Individual Assignment Submission date: 8 May 2015 (Friday)

KNJ1433 Engineering Materials – S1 2014/2015

Assignment 1

Part A (50%) CO1

1. Below are listed the atomic weight, density, and atomic radius for three hypothetical alloys.

For each determine whether its crystal structure is FCC, BCC, or simple cubic and then justify

your determination. A simple cubic unit cell is shown in Figure 1.

Figure 1: Hard-sphere unit cell representation of the

simple cubic crystal structure.

Alloy Atomic Weight (g/mol)

Density (g/cm3)

Atomic Radius (nm)

A 43.1 6.40 0.122

B 184.4 12.30 0.146

C 91.6 9.60 0.137

(15 marks)

2. Determine the expected diffraction angle for the first-order reflection from the (110) set of

planes for FCC aluminium when monochromatic radiation of wavelength 0.1431 nm is used.

(8 marks)

3. There are several techniques in strengthening single phase metals.

I. Explain what Strain Hardening is and why it is termed ‘cold-working’? With the aid of

diagram, explain how elastic strain recovery influences the yield strength.

(10 marks)

II. Explain what Solid Solution Strengthening is? With the aid of diagram, explain how the

impurity plays the role in strengthening a metal.

(10 marks)

4.

I. A 20 mm in diameter cylindrical specimen of some metal alloy is stressed elastically in

tension. A force of 18,000 N produces a reduction in specimen diameter of 7 x 10-3 mm.

Find the Poisson’s ratio for this material if its elastic modulus is 100 GPa.

(2 marks)

II. Consider the brass alloy for which the stress–strain behaviour is shown in Figure 2. A

cylindrical specimen of this material 10.0 mm in diameter and 101.6 mm long is pulled in

tension with a force of 10,000 N. If it is known that this alloy has a value for Poisson’s ratio

of 0.35, compute (a) the specimen elongation, and (b) the reduction in specimen diameter.

(5 marks)

Part B (25%)

5. An electrochemical cell is composed of pure copper and pure lead electrodes immersed in

solutions of their respective divalent ions. For a 0.6 M concentration of Cu2+, the lead electrode

is oxidized, yielding a cell potential of 0.507 V. Calculate the concentration of Pb2+ ions if the

temperature is 25oC.

(5 marks)

6. For each form of corrosion - (a) stress corrosion, and (b) erosion-corrosion, do the following: - Describe (i) why, (ii) where and (iii) the conditions under which the corrosion happens;

and

- Give method(s) that may be taken to prevent or control it.

(20 marks)