# Engineering materials

**Paper , Order, or Assignment Requirements**

Section A (answer all the questions):

- At constant pressure, for a ternary A-B-C alloy, if F = 2, answer how many phases coexist?

[3 marks]

- The microstructure of a binay Al-Si cast alloy at 575C consists of primary α (maximum Si

solibility 1.6wt%) and eutectic (12.6wt%Si) structures. If the mass fractions of these two

microconstitutients are 0.45 and 0.55 just after equilibrium solidification, respectively,

determine the composition of the alloy and the fraction of Si in the alloy.

[7 marks]

- Explain why yield strength is a structure-sensitive property. [7 marks]

- A single-phase zone-refining operation is to be carried out on a uniform and long bar (1000

mm). The zone is 20 mm long. Assuming the initial impurity concentration Co = 0.01% and

the partion ratio k = 0.01, determine the distance over which the resulting impurity

concentration is below 0.007% after first pass.

[6 marks]

- Liquid Al (melting temperature 660C) is undercooled to 640C at which solid Al nuclei are

assumed to form homogeneously. Find the critial radius of nuclei, given liquid-solid surface

energy is 0.167 J/m3, latent heat of fusion is 321 kJ/kg, density is 2,700 kg/m3. Estimate the

number of Al atoms needed to make a critial sized nucleus during this homogeneous

nucleation, given atomic weight of Al is 26.9815 amu.

[6+6 marks]

- Explain how tensile strength of a polymer depends on (a) temperature, (b) strain rate, (c)

molecular orientation and (d) degree of polymerization

[8 marks]

- A unidirectional fiber composite consists of 55% by volume of continous type 1 carbon

fibers in a matrix of epoxy. Find the maximum tensile strength of the composite, assuming

that the matrix yields in tension at a stress of 60 MPa.

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