Typical Exam Questions Solutions In solutions to Questions 2 and 3 the underlined areas represent items required in answer for one mark.

These do not need to be word for word – as long as it is clear that the student has understood the

concept

Question 1.

These are short answer questions (Eg. 1 word – 1 or 2 sentences).

Each question carries 2 marks.

a) From a basic tensile stress-strain curve for a material, how would you

determine the value for Young’s Modulus?

By determining the gradient of the linear (elastic) portion of the curve

b) Give a brief description of fretting corrosion.

Damage as a result of reciprocating relative movement, of very small amplitude, between two contacting surfaces

c) Define hardness of a material.

Resistance to plastic deformation (also resistance to abrasion or scratching)

d) If a tensile load of 6.0 newtons is applied to a flat tensile specimen of length

60 mm, width 10mm and thickness 3.0 mm, what is the tensile stress, in MPa,

on the specimen?

0.2 MPa

e) What is the difference between ferrite and martensite?

Ferrite has a Body Centred Cubic (BCC) crystal structure and is soft Martensite has a Body Centred Tetragonal (BCT) crystal structure and is hard

f) Work hardening (or strain hardening) is a useful strengthening mechanism but

it has which two of the following disadvantages?

A. is only useful for two-phase materials

B. decreases the ductility of the material

C. is not suitable if the material is to be used at elevated temperature

D. is only applicable to single crystals

B and C

g) Why are alloys of eutectic composition attractive for casting and as filler

metals in soldering and brazing?

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Because they have the lowest discrete melting point of the given alloy system, and solidify to a relatively high strength structure

h) What is the common name given for a ceramic material in the non-crystalline,

or amorphous, state?

Glass

i) Name two necessary constituents of concrete.

Any two of; Cement paste Aggregate Water Pores

Question 2.

a) In relation to the metal alloys, describe the following conditions;

i. Complete solid solubility

ii. Partial solid solubility

iii. Insolubility

Complete Solid Solubility implies complete solubility (mixing and interchangeability) of the atoms of the alloying species. The two (or more) types of atoms have to be able to exist in the same crystal structure. Atom sizes and valence electron structure of the species or elements need to be similar. Partial Solid Solubility results when there is saturation limit for one of the alloying species in the other, and this saturation limit depends on temperature. So, as the temperature is lowered and solubility decreases a two-phase material forms from the initial single phase. Insolubility means that the materials are so different in nature (atomic size, valence electron structure, etc.) that they are insoluble – ie. Separate into two distinct elemental phases.

b) Name and describe the four single phases that can be found in the steel portion

of the iron – carbon equilibrium phase diagram.

The four phases are; Ferrite (α – iron), which is the low temperature BCC structure of carbon interstitially dissolved in iron

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Austenite (γ – iron), which is the elevated temperature FCC structure of carbon interstitially dissolved in iron Delta – Ferrite (δ – iron), which is the elevated temperature BCC structure of carbon interstitially dissolved in iron Cementite (Fe3C), which is the iron-carbon intermetallic compound that occurs with 6.67% Carbon content.

c) What is the primary strengthening mechanism in the high strength “aircraft

quality” aluminium alloys?

The primary strengthening method is Precipitation Hardening (or Age Hardening) produced by heat treatment

Question 3.

a) Briefly explain why there may be significant scatter in the fracture strength for

some given ceramic material, and why fracture strength increases with

decreasing specimen size.

Because the mechanical properties of ceramics are dependent on the presence and size of internal and surface flaws and cracks. The lower the volume of the ceramic, ie. the smaller the piece, the lower the probability that the piece contains a critically sized flaw and therefore less chance of a reduction in strength – so an increase in strength is observed. Hence the introduction of a probability factor requires the use of a statistical analysis and therefore the observed scatter in data on fracture strength.

b) In a fibre-reinforced composite, what are the primary roles of;

i. the matrix?

ii. the fibres?

The Matrix supports and transmits loads to he fibres and provides ductility and toughness The fibres provide strength by carrying most of the load

Question 4.

Draw a phase equilibrium diagram representing the binary alloy system between two

metals „A‟ and „B‟. The following data apply;

i. Pure „A‟ melts at 960º C and pure „B‟ melts at 1320º C

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ii. Both elements have limited solid solubility in each other, and complete liquid

miscibility. The limit of solid solubility of „B‟ in „A‟ is 15% at a eutectic

temperature of 640º C. The limit of solid solubility of „A‟ in „B‟ is 7% at

640º C.

iii. Solubility of „B‟ in „A‟ reduces to 10% at 400º C and 6% at 0º C.

iv. Solubility of „A‟ in „B‟ reduces to 3% at 300º C and to zero at 20º C.

v. The system has one eutectic composition at 63% „B‟, 37% „A‟

vi. An isothermal line drawn at 860º C will intercept solidus lines at compositions

96% „B‟ and 4% „B‟, and liquidus lines at 35% „B‟ and 72% „B‟.

Assume the solid solution of „B‟ in „A‟ is known as - phase, and the solid solution

of „A‟ in „B‟ is known as - phase. Label each phase field and boundary line in the

diagram.

From the phase diagram that you have constructed, for the specific alloy composition

20% „B‟ 80% „A‟, at the temperature of 800º C;

a) Indicate the composition of the - phase that would be solidifying at that

temperature

b) Indicate the composition of the liquid phase remaining at that temperature

c) Calculate the proportions of - phase and liquid phase that would be present

at that temperature.

Solution

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(NB. Maximum marks for this part is 14, not 12 as incorrectly stated above)

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