Malmö högskola Avd. För Materialvetenskap

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1 Malmö högskola Avd. För Materialvetenskap Ämneskod-linje MT7150 Tentamensdatum Skrivtid Omtentamen I Tillämpad Materialteknik The 4th Examination in: Applied Engineering Materials Number of questions: 9 Examiner: Liu-Ying Wei (telephone /57136) Aids: pocket calculator, ENGLSK-SVENSK ORDLISTA Summary of questions: I. Crystalline structure 7p II. Stress and strain 8p III. Strengthening of superalloys 9p IV. Polymers 7p V. Titanium alloys 15p VI. Fe-C system 10p VII. Creep 10p VIII. Precipitation strengthening 6p IX. Ceramics 8p Total Pass minimum: 80P 38P Mark: 5: >66P; 4: >53P; 3: >38P

2 I. Crystalline structure (7p) 1. Copper (Cu) has a face-centered cubic (fcc) structure, the atomic radius of an Cu atom r = nm, calculate the cube edge length a of the unit cell. (3p) 2. Sketch a (212) plane and a [120] direction in a cubic unit cell (see Fig. 1). (4p) z y X Fig. 1 A cubic unit cell 1

3 II. Stress and strain (8p) 1. Fig. 2 is a stress-strain curve obtained from a tensile test of a type 304 stainless steel. The inserted curve gives the detailed results in the early stage of the test. Calculate the elastic modulus E of the steel; (2p) 2. Determine the yield strength (YS) and tensile strength (TS) of the Al alloy (see Fig. 2); (2p) 3. In the tensile test a cylindrical specimen was used (see Fig. 3), 12.7 mm in diameter (d) and 80 mm in length (l). When a load (P) of N is applied, determine: (1) the strain of the specimen; and (2) its elongation (D l). 1MPa = 1N/(mm) 2. (4p) P 600 PLot of stress vs. strain 500 Stress, MPa Stress, MPa PLot of stress vs. strain l Strain, mm/mm Strain, mm/mm d P Fig. 2 Tensile stress-strain behaviour for Fig. 3 A cylindrical specimen in 304 stainless steel. the tensile test. 2

4 III. Strengthening of superalloys (9p) 1. The composition of Inconel 718 superalloy is as bellow: Ni-0.04C-0.2Si-0.2Mn-19Cr-19Fe-0.9Ti-0.5Al-5.1Nb Discuss functions of the four alloying elements respectively (1) Cr, (2) Al, (3) C and (4) Nb. (6p) 2. Fig. 4 shows the microstructure of superalloy U 500 (Ni-0.1C-0.3Si- 0.2Mn-18Cr-2Fe-2.9Ti-2.9Al-18Co-2Mo) after aging for 1400h at 840 C. Give the names of phases pointed by arrows in the picture. (3p) Phase I Phase II Fig. 4 Microstructure of superalloy U500 after aging at 840 C for 1400h. SEM micrograph. 3

5 VI. Polymers (7p) 1. Give at least 5 typical characterises of polymers? (5p) 2. The following is the structure of a monomer for formaldehyde (CH 2 O), give the structure of polyacetal (CH 2 O) n. (2p) H C = O H 4

6 V. Titanium alloys (15p) 1. The following Ti-Al-V diagram plots transformation temperature versus V concentration for two fixed level of Al, 7%Al and 4%Al. (1) Distinguish the two sets of phase boundaries (in solid line and in dash line) which belong to 7%Al and 4%Al respectively; (2P) (2) Explain the reason. (2P) 2. A titanium alloy Ti-6Al-4V was subjected to the following heat treatments: (1) held at 1020ºC for 2 hours; (2) quench to 900 ºC and held at 900 ºC for 2h; (3) slowly cooled to room temperature, using the attached (Ti-6Al)-V phase diagram (Fig. 5) answer the following questions: a. Give the composition of each phase in the alloy when the alloy has been held at 900 ºC for 2h. (2p) b. Calculate the amount of each phase in the alloy when the alloy has been held at 900 ºC for 2h (2p) c. Sketch the microstructure of the alloy when the alloy has been held at 900 ºC for 2h (2p) d. Sketch the final microstructure of the Ti alloy when it has been slowly cooled down to room temperature; give the chemical composition for each phase in the alloy. (5p) 5

7 Temperature (ºC) 1000 b a 500 a+b M s M f Vanadium wt% Fig. 5 (Ti-6Al)-V phase diagram 6

8 VI. Fe-C system (10p) 1. Fig. 6 (a-b) illustrates the microstructures for grey iron and white iron. Give the name of phases pointed by arrows in the pictures. (3p) Phase 1 Phase 2 Phase 3 a b Fig. 6 Typical microstructures of (a) grey iron; (b) white iron 2. Fe-Fe 3 C phase diagram is shown in Fig. 7. What phases are to be found in an alloy containing 2% C (wt %) at (i) 1300 ºC (ii) 1000 ºC (iii) at 600 ºC. (3p) 3. A TTT diagram for AISI-SAE 1080 steel (containing 0.8 wt% C) is shown in Fig. 8. Design a process for heat treatment in order to obtain a final microstructure of the 1080 steel containing about 50% fine pearlite (a+fe 3 C) and 50% martensite. (4p) 7

9 Fig. 7 Fe-Fe 3 C phase diagram Fig. 8 TTT diagram for 1080 steel (the dash-line is the 50% completion curve) 8

10 VII. Creep (10p) 1. A common creep requirement is a 100h creep life to 0.2% strain, calculate the strain rate (e& ) for this creep requirement. (2p) 2. From the figure below make a comparison of the relative creep resistance of three Ti alloys 6242, 6246 and Ti-5Al-2.5Sn by estimating their lifetime to failure at 450ºC 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. (5p) 3. What is the service temperature limitation for alloy 6242 if the lifetime is ten times long as in question 2 provided that the applied stress remains at 300 MPa. (3p) Temperature (ºC) for total plastic strain 0.2% in 100 h Stress MPa Ti-5Al-2.5Sn Larson Miller parameter, T(20+logt) x

11 VIII. Precipitation strengthening (6p) Cu-containing Al alloys are usually precipitation strengthened. With help of the phase diagram in Fig. 10, describe how to perform a precipitation strengthening (or age-hardening) treatment for alloy Al-4Cu. (Do not forget to give the suggested temperature and time for each step) a+l a a(al)+q (Al 2 Cu) time Fig. 10 Al-Cu phase diagram 10

12 IX. Ceramics (8p) 1. Give at least 4 typical characteristics of ceramic materials? (2p) 2. What is the essential difference between a ceramic and a glass? (2p) 3. Fig. 10 (a) and (b) are two stress-strain curves obtained from tensile tests of a typical metallic material and a ceramic material. Identify the curve that is obtained from a ceramic material and give the reason. (4p) (a) (b) Fig. 10 (a-b) Stress-strain curves obtained from tensile tests 11