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Advanced Engineering Materials
Question 1 (15marks)
Determine, for the tensile curve shown in the following Figure.
(Sample dimensions L0=10mm, D0=5mm, D=3mm)
(1) Young’s modulus
(2) The ultimate tensile strength
(3) The yield strength (with a 0.2% offset)
(4) The uniform strain
(5) The total strain
(6) The reduction in area at the fracture
(7) The engineering stress-strain curve
(8) The true stress-strain curve
(9) The true uniform strain. Can you get the true fracture strain? If no, please explain why?
Question 2 (5 marks): If you could produce Fe-18%Ni alloy with a grain size of 50 nm, what would be the expected yield strength, assuming a Hall-Petch relationship.
Question 3 (5 marks): An iron-carbon alloy with the carbon content of 0.76wt% (the eutectoid composition) was held at 800ºC for long enough time to acquire a complete and homogeneous austenitic structure followed by one of the following 5 heat treatment processes.
(A) Quench to room temperature ……………………..( ) (B) Cool in air to room temperature ………………….( )
(C) Cool in furnace to room temperature …….……….( )
(D) Quench to room temperature and then reheat to 350ºC for 5 minutes ……………( )
(E) Quench to room temperature and then reheat to 700ºC for 20 hours …………….( )
Please put number 1, 2, 3, 4 or 5 in the parentheses above so that the process that produced the softest structure is marked with 1 and the hardest marked with 5.
Question 4 (10 marks)
The following Figure shows the Pb-Sn diagram.
(a) For a 40%Pb-60%Sn alloy at 220 ?, please determine (1) the phase present, (2) the phase composition (3) the relative amount of each phase
(b) For A Pb-Sn alloy with 80wt% Pb, please determine (1) the composition of the phase/s at immediately below 183?; (2) the mass fraction of the phase/s at immediately below 183?
(c) For a Pb-Sn alloy with 10 wt% Pb, heating it to 380 ?, please draw the typical microstructures with cooling
(d) For a Pb-Sn alloy with 61.9wt% Pb, heating it to 260 ?, please draw the typical microstructures with cooling
Question 5 (10 marks) For an alloy containing AlMg precipitates, calculate the critical spacing of precipitates at which the mechanism of hardening changes from particle shear to particle bypass. Take gAlMg=1,200mJm-2, Atomic radius (Al) = 0.143 nm, GAl=27Gpa
Question 6 (10 marks) The Al--Mg phase diagram is shown as follows. For an alloy with 5% Mg by weight, calculate the Al2Mg (ß) equilibrium volume fraction of precipitate if the densities of Al and Al2Mg are 2.7 and 2.3 g/cm3, respectively.
Question 7 (7 marks)
Explain (1) why martensite occur in the form of thin plates; (2) an Fe-30Ni wt% martensite is weak compared with martensite in Fe-0.2Cwt% alloy.
Question 8 (8 marks) A Fe-C alloy with the carbon content of 0.3 wt% is heated to 800°C until an equilibrium structure is obtained and then rapidly quenched to room temperature. (a) Please explain if this process can produce ferrite and Fe3C and why. (b) What is the purpose of tempering heat treatment of martensitic steels? (c)Explain why martensite does not present in the Fe-C phase diagram.
Question 9 (10 marks)
For a Ti alloy, when subjected to fatigue under a ???=140??????, the alloy showed the following Paris-type fatigue crack propagation relationship: da/dN (m/cycle) = 0.26 * 10-8(???)1.25
Where ??? is in MPa m1/2. Estimate the number of cycles required for the crack to grow from 1.2mm to 6.6mm.
Question 10 (20 marks)
In vehicle suspension design it is desirable to minimise the mass of all components. You have been asked to select a material and dimensions for a lightweight spring to replace the steel leafspring of an existing truck suspension.
The existing leafspring is a beam, shown schematically in Fig. B2. The new spring must have the same length L and stiffness S as the existing one, and must deflect through a maximum displacement d without failure. The width b and thickness t are not constrained.
Derive a performance index for the selection of a material for this application. Note that this is a problem with two free variables: b and t; and there are two constraints, one on safe deflection d and other on stiffness S. Use the two constraints to fix the two free variables.
Materials r [g/cm3] E [GPa] su [MPa]
Steel (4340) 7.85 207 1280
Aluminum (7075) 2.85 71 550
Titanium (Ti-6Al-Av) 4.43 114 900
Magnesium (AZ31B) 1.81 45 250
Nickel-based Super Alloy 8.89 204 950
Polycarbonate 1.20 2.4 67
60%-CF/EP 1.7 220 1200
SiC (sintered) 3.3 350 300
Red Oak 0.64 12.5 112
A truck spring. The spring must have a given stiffness S and be capable of deflecting through d without failure. The objective is to minimise its mass, m.