Question 1 You are employed as a metallurgist by a motor car manufacturer that uses a very low carbon ferritic sheet steel for its body panels. The general manager calls you in and tells you that they have found a motor car paint that needs drying only at 60C for about 10 minutes instead of 170C for 20 minutes and that the company can save both capital and running costs by using the new paint. a. What would your response be? Motivate your answer and where possible quantitatively justify your response. b. Would a change in steel alloy content and/or processing cycle for such a new paint process be possible to consider? Motivate your answer. Question 2 a) Cross slip and climb are important phenomena which govern work hardening but also recovery and recrystallisation processes in metals. Discuss (be brief and to the point). b) The Figure below shows a micrograph of Al showing slip lines and grain boundaries after being subjected to plastic deformation. Answer the following questions. (i) Why does slip prefer to take place in close-packed planes and direction? (ii) What are the three stages of deformation that can possibly be observed in this material? Explain briefly. (iii) What is the most likely stage of slip is observed in this micrograph? Motivate. (iv) How many slip systems are present in this material? Give two examples of valid slip systems in this material. (4) (v) Give the number of active/observed slip systems in each of the grains 1 to 4. (vi) Comment on the orientation relationship between grains 1 and 2, and between 2 and 3 . (vii) Slip direction is always in the direction of closest packing. Why? (viii) If one of the grains above is (111), what are the directions of slip? (3) Question 3 A 50mmm long circular rod of the AISI 1015 hot rolled steel with a diameter of 12.5mm is loaded in tension to failure. From the test it is learnt that the yield strength Rp=315MPa, the tensile strength Rm =420MPa, the elongation at fracture ef=39% and the reduction in area Z=51%. It is also observed that 45% of the total elongation was uniform in nature prior to the onset of necking. a) What is the true strain at fracture? (5) b) Calculate the true stress at the point of onset of plastic instability (10) c) What is the Hollomon strain hardening exponent n and the strength coefficient K for this steel? (5) [20] Question 4 A component that is made of Nichrome-2 alloy with the properties below, is subjected to an uniaxial stress of 30MPa at a temperature of 800C. Grain size: 100m Atomic volume: b3=1.11029m3 Activation energy for creation of vacancies: 285000J/mol Diffusion constant D0=1.7104m2/s Grain boundary diffusion rate at 800C:2.81015m2/s Power law creep activation energy: 285000J/mol Stress exponent for creep n:4.6 Structure factor S=0.5 for in MPa and time in s R=8.316J/molK Boltzmann's constant k:1.3811023J/K a) Calculate the steady state creep rate for power law creep and for diffusion creep by both the mechanisms of lattice vacancy diffusion as well as grain boundary vacancy diffusion. (15) b) What is the dominant mechanism for creep in this alloy at 800C at a stress of 30MPa ? (10) [25] a) The maximum solubility of C in -Fe is 2.14wt% at 1147C whilst that in -Fe is only 0.022wt% at 727C. However, the atomic packing factor of -Fe (74%) is higher than that of -Fe (68%) meaning -Fe has more voids and should have higher solubility of carbon. Give a brief explanation why the solubility of C is higher in -Fe than that of Fe. [2 marks] b) Explain why creep rate is higher in ferrite compared to austenite. [2 marks] c) Draw graphs for the following relationship: Hardness (H) vs time (t) for 100% recrystallization for the two steels and explain the shapes of the graphs in detail: i. 20% cold worked FeC alloy and ii. the same FeC alloy but now 40% cold worked, on the same axis. [3 marks] d) Repeat question (b) but by plotting the relationship of Ductility (D) vs time ( t ) for 100% recrystallization for the two steels after the same cold rolling conditions. [3 marks] FIGURE 9.I1 Adjusted energy-temperature curves and shear fracture-temperature curves for 38-mm-thick plate of A283 steel tested with Charpy V-notch specimens of various thicknesses. Absorbed energy defined at 5.2J/2.5mm(3.8ftlb/0.1in.) of specimen thickness. 