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Physical Metallurgy 8th Lecture

Physical Metallurgy 8th Lecture. MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140. Review. Melting point of compounds increases with increasing negativity difference. Electro- negativity. a) T m. Memory Help : T melt up ‘cause more covalent/ionic.

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Physical Metallurgy 8th Lecture

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  1. Physical Metallurgy8th Lecture MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140

  2. Review Melting point of compounds increases with increasing negativity difference Electro- negativity a) Tm Memory Help : Tmelt up ‘cause more covalent/ionic

  3. Decreases with increasing electronegativity difference b) Solubility HW 8-1 Using the table of electronegativities given in lecture 7, what is the electron negativity difference of Cu-Zn and Cu-Sn ? Is above rule true ? A) If yes, why B) If not, why not Repeat above exercise with the melting point of the beta phase

  4. The more stable an intermetallic phase, the more limited the solid solubility For your own amusement (or if you are a born metallurgist) you may want to check this in the Cu-Zn and Cu-Sn system by looking at the existence width of the beta phase :-) Memory help : Ni3Al

  5. Higher valence more soluble in low valence than reverse Memory help : “Blowing up the Fermi sphere till it hits !

  6. Phase Boundaries To turn you into metallurgists.. A “lingo” slide =>

  7. The idea that the solid solubility always increases with temperature (because temperature favors entropy) is true for almost all systems but fails at high temperatures in a technically very important system: Dopants in Silicon

  8. Ouch…. Above ~ 1250 C the solubility of solutes in Si falls with increasing temperature. True for all elements, except Boron and Carbon which behaves like expected from thermo... Ask Professor Thompson why that is so…..

  9. First: Do not confuse above bwith the b in Pb-Sn. It refers to an intermediate, not terminal phase. Second: The temperature dependence of solubility is the foundation of precipitation hardening Third: The reverse, it’s the cause of an important failure mode !

  10. Short excursion into the real world of metallurgy :-) Memo: G-P zones !

  11. The good Semisolid casting is the forming of near net shape metal parts by using semisolid raw material Such alloys are great stuff !!!! Phase diagram for aluminum and silicon showing the shaded semisolid region and a line showing the fixed composition of aluminum alloy 356

  12. Bonus HW problem google Aluminum Silicon + Head + Porsche 911 + performance handbook I am a firm believer to let students discover interesting engineering on their own .. Why does Porsche like Si to be in the Al it uses for cylinder blocks ?

  13. The bad and the ugly • Overaging (coherent precip to incoherent precip transition) • Spatially inhomogeneous dissolution Al contacts to Si get involuntarily heat treated during the contact anneal (Why is that necessary). Looking at the phase diagram, would you worry ?

  14. Bronze and the bronze age If you study ancient metallurgy you will find: They knew an amazing amount !

  15. And went to great length to get the ingredients: ….they were addicted to tin as we are to oil, importing it at great cost over (then) incredible distances… from the edge of the known world !

  16. One more excursion into bronze…. Four bronze horses pulling a bronze chariot Qin dynasty, Xi’an Pretty nifty work with bronze, 1500 years ago !

  17. Phase boundaries I hope you know that by now !

  18. By far the most important interstitial is C in Fe so I will spent some time on it

  19. We note: 1. Maximum eq. solubility, fcc,(g, austenite) 2.08 wt % 2. Maximum eq. Solubility, bcc, high T,d, 0.09 wt%, low T,a, (ferrite) 0.025 wt% 3. Carbon in excess of equilibrium solubility can be quenched in, most interesting “stuff” results. However, the structures are thermodynamically unstable Heat will “do them in” which is why the World Trade Center collapsed.

  20. There is no room, thus, the fcc cell is distorted. Distorted cells are good dislocation obstacles. Why ? Hint: Think coherent, think stress field

  21. There are two (2) possible sites, known as tetrahedral and octahedral. Carbon prefers octahedral

  22. Tetrahedral Carbon in ferrite Not a preferred position, isotropic stress field The pics are from QM calculations of the Cambridge (UK) group

  23. Octahedral Carbon in Ferrite X => C interstitial positions Preferred position Directional stress field “Stress dipole”

  24. Cancellation of long range stress fields HW 8-2 Make a drawing of several unit cells, containing 2 C atoms, such that their long range stress field partially cancels.

  25. H-R phases Electro-negativity Size factor

  26. H-R (Hume Rothery) phases Memo: In modern theory it’s the interaction of 2kf with the reciprocal lattice - think Friedel oscillations ! Did you find that slide in the appendix of lecture 5 ?

