The structure of cast metals

Transcription

1 The structure of cast metals Ralph E. Napolitano Department of Materials Science & Engineering Iowa State University Ames, Iowa Metals June 1, 2005 IOWA STATE UNIVERSITY Materials Science & Engineering

2 Let s do an experiment. Let s heat a pure material so that it is a liquid at a uniform temperature, let it cool uniformly, and measure the temperature vs time. T ( C) T m Freezing begins Freezing ends t (sec) If we cool very slowly so that the system is always at equilibrium, then freezing will occur isothermally at T m.

3 Let s do an experiment. Realistically, we do not observe an isothermal arrest. T ( C) T m T t (sec) Even at the same temperature, the liquid phase contains more heat than the solid. This heat Is liberated upon freezing.

4 Driving force and the importance of rate

5 Driving force and the importance of rate In our freezing example, the heat may be liberated too quickly to be liberated efficiently. Still Mother Nature tries to optimize this efficiency using any and all means available. You are all very familiar with one consequence of such optimization

6 How do metals freeze? Metals freeze in much the same way that water freezes into the familiar snowflakes. The Rasmussen & Libbrecht Collection

7 Goals for this lecture I. Fundamentals of solidification II. The structure of cast metals III. A brief history of casting technology IV. Modern casting techniques

8 How do metals freeze? Here we compare the snowflake structures to a transparent organic metal-analog. M.E. Glicksman, NASA-IDGE, The Rasmussen & Libbrecht Collection

9 Perspective What is so special about the solid-liquid interface in metals?

10 Early observation of dendrites

11 Early observation of dendrites

12 Critical Issues The critical issues are essentially the same for all (most) phase transformations Thermodynamics Kinetics Phase stability (phase diagrams) The energy of interfaces Quantification of driving forces Thermal and chemical partitioning The diffusion of heat and solute The kinetics of atomistic processes Nucleation kinetics Interface kinetics

13 Critical Issues The objective for today is to look at the evolution of cast microstructures from what may be a new viewpoint. Competition Selection Instability (dynamic) Local equilibrium

14 Natural selection If you want to study genetic would you use antelope?

15 Natural selection Fruit flies Atoms vibrate at ~10000 GHz, - quite a prolific fruit fly!

16 Competition and natural selection In nature, everything is a competition, with many phenomena occurring simultaneously.

17 Dynamic Instability BUT This is only a side view.

18 Dynamic Instability Side view Front section view

19 Dynamic Instability Side view Front section view

20 Dynamic Instability Side view Front section view

21 Dynamic Instability Side view Front section view

22 Dynamic Instability Side view Front section view

23 Dynamic Instability Side view Front section view

24 Dynamic Instability Side view Front section view

25 Dynamic Instability Side view Front section view

26 Dynamic Instability Top view Side view Front section view

27 Dynamic Instability Small fluctuations or perturbations are NOT reinforced. Instead, they are counteracted, and the ball is returned to the original path. Top view A stable process Side view Front section view

28 Dynamic Instability What happens in this case? The path might be straight. Top view Side view Front section view

29 Dynamic Instability Any small perturbations would be reinforced, and the path would diverge. Top view An unstable process Side view Front section view

30 Lesson learned During phase transformations (actually always) - The system relentlessly seeks the best path. - Perturbations are ubiquitous.

31 A simple (but useful) example The evolution of a grain structure illustrates instability, competition, and selection.

32 A simple example The evolution of a grain structure:

33 A simple example The evolution of a grain structure:

34 A simple example The evolution of a grain structure:

35 A simple example The evolution of a grain structure:

36 A simple example The evolution of a grain structure:

37 A simple example The evolution of a grain structure: The size distribution is governed by the competition between nucleation and growth. Both depend on T and alloy variables in different ways.

38 Competition within a single grain During the growth of any given grain, every location is competing with every other location. Which ones win and which ones lose depends on interfacial properties and how the crystal interacts with its surroundings.

39 A simple (but useful) example The evolution of a grain structure illustrates instability, competition, and selection.

40 A simple example The evolution of a grain structure:

41 A simple example The evolution of a grain structure:

42 A simple example The evolution of a grain structure:

43 A simple example The evolution of a grain structure: The size distribution is governed by the competition between nucleation and growth. Both depend on T and alloy variables in different ways.

44 Solidification morphologies It is this competition within a growing grain that ultimately