6 (Reprinted from Welding Journal by permission of the American Welding Society.) (e) Consider a center cracked plate of Al alloy 7075-T6 subjected to tensile loading with the following characteristics: 2a(cracksize)=254mmw(width)=1.27mB(thickness)=12.7mmYS=531MPaUTS=579MPaKIC=33MPam1/2 i) Determine whether the plate will sustain the load of F=712000N, ii) If so, determine the safety factor, iii) the critical crack size for this load, iv) The plastic zone at the crack tip, v) If this alloy was used for a pressure vessel, would it meet the "leak-before-break" criterion? Motivate. (5) [50] Question 6 a) Name at least three BCC metals. (3) b) On which general slip system do BCC metals slip and why? c) Sketch a typical slip plane from a BCC lattice and indicate which atom will actually touch each other on this plane and which ones will not touch. (4) d) Determine whether the specific system [111](110) is a valid slip system in BCC metals. Use (i) a mathematical proof as well as (ii) a sketch of a BCC lattice unit cell. (6) e) Why do we find "wavy" slip bands on the surface of a BCC metal that has been plastically deformed? f) By using sketches, show how a (i) edge and (ii) a screw dislocations can respectively overcome work hardening obstacles. g) You are given specimens of Al, one fully annealed with a dislocation density of about 107cm/cm3 and the other a cold worked sample with a dislocation density of about 1012cm/cm3. For Al the shear elastic modulus GM=28.4GPa and the lattice constant a=0.405nm. Calculate in both cases the following: (a) The average distance between adjacent dislocations in both samples; and (10) (b) The expected driving force (energy in Joules per unit volume) for recrystallisation when annealing the cold worked material. Assume that the elastic energy locked up in a dislocation equals E=GMb2 per unit line length of the dislocation, where b is the Burger's vector and GM is the shear modulus. (8) [40] Question 7 A turbine blade in a jet engine operates at a peak temperature of 700C. Due to the high speed rotation of the engine during operation the blades are subject to an axial tensile stress of 200MPa. From the literature it appears to you that the blade at this temperature, obeys the creep law of: =Anexp[G/RT] where G (the activation energy for thermally driven creep) =200kJ/mol, A (a structure constant) =12.1s1MPan,R is the universal gas constant =8.316J/molK and n (the stress sensitivity) =1, is the axial stress in MPa and is the creep strain rate in s1. There is a clearance of 4mm between the casing surrounding the blade and the end of the 200mm long blade, measured from its root fixture on the turbine rotor to its tip. (i) What typical alloys are used in the turbine blade industry? (2) (ii) Where will the maximum stress arise within the blade and which type of stress(es) will these be? (5) (iii) You are requested to specify the most suitable inspection intervals and the expected life time of this blade. Motivate your answers and state your assumptions clearly. (12) (iv) Your company's chief designer wishes to increase the performance limits of the engine through either increasing the operating temperature by 100C or by increasing the rotation speed by an amount that will increase the maximum stress by 20%. Which will be the better choice in terms of the life time of the blade? Motivate your answer. (10) (v) You have made your recommendation in question (iv) above but in a specially designed test engine on a test bench, it was found that the blade failed prematurely. From your knowledge of deformation mechanism maps and the different creep mechanisms, speculate on the likely reasons for the early failure? Can you recommend steps to be considered to meet with the higher performance standards of the engine? Also sketch a likely creep deformation mechanism map for this alloy. (16) [45] Question 8 In brief discuss in bullet form how each one of the following factors will influence the toughness of steel: a) Ferrite grain size b) Nature of inclusions and precipitates c) Phase transformations during processing d) Specimen size Where possible use schematic diagrams [20] Question 9 An infinitely large sheet is subjected to a gross stress of 350MPa. There is a central crack 5/cm long and the material has a yield strength of 500MPa. a) Calculate the stress intensity factor at the crack tip (8) b) Calculate the plastic zone size at the tip of the crack (8) c) Your new assignment as a failure analyst is to evaluate critically the introduction of fracture mechanics concepts in place of current Charpy impact test procedures. Consider both advantages and disadvantages of such a decision in your brief summary to your supervisor. You answer should be brief and in bullet form. (9) [25]