  27. Intermetallic compounds The famous Ni3Al intermetallic compound High T super alloys are based on the Ni-Al-Ti phase diagram, which is known in great detail for different temperatures. Example below. http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html

  28. You want g’ not g . About Ni0.76Al16Ti8

  29. Solid solutionOrdered alloy HW 8-3 What is the composition of an alloy represented by the red dot ? (Practical metallurgy 101 .. How to read a phase diagram…… :-) You wonder why g’ face is cubic ? It tells you that the interfacial energy with g is very low. That also helps to suppress Oswald ripening

  30. One more interesting intermetallic compound Gallium is a metal. Arsenic is a metal … remember ? … the existence range of their intermetallic compound is next to zero ! HW 8-3 Using the table provided in lecture ? , what is the difference in electronegative between Ga and As ? What is it between Ni and Al ? How good are the rules ? =>

  31. Size factor compounds • Hydrides, Carbides, Borides, Nitrides…. • Rules of thumb • May be solid solutions, may be interstitial compounds • Nice fit ri < 0.91rm (m => matrix, host) • Distorted fit 0.41rm < ri 0.59 rm

  32. I have a slightly different view why the transition metal carbides, borides tend to be cubic • The transition metals easily give up electrons • Carbon, Boron want electrons to fill their shell • => Strong ionic component • Ionic compounds are cubic

  33. Fe3C has two important and different crystal structures: Cementite (oP16, Do11), Bainite (hP8 Space Group: P632 ) see appendix, if interested Fe3C (cementite) Black => cementite White => ferrite If you wish you can think about Fe3C as “the end point of a standard binary” phase diagram, in which the “liquid” is g and the eutectic is a eutectoid…... ~ 0.8% C Loves to form at grain boundaries of austenite when hypereutectoid steel is cooled..

  34. TiN has the NaCl structure. • TiNx compounds with x ranging from 0.6 to 1.2 are thermodynamically stable. • Lots of uses. • Tools left • Jewlery • Diffusion barrier in IC against Cu TiN

  35. Laves Phases

  36. Definition: Laves phases have composition AB2, where the A atoms are ordered as in diamond, hexagonal diamond, or a related structure, and the B atoms form tetrahedra around the A atoms. (Barrett and Massalski, pp. 256-9), http://cst-www.nrl.navy.mil/lattice/struk/laves.html

  37. Mg-Zn Solid solutions plus Laves phase. Mg-Zn no commercial applications. Mg with Al and Zn (ternary) is used for low T, light weight castings. See appendix

  38. The term is sort of a predecessor of the H-R rules based on the observation that CuZn, Cu3Al, Cu5Sn all had the b brass structure. The all had 1.5 valence electrons per atom (we now know why). This led to “guessing new phases” in copper, Ag and Au, from known ones by postulating that the “unknown should have the same phase as the “known” provided it had the same e-/atom ratio. That led the term “electronic phase” and “electronic compound”

  39. d-electron alloy design. • It is a semi-empirical method based on • bond order • d-orbit energy • The bond order is the number “bonds”* as a function of electron density ; a concept extended from molecules to crystals. The bond energy** is then the bond order times the bond strength , that is the hopping integral • * in a H2 molecule the bond order 0 if you have o electrons for the two protons, 0.5 if you have only 1 electron for the two protons,, 1 if you have 2 electrons for the two protons, 0.5 if you force 3 electrons in (one is now antibonding) and 0 if you force in four electrons. • ** a factor 2 too tedious to explain here Modern extension Plot for Ti alloys Existence range for a and b phase in a bond order vs d-orbital energy level diagram. Arrows indicate how these ranges move with alloying

  40. The End

  41. Lightweight, low cost, die Mg alloy castings • Mg matrix + aluminum + and up to 1% zinc (AZ alloys) or aluminum and magnesium without zinc (AM alloys) offer a good compromise of strength, castability and corrosion resistance. Recently the became famous for superplasticity • Drawbacks; • Can’t be welded • Poor creep resistance and poor high temperature strength • Strength comes from Mg17Al12 intermetallic precipitates (ß-phase, cubic) with an amazingly low (for I.M. melting point (462° C.) * Google it. Of interest only to born metallurgists interested in low cost forming. These alloys do grain boundary sliding

  42. Wow…. In Japan , the University President is a co-author !!!!

  43. Crystal structure of cementite (DO11) cst-www.nrl.navy.mil/ lattice/struk/Bainite.html

  44. Basis Vectors of Cementite

  45. Crystal Structure of Bainite

  46. Basis Vectors Bainite

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