Malmö högskolaAvd. För Materialvetenskap

Ämneskod-linje MT7150Tentamensdatum 2006-01-12Skrivtid 14.15-18.15

Tentamen I Tillämpad MaterialteknikExamination in: Applied Engineering MaterialsNumber of questions: 8Examiner: Liu-Ying Wei (telephone 57136)

Aids: pocket calculator, “ENGLSK-SVENSK ORDLISTA”

Summary of questions:I. Crystalline structure 3p

II. Stress and strain 7pIII. Fe-C system 12pIV. Cast irons 6PV. Creep 10P

VI. Ni-base superalloys 6PVII. Polymers 3PVIII. Titanium alloys 12P

Total 59P

Pass minimum: 28P

Mark: 5: >49P; 4: >39P; 3: >28P

I. Crystalline structure (3p)

Sketch a (210) plane and a [112] direction in a cubic unit cell (see Fig. 1).

Fig. 1 A cubic unit cell

II. Stress and strain (7p)

Fig. 2 is a stress-strain curve obtained from a tensile test of Al alloy 2024. 1. Calculate the elastic modulus E of the alloy; (2p)2. Calculate the yield strength (Y.S), tensile strength (T.S) and percent

elongation of the alloy; (3p)3. Calculate the elastic recovery for the specimen upon removal of the

load at point A (see Fig. 2) in the curve, what is the plastic deformation at point A? (2p)

Fig. 2 stress-strain curve of Al alloy 2024

y

z

x

fracture

A

III. Fe-C system (12p)

1. Fe-Fe3C phase diagram is shown in Fig. 3; fill the phase name(s) in the regions (1) – (8). (4p)

2. What is the maximum solubility of carbon in the austenite? (1p)

3. What is a eutectoid reaction? Using the attached Fe-Fe3C phase diagram give the eutectoid temperature, eutectoid composition and the product of the eutectoid reaction in the Fe-C system. (2p)

4. A TTT diagram for AISI-SAE 1080 steel (containing 0.8 wt% C) is shown in Fig. 4. Predict the microstructures of the steel subjected to the following treatments i) rapidly quenched from austenitic region to 470ºC, ii) held for 4 seconds, iii) quenched to room temperature, iv) reheated to 300 ºC for 5 hours, v) cooled to room temperature. (5p)

Fig. 3 Fe-Fe3C phase diagram

1

2 3

45

6 78

Fig. 4 TTT diagram for 1080 steel

IV Cast irons (6p)

1. What is the function of silicon (Si) in grey iron? (1p)

2. What is the function of magnesium (Mg) in ductile iron? (1p)

3. Fig. 5 (A-D) illustrates the microstructures for four types of cast irons; identify which one is grey iron and which is ductile iron. (2p)

4. Describe the differences of the two cast irons, in microstructure and in mechanical property. (2p)

Fig. 5 Four different types of cast irons. Optical micrographs.

A B

C D

V. Creep (10p)

1. A common creep requirement is a 100h creep life to 0.2% strain, calculate the strain rate (ε ) for this creep requirement. (1p)

2. From the figure below (Fig. 6) make a comparison of the relative creep resistance of IMI Ex 834, 6246 and 64 (Ti-6Al-4V), by estimating their lifetime to failure at 500ºC and an applied stress of 300 MPa. Note that the Larson Miller parameter (LM = T(20+logt)) refers to temperature in Kelvin and time in hours and that the criterion for life is 0.2% strain. (4p)

3. What is the lifetime for alloy 6246 if the service temperature decreases to 450 ºC provided that the applied stress remains at 300 Mpa. (2p)

4. Explain qualitatively why, in general, short-term yield strength increases while creep strength decreases with decreasing grain size. (3p)

Fig. 6

III Creep

VI. Ni-base superalloys (6p)

1. The composition of the alloy Hastelloy X is as follow,Ni-0.1C-1Si-22Cr-18Fe-1.5Co-9Mo-0.6W

What are the main strengthening mechanisms in the alloy? Why? (3p)

2. What is the function of the γ’ phase in Ni-base superalloys, give the compound formula and draw a schematic unit cell for the γ’ phase. (3p)

VII Polymers (3p)

1. What are thermosetting polymers? (1p)

2. Vinylidene fluoride, as shown below, is one of the common thermosetting polymers, give the formulas for the mer and the monomer of vinylidene fluoride. (2p)

HF

- C - C F H

HF

- C - C - F H

HF

- C - C F H

HF

- C - C F H

HF

- C - C F H

HF

- C - C F H

VIII Titanium alloys (12p)

1. What is β stabilizer in titanium alloys? Give two examples of the alloying elements for the β stabilizer. (2p)

2. Using the attached (Ti-6Al)-V phase diagram (Fig. 7) to predict the microstructure of the alloy Ti-6Al-4V subjected to the following three different treatments, draw schematically the final microstructure, indicate the composition of each phase in the final microstructure and calculate the amount of each phase:

a. Held at 1020ºC for 2h + water quench to room temperature, (2p)b. Held at 1020ºC for 2h + slowly cooled to room temperature, (3p)c. Quench to 950 ºC + held at 950 ºC for 2h + air cool to room

temperature. (5p)

Fig. 7 (Ti-6Al)-V phase diagram

Vanadium wt%5 10 20

Temperature (ºC)

500

1000 β

α

α+β

Ms

Mf