Dispersion Strengthening and Eutectic Phase Diagrams

1. Dispersion Strengthening and Eutectic Phase Diagrams Chapter 10 – 4th Edition Chapter 11 – 5th Edition

2. In the last chapter… • We talked about alloys that form from materials that are completely soluble in each other • We also learned not all materials are completely soluble • In this chapter we’ll talk about what happens when there are multiple solid phases present

3. Dispersion Strengthening • The existence of two or more phases can strengthen a material • When that happens it’s called dispersion strengthening • Matrix – present in large amounts • Precipitate – present in smaller amounts

4. Guidelines • Matrix should be soft to provide ductility • Precipitate should be hard to provide strength – more precipitate results in higher strength • Precipitate should be discontinuous Precipitate Matrix

5. Guidelines • Smaller, more numerous precipitate particles give higher strength because they interfere with slip • Round particles are better than sharp particles – Sharp particles can initiate a crack Not so Good Good

6. Guidelines • Smaller, more numerous precipitate particles give higher strength because they interfere with slip • Round particles are better than sharp particles – Sharp particles can initiate a crack Not good at all Good

7. Intermetallic Compounds • Often precipitates are intermetallic compounds – metals that are bonded to each other • Stoichiometric Compounds • Non-stoichiometric Compounds have a range of compositions (often a blend of two or more stoichiometric compounds)

8. 3-Phase Reactions • In order to have a precipitate we must have 2 solid phases • Matrix • Precipitate • There are a number of ways to create these phases – but we are going to look at the following 3-Phase reactions • Eutectic L-> S1 + S2 (this chapter) • Eutectoid S1 -> S2 + S3 (next chapter)

9. Let’s start with the Eutectic L S2 S1 S1 + S2 L → S1 + S2

10. Eutectoid S1 S3 S2 S2+ S3 S1→ S2+ S3

11. Liquid Eutectic Solidus Solidus Liquidus a Temperature b Solvus Solvus Y Wt% Y X The Eutectic Phase Diagram a + L b + L a + b

12. Liquid a Temperature b Y Wt% Y X The Eutectic Phase Diagram a + L b + L a + b In this portion of the diagram only a 2 phase reaction occurs – similar to what we observed in Chapter 9 The strengthening mechanism is solid solution strengthening

13. Lead – Tin Phase Diagram Liquid a a + L b + L Temperature b a + b Amount of a Amount of b Y Wt% Y Sn Pb X Sn

14. Liquid Liquid + a a Cooling Curve Temperature a + b Time

15. a Solidification of a Lead-Tin Alloy Lead – Tin Cooling Curve Lead – Tin Phase Diagram Liquid Liquid Liquid + a Temperature a a + L Temperature b + L b a + b a + b Amount of a Amount of b Y Time Sn Wt% Sn X Pb

16. a Liquid L + a a + b How Does the Solid Form? We’ll talk in the next chapter about how to manipulate the particle growth to achieve the optimum distribution of the precipitate This alloy is dispersion strengthened

17. Lead – Tin Phase Diagram Liquid Eutectic a a + L b + L Temperature b a + b Amount of b Amount of a Y Wt% Y Sn Pb X Sn

18. Liquid a + b L + a + b Cooling Curve for a Hypoeutectic System Liquid + a Temperature Time

19. Lets not look at how the solid forms just yet – it’s a little complicated • Instead lets look at what happens if your over all composition is the eutectic composition

20. Lead – Tin Phase Diagram Eutectic Composition Liquid a a + L b + L Temperature b a + b Y Wt% Y Sn Pb X

21. Liquid a + b L + a + b Cooling Curve for a Eutectic System Plumber’s solder is a eutectic alloy of Pb and Sn. Why? Temperature Low, sharp melting point Time

22. Liquid How Does the Eutectic Solid Form? L + a + b Eutectic Solids are strong but generally have little ductility

23. Strength of Eutectic Alloys • Each phase is solid solution strengthened • Grain size affects the strength – well inoculated melts have smaller grain size • Interlamellar spacing

24. Cobalt-Carbon Eutectic Scanning electron microscope image of cobalt-carbon eutectic. There is an irregular arrangement of graphite needles in a cobalt rich-phase matrix. http://www.npl.co.uk/server.php?show=conMediaFile.1613

25. Interlamellar Spacing Interlamellar Spacing • The spacing is controlled by how long the grains are allowed to grow • You can limit the spacing by reducing the solidification time – by removing heat faster • Small interlamellar spacing results in high strength

26. Lead – Tin Phase Diagram Liquid Eutectic a a + L b + L Temperature b a + b Amount of b Amount of a Y Wt% Y Sn Pb X Let’s go back to the hypo eutectic composition Sn

