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Examination (open book) – June 2022

QUESTION 1
You are hardening a very thick plate of bcc Fe by increasing the carbon content in its surface layer. The plate is heated to a set temperature and kept for a set period of time in the presence of a C-rich gas (e.g., methane) which maintains a constant C concentration at the surface (this is the original treatment). Schematically draw the C concentration profile (i.e., the C concentration as a function of the distance from the surface) at the end of the treatment. Where will the greatest diffusion flux be found and where the smallest (you are required to justify)? The thickness of the hardened layer is defined as the distance from the surface within which the C concentration is above a set value. How to double the thickness of the hardened layer achieved by the original treatment if the set temperature is not to be changed (you are required to show details of your calculation/reasoning)? Add the corresponding C concentration profile in the original diagram with the set C value and the thicknesses of the hardened layer in the two treatments clearly marked. In another case, the original set time is not to be changed but the temperature can be set at a different value. Estimate the new set temperature to achieve tripling of the original hardened layer thickness if the set temperature in the original treatment is 600 °C. Hint: Your estimation should be based on the figure below, showing a plot of the logarithm of the diffusion coefficient versus reciprocal of absolute temperature for a number of diffusion systems.

QUESTION 3
Referring to the Pb-Sn phase diagram below, for an alloy of Pb-30 wt% Sn, calculate the amount of liquid following equilibrium cooling from 300 °C to just reaching the eutectic temperature. If the total amount of the alloy is 100 g, how many grams of Sn are in the solid phase at this moment?
For an alloy of Pb-15 wt% Sn, sketch the microstructure following equilibrium cooling from 300 °C to room temperature? Under non-equilibrium cooling, however, eutectic structure has been observed in the microstructure at room temperature. Explain why (you may draw on the diagram).

QUESTION 6
For engineering alloys, what are the most common strengthening mechanisms? Which metallic alloy shows the highest yield strength? (Hint: Consult the various tables for Al, Ti, Fe and Ni alloys.)
(3 marks)
QUESTION 7
(a) Plot two fcc unit cells for anions. Identify a tetrahedral interstitial site in one and an octahedral site in the other. Identify two adjacent layers of (111) planes in each to show that the two types of interstitial sites are formed between them. Why can it be said that the interstitial structures in the fcc and hcp structures are the same?
(4 marks)

Cite the phases that are present and the phase compositions for the following alloys:
(a) 15 wt% Sn – 85 wt% Pb at 100°C.
(b) 25 wt% Pb – 75 wt% Mg at 425°C.
(c) 85 wt% Ag – 15 wt% Cu at 800°C.
(d) 55 wt% Zn – 45 wt% Cu at 600°C.
(e) 1.25 kg Sn and 14 kg Pb at 200°C.