Hallo allerseits,
nachdem ich mich nichtmehr an alle Fragen erinnern kann, würde ich mich freuen, wenn wir hier zusammen ein Gedächtnisprotokoll auf die Beine stellen könnten, das Forum und die Filebase zur Vorlesung sind ja bisher doch eher mau.
Allgemeines: Die Prüfung fand dieses Jahr schriftlich statt, da wesentlich mehr als 30 Studierende angemeldet waren und eine mündliche Prüfung so für Professor Oechsner nichtmehr sinnvoll bewältigbar ist. Die Prüfung dauerte 60 Min, es gab insgesamt 100 Punkte. In der Regel werden 48 Punkte zum Bestehen gebraucht, Prof. Oechsner hat betont, dass die leichten Fragen in Summe bereits mehr als 48 Punkte ergeben und die Klausur damit eigentlich kein Problem darstellen sollte. Ich fand die Klausur wie erwartet sehr fair: Viele Fragen lassen sich mit angemessenem Lernaufwand gut beantworten und einer guten Note sollte nicht viel im Wege stehen, es gibt allerdings auch ein paar schwierigere Fragen, die wohl eine Differenzierung zu sehr guten Noten ermöglichen sollen. Es gab insgesamt 21 Fragen auf 11 Seiten.
english: This year's exam took place in a written format, as more than 30 students declared interest in attending the exam and that's his usual threshold where oral exams are not doable anymore. The timeframe was 60 minutes, the exam had a total score of 100 points. 48 points are usually used as minimum value to get a passing grade, he emphasized that easy questions make up more than 48 points though, so it shouldn't be too hard. All in all, I'd say the exam was more than fair and there's only a few tough questions to distinguish between good and very good grades. There were 21 questions on 11 pages in total.
Questions:
1. Define high temperature materials
2. Please draw the strain-time diagramm for creep and the according strain-rate - time diagramm below and name the three notable zones - primary, secondary and tertiary creep
3. Please name three reasons why Ni-alloys are preffered over Fe-alloys for temperatures above 700°C - e.g. Nickel has better creep properties than Fe, Nickel stays in fcc unit cell -> no volume change, ...
4. Why does hot corrosion (of Type I) have a temperature range in which it occurs, that has an upper limit? - Molten salts are required for the reaction to happen, these might become gaseous at a certain temp -> no reaction possible above that point
5. Look at these two coating structures and name the application process that was used to create them. - APS and EB-PVD, same image as in lecture slides
6. There are monolithic and CMC ceramics. Which tend to show brittle behaviour under uniaxial load? - Monolithic
7. A Strain (in precent) - Cycle Number diagramm was given with two graphs for 200°C and 500°C curves (500°C was lower), a strain limit of 1% is given. Calculate Dj (Damage according to Palmgren-Miner) for both states
8. What problem can occur if two materials are in contact that show different diffusion coefficients (phrasing was more complex), explain! - kirkendall effect
9. Explain why oxidation reactions might happen more quickly at higher temps even though the ellingham diagram tells us it is easier at lower temps and name the relation Ellingham is based on - Gibbs-Helmholtz-Equation, mass transfer through oxide scale limits reaction rate, mass transfer, especially diffusion, profits a lot from higher temps -> Arrhenius law!
10. Name 3 phenomena that can be observered above the transition temperature (time dependent phenomena) - Creep, Fatigue, Microstructural Changes, Corrosion/Oxidation
11. Define the transition temperature and explain what it means - Refer to diagram in lecture slides, T trans is the temperature at which the Graph for Ru10000h meets the graph of Rp0,2, from that point on time dependent mechanisms govern component life, not yield strength
12. Explain the difference between chemical and physical bonds between atoms, please explain what happens with electrons for each process - Chemical bonds mean sharing or handing of electrons between atoms while physical bonds are governed by intermolecular forces and partial charges of atoms and the resulting repelling/attracting forces
13. How can you increase the diffusion flux while respecting thermal boundaries - increase concentration delta or low activation energy needed for diffusion paths, e.g. lower bonding force, less densely packed lattice etc.
14. Name one defect for each dimension (0-D,1-D,2-D) - Interstitial atoms, screw dislocation, grain boundary
15. What is the thermodynamic process for equalizing a temperature gradient in a solid material called - heat conduction
15b What material parameter influences that process? - heat conductivity a/lambda
16. How does the diffusion rate change for an iron based material when changing the lattice structure from bcc to fcc - fcc is more densely packed, diffusion rate is lower as it gets more difficult to pass through grain boundaries
17. At higher temperatures, materials change their grain size, do they grow or shrink? why does this happen? - they grow naturally, ostwald-ripening t^(1/3) growth law -> larger grain boundaries throughout the component means less surface area and therefore less surface energy -> energetically preffered
18. Picture of a load cycle with all variable names removed (9 gaps), name them accordingly! - sigma_max, sigma_min, sigma_m, sigma_a, sigma_a, T, delta_sigma + axis: sigma & time t
That's all I can remember for now. It would be great if people who also attended today's exam could add the questions missing and let me know about different solutions/errors in my notes 