27. How does solidification occur in a hypoeutectic system? L + a + b Liquid L + a Primary Phase is a (Proeutectic) Eutectic Microconstituent

28. This image is 60-40 lead-tin solder showing the dark dendrites of primary lead surrounded by Pb-Sn eutectic. Scale bar is 667 micrometers Pb-Sn HypoEutectic Composition Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University

29. Higher magnification of solder showing varying structure of the Pb within the two phase Pb-Sn eutectic, which surrounds the primary lead dendrites. Scale bar is 100 micrometers long. Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University http://www.mse.mtu.edu/slides/slide_2.html

30. Lead – Tin Phase Diagram Liquid Eutectic a a + L b + L Temperature b a + b Amount of a existing as the proeutectic or primary microconstituent Y Wt% Y Sn Pb X As the alloy cools from the eutectic temperature, there is a rearrangement WITHIN each microconstituent of the amount of  and β Amount of Eutectic Microconstituent Sn

31. Lead – Tin Phase Diagram Liquid Eutectic a a + L b + L Temperature b a + b Y Wt% Y Sn Pb X As the alloy cools from the eutectic temperature, there is a rearrangement WITHIN each microconstituent of the amount of  and β The amount of βexisting in the eutectic microconstituent changes with temperature The amount of existing in the eutectic microconstituent changes with temperature Sn

32. Lead – Tin Phase Diagram Liquid Eutectic a a + L b + L Temperature b a + b Y Wt% Y Sn Pb X The PRIMARY microconstituent has no beta at the eutectic temperature, but particles grow within in it as the temperature decreases Amount of β in the primary phase Amount of  in the primary phase Sn

33. 50-50 alloy of Pb and Bi - HypoEutectic Note: This is lead and bismuth – but the idea is the same EutecticMicroconstituent Primary – Lead rich phase Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University http://www.mse.mtu.edu/slides/slide_2.html

34. Lead – Tin Phase Diagram Liquid a a + L b + L Temperature b a + b Y Wt% Sn Pb X Hypereutectic Composition

35. What happens during the solidification of a hypereutectic system? L + a + b Liquid L + b Primary Phase is b Eutectic Microconstituent

36. Hypoeutectic Hypereutectic Dispersion Strengthening Solid Solution Strengthening Which is Best? • It depends on your design requirements Eutectic Hypothetical Alloy Strength Composition

37. Let’s do some calculations • Assume you have an alloy that is • 40 wt% Sn • 60 wt% Pb • What is the composition at room temperature of the •  (Pb rich) phase? • β (Sn rich) phase? • What fraction of the alloy is • ? • β ? • What fraction is • Eutectic microconstituent? • Proeutectic microconstituent?

38. This version of the phase diagram will be easier to use At room temperature the Pb rich phase is approximately 2% Pb At room temperature the Pb rich phase is approximately 2% Sn At room temperature the Sn rich phase is approximately 100% Sn

39. Fraction of each phase At room temperature the Pb rich phase is approximately 2% Pb  Fraction Sn rich phase (β) Fraction Pb rich phase ()

40. Fraction of each microconstituent At room temperature the Pb rich phase is approximately 2% Pb  Fraction Proeutectic microconstituent Fraction Eutectic microconstituent

41. Other Phase Diagrams Containing 3-Phase Reactions • All we’ve looked at are phase diagrams with a eutectic • Remember, a eutectic is a point where L-> a + b • There are lots of other possible 3 phase reactions, and lots of much more complicated phase diagrams

42. 3-Phase Reactions • Eutectic L-> S1 + S2 • Eutectoid S1 -> S2 + S3 • Peritectic S1 + L1 -> S2 • Peritectoid S1 + S2 -> S3 • Monotectic L1 -> S1 + L2 • We will be primarily concerned with Eutectic and Eutectoid Reactions

43. Consider the following hypothetical phase diagram taken from Askeland (pg 270)

44. Monotectic Eutectic Peritectic Eutectoid Peritectoid L1  + L1 d g + d L1 + g L2 L1 + L2 L2 + b L2 + g g b Temperature g + b a + g a a + b a + m m m + b X Y

45. Three Component Phase Diagrams • So far we have only looked at 2 component alloys • Three component phase diagrams can be very complicated – they require a three dimensional phase diagram • A 2-D slice can tell us a lot though

46. Z 20 % 80% 40 % Wt% X 60% 60 % 40% 80 % 20% 20% 40% 60% 80% Wt % Y Wt% Z Y X

47. Ternary Phase Diagrams • Used to plot the liquidus • An isothermal plot shows the phases present

48. http://sti.srs.gov/fulltext/ms9900751/ms9900751.html