gibbs free energy, the ideal solution contains only the term corresponding to the (b1) to account for the effect of mixing, whereas the non-ideal solution contains an additional term. This is because the ideal solution ignores (b2) whereas the non-ideal solution takes it into account, and it appears as (b3). When the value of (b3) is significantly positive, it may cause
a separation of the solution phase, which is called (b4). [4 marks]
(b) According to the information shown in Figure 3, which phase(s) are forming at 1000oC when the compositions of C and D are 30 and 70 mol%, respectively? Name the phase(s) and give their fractions present. Also, estimate the
composition of C and D in the phase(s). [5 marks]
(c) What are the ideal compositions of C and D to obtain a mixture of liquid and solid solution phases at a ratio of (liquid):(solid solution) =1:3 at 1000 oC? [3 marks]
(d) Which of the figures 4(e1) ~ 4(e6), indicates the correct relation between Gibbs energy curves of the liquid phase (GL) and solid solution phase (GS) at 600 oC, 1000 oC and 1400 oC? Select one figure for each temperature. [3 marks]
Figure 4 - Gibbs energy curves of Solid solution and Liquid phases.
(e) Compare the chemical potentials of substance D at 1000 oC when the
composition of D is 40 mol% and 60 mol%, and describe the relation between them. Also, how would this change when the temperature is increased to 1400 oC? [5 marks]
SECTION B
Fundamental Constants:
Electron mass, me = 9.11 x 10-31 kg
Electron Charge, e = 1.6 x 10-19 C
Speed of light, c = 3 x 108 m/s
Planck’s Constant, h = 6.63 x 10-34 J.s
Boltzmann Constant, kB = 1.38x10-23 J/K
Wien Constant, k = 2.9 x 10-3 m K
Rydberg constant, RH = 109677 cm-1
Permittivity of free space, εo = 8.854 x 10-12 F/m
3. (a) Describe the main components of a scanning electron microscope (SEM), starting from the top and working down the chamber, explaining why each of them is required. [8 marks]
(b) Describe the two different types of electron guns used in SEMs, including the
differences between them. [4 marks]
(c) Explain how changing the gun parameters changes the quality of a SEM image. [4 marks]
(d) From the table below, determine the divergence semi-angle for each of the guns. [4 marks]
Cathode Material |
Brightness (A/cm2.sr.kV) |
Current Density (A/cm2) |
Lifetime (hr) |
W |
1x104 |
3 |
100 |
ZrO/W(100) |
1x107 |
1000 |
>5000 |
4. (a) A transmission electron microscope (TEM) provides two different methods for determining a sample’s composition, EELs and EDX. Explain each technique, including in your answer details of where the detector is positioned, the required thickness range of the sample, and any advantages of each technique. [10 marks]
(b) For EDX, the Cliff-Lorimer equation has to be used. Give the equation, and
explain why it is required. [4 marks]
(c) Figure 5 below shows the EDS spectra for (i) Fe50Co50 and (ii) Ni81Fe19
For each material determine the Cliff-Lorimer factor [6 marks]
(i)
(ii)
Figure 5: Intensity of EDX for (i) FeCo and (ii) NiFe
SECTION C
5. (a) Using the TTT diagram provided in Figure 6, for a plain carbon steel with 0.8wt.% C, define the constituents of the final microstructure for the following time-temperature heat treatments. In each case, assume that the specimen was austenitised at 900°C and held at this temperature long enough to achieve a homogeneous austenitic structure prior to cooling.
i. Rapidly cool to 400°C, hold for 1000s then water quench to room temperature. [2 marks]
ii. Quench to room temperature. [2 marks]
iii. Rapidly cool to 600°C, hold for 1 hour and water quench to room temperature [2 marks]
(b) Explain the reasons for the shape of the TTT diagram for the austenite to
pearlite and bainite transformation as a function of temperature, as shown in
[5 marks]
Figure 6 - TTT diagram of a 0.8%C steel
(c) How does the TTT diagram for an alloy steel differ from that of a eutectoid steel? Therefore, explain why it is easier to form a fully martensitic structure in the alloy steel. [3 marks]
(d) What is the effect of Cr and Mo additions to the steel on the hardness developed during tempering of the martensite? Explain the reasons for the differences in tempering response between a plain carbon steel and a steel alloyed with Cr or Mo. [6 marks]
6. (a) Figure 7 shows low and high magnification images for a nickel base superalloy. Define the phases labelled 1, 2, 3. [3 marks]
(b) What is the structure of the main precipitate phase? Why is it important for high
temperature strength? [4 marks]
(c) Describe the interface structure between the precipitate and the matrix. Define the energies associated with this interface and briefly indicate what effect the interface structure has on these energies. [6 marks]
(d) In a Ni based superalloy the lattice parameter of the matrix and the precipitate are chosen to be as close as possible. Explain why this is and explain what effect a larger difference in lattice parameter would have. [7 marks]
Figure 7 - Low and high magnification images of a nickel base superalloy
SECTION D
7. (a) i. Describe the evidence available that indicates that the martensite transformation is diffusionless. [5 marks]
ii. Indicate why, in steels, as-quenched martensite is hard and brittle. Describe the process by which the brittleness of martensite is removed. [5 marks]
(b) i. Name and briefly describe two point defects found in ceramics. How do
these differ from point defects in metals? [2 marks]
ii. What causes some phases to appear brighter under an optical microscope than others? Why do pores appear dark? [3 marks]
iii. What is the difference between thermal etching and chemical etching? What features does each process reveal in a microstructure and why? [5 marks]
8. (a) Provide the correct information for (A)-(F) for Figure 8, below, which describes distinct sintering mechanisms seen in ceramic processing.
Figure 8
[6 marks]
(b) Which sintering mechanism is commonly utilised in the densification of
electroceramics? Describe the effect of rapid and slow cooling on the phases
present within the microstructure. [4 marks]
(c) What is a glass-ceramic? Briefly discuss why a two-stage heat treatment process is required to achieve the desired microstructure. [4 marks]
(d) The schematics in Figure 9 below show common glass-ceramic microstructures. Name each type of microstructure and briefly comment on the effect each has on the physical properties (mechanical properties, transparency, thermal expansion etc.) of the final material.
[6 marks]
Figure 9
2022-08-19