MECH 340 Final, Spring 2006

Introduction To Materials Science, Chapter 1, Introduction University of Virginia, Dept. of Materials Science and Engineering 18 Polymer composite materials: reinforcing glass fibers in a polymer matrix. Composites University of Virginia, Dept. of Materials Science and Engineering 24 Thin crystalline platelets grown from solution - chains fold back and forth: chain-folded model Polyethylene The average chain length is much greater than the thickness of the crystallite University of Virginia, Dept. of Materials Science and Engineering 9 Cr, �-Fe, Mo have this crystal structure Spring 2006 Final Exam Tuesday, June 6, 2006 Duration: 3 hours Open Book Exam Write clearly your derivations and answers on the answer sheet Name: ID#: Engineering Material Spring 2006 Problem I [20 Pts] 1. Explain why a thermoplastic polymer with high crystallinity is desirable for conditions of exposure to high temperatures 2. When nylon reinforcing cord was first used in rubber tires, the tires developed “flat spots: as the vehicle remained stationary overnight. What accounts for this phenomenon, and why is the return to the round shape more rapid in the summer that in the winter 3. Explain the following effects with respect to the glass transition temperature Tg (a) an increase in pressure on the polymer increases its glass transition temperature (b)The ration Tg/Tm increases as the percentage of crystallinity decreases in a copolymer 4. Explain why a high degree of crystallinity may be difficult to achieve in brached polymers, in atactic polymers and in copolymers 5. Name three mechanisms of plastic deformation. 6. Name four main mechanisms of strengthening for metals. 7. What is recrystallisation? Is it possible to use the phenomenon of recrystallisation for the benefit of strengthening a metal? If yes, explain why. 8. Discuss the mechanical properties of ceramics and explain the brittleness of ionically bonded ceramics. Mech340: Engineering Material Spring 2006 Problem II [15 Pts] Shown below is a face-centred cubic unit cell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a) Write down the miller indices of the shaded plane according to the atomic co- ordinate system given. (2 points) (b) Mark ALL <111> directions in this unit cell and label them with their indices. (4 points) (c) Calculate the atomic packing factor of this structure (show your calculation procedure). (4 points) (d) Pure iron undergoes an allotropic phase transformation from fcc (austenite) to bcc (ferrite) on cooling at 912°C. Calculate the percent change in atomic size accompanying this transformation, assuming that the volume change is zero. (5 points) Mech340: Engineering Material 3/7 Spring 2006 Problem III [20 Pts] (a) Explain phases of austenite and ferrite in terms of their structures and chemical make- ups. What is pearlite? (b) What are the solubility limits of carbon in the austenite and ferrite? (c) Where do carbon atoms reside inside austenite (name(s) of the position(s))? • Use a graphical presentation to illustrate the position(s) (illustrate only one if more than one). • How many of these positions are there in one unit cell? • If all these positions are occupied by carbon atoms, what would be the solubility limit, in terms of weight percentage (show your calculation)? • If this value (the value calculated right above) is drastically different from that read from the phase diagram, offer your brief explanation. (d) Calculate the density of pure Fe at room temperature, assuming an atomic radius of Fe of 0.124 nm. (e) What would you call a Fe-0.4%C alloy (in engineering terms)? What phases will it contain when cooled from molten state to (i) 1000 °C, (ii) 730 °C, and (iii) 600 °C (you do not need to determine the portions of each phase)? (f) Determine the weight percentages of pro-eutectoid ferrite and pearlite in this alloy at room temperature. Mech340: Engineering Material 4/7 Spring 2006 Problem IV [15 Pts] A cylindrical piece of steel 25 mm in diameter is to be quenched in moderately agitated oil. Surface and center hardnesses should b at least 55 and 50 HRC, respectively. Which of the following alloys will satisfy these requirements 1040, 5140, 4340, 4140 and 8640? Justify your choices 314 Tin would almost always be hot-worked. Even deformation at room temperature would be considered hot-working inasmuch as its recrystallization temperature is below room temperature (Table 7.2). Tungsten is hard and strong at room temperature, has a high recrystallization temperature, and experiences oxidation at elevated temperatures. Cold-working is difficult because of its strength, and hot-working is not practical because of oxidation problems. Most tungsten articles are fabricated by powder metallurgy, or by using cold-working followed by annealing cycles. Most aluminum alloys may be cold-worked since they are ductile and have relatively low yield strengths. Magnesium alloys are normally hot-worked inasmuch as they are quite brittle at room temperature. Also, magnesium alloys have relatively low recrystallization temperatures. A 4140 steel could be cold-worked in an over-tempered state which leaves it soft and relatively ductile, after which quenching and tempering heat treatments may be employed to strengthen and harden it. This steel would probably have a relatively high recrystallization temperature, and hot-working may cause oxidation. 11.D8 A one-inch diameter steel specimen is to be quenched in moderately agitated oil. We are to decide which of five different steels will have surface and center hardnesses of at least 55 and 50 HRC, respectively. In moderately agitated oil, the equivalent distances from the quenched end for a one-inch diameter bar for surface and center positions are 3 mm (1/8 in.) and 8 mm (11/32 in.), respectively [Figure 11.16(b)]. The hardnesses at these two positions for the alloys cited (as determined using Figure 11.13) are given below. Surface Center Alloy Hardness (HRC) Hardness (HRC) 1040 50 30 5140 55 47 4340 57 57 4140 56 54 8640 56 52.5 Thus, alloys 4340, 4140, and 8640 will satisfy the criteria for both surface and center hardnesses. 11.D9 (a) This problem calls for us to decide which of 8660, 8640, 8630, and 8620 alloys may be fabricated into a cylindrical piece 57 mm (2-1/4 in.) in diameter which, when quenched in mildly agitated water, will produce a minimum hardness of 45 HRC throughout the entire piece. The center of the steel cylinder will cool the slowest and therefore will be the softest. In moderately agitated water the equivalent distance from the quenched end for a 57 mm diameter bar for the center position is about 11 mm (7/16 in.) [Figure 11.16(a)]. The hardnesses at this position for the alloys cited (Figure 11.14) are given below. Center Alloy Hardness (HRC) 8660 61 8640 49 8630 36 8620 25 Mech340: Engineering Material 5/7 Spring 2006 Problem V [10 Pts] A solder manufacturer wishes to make a batch of solder having the eutectic composition. On hand is a supply of 250 kd of electrical solder scrap containing 60 wt% Sn-40wt% Pb, and 1250 kg of plumbing solder scrap containing 60wt%Pb -40wt%Sn. If all o the scrap is to be melted, how much pure Sn should be added to achieve the desired Sn composition. Mech340: Engineering Material 6/7 Spring 2006 Problem VI [20 Pts] Iron Titanate, FeTiO3 forms in the ilmenite crystal structure that consists of an HCP arrangement of O2- a. What type of interstitial site will the Fe2+ ions occupy, explain. b. Which type of interstitial site will the Ti4+ ions occupy, explain c. What fraction of the total tetrahedral sites will be occupied d. What fraction of the total octahedral sites will be occupied Mech340: Engineering Material 7/7