a. You recall that the aluminum alloy has a stiffness of 10 E6 psi that steel has a stiffness of 30 E6 psi. To think about and compare the properties of these two materials:a. sketch a NEAT tensile engineering stress-strain curve where the two materials are placed on the same diagram; use a solid line (______) for the aluminum material and a dashed line (- – – – – – ) for the steel. (6 points)
b. label the ultimate tensile strength on the aluminum curve – not the value … just the place on the curve. (1 point)
c. label the offset (0.2%) yield point on the aluminum curve. (1 point)
d. label the elastic modulus (don’t need to put the value of this, just the “definition”) on the aluminum curve. (2 points)
e. how might you be able to compare the toughness of the two materials since you are concerned that they might get banged around a lot in this application (describe as it relates to the stress strain curve). (3 points)

b. You also remember that one of these materials (1100 Aluminum or 1040 Steel) will respond better to cold working processes than the other. Which material is this and why does it respond better to cold working? (3 points)

c. On the stress-strain curve above, using a dotted line (….), show how the shape of the stress-strain curve will change for the material you chose after it had been cold formed (lets say to 50% but it really doesn’t matter that much) into the product. If these properties changed (UTS, offset yield and modulus), what was the microstructural mechanism responsible for the change in properties? (6 points)
a. UTS
b. Offset yield strength
c. Modulus

d. You also recall that if needed, you can form the material by cold working and then annealing it. You recall that annealing has three steps. List these steps, describe the microstructural changes that occur in the material at each of these three steps, and how they affect the strength and modulus of the material. (12 points)

Stage Microstructural changes Affect on strength
(+/=/-) Affect on modulus (+/=/-)

baseline baseline

e. Considering that this is an outdoor application, there may be some advantage to subjecting the material to only the first step of the annealing process. Why? (3 points)

f. You also remember that recrystallization involves a nucleation and growth process where the new grains nucleate on the boundaries of the dislocations.

a. What type of nucleation is this? (1 point)

b. If you were to cold work the material only 20% and cold work the material 70%, how would this increase in % cold work affect the recrystallization temperature and why? (3 points)

c. Comparing the steel and the aluminum’s melting point, and assuming they would be exposed to the same amount of cold working, which material would have the higher recrystallization temperature and how did you determine this? (3 points)

d. Nucleation of the new grains requires diffusion of atoms to the potential nucleation sites. All else being equal, which material, the steel or the aluminum, would experience easier nucleation as a result of faster diffusion of the atoms? Why? (3 points)

e. Thinking more about the role of diffusion in nucleation, what are the mechanisms and types of diffusion and how do these affect the rate of diffusion? (15 points)
Name Brief description Relative rate, high, med or low (compared to other mechanism or type)
Mechanism

Mechanism

Type
Type
Type
g. Your final selection. Fill out the chart. Rank the material as 1 or 2 with 2 being the desirable (good) quality and 1 being the undesirable quality. (12 points)

Property Weight factor Steel Steel (score = rank * weight factor) Aluminum Aluminum (score = rank * weight factor)
Light weight 5
High strength 7
Good corrosion resistance 8
Stiffness 5
Ability to be cold formed 10
Total (add weight factor * rank for each property)

h. Which material do you select for this application? (2 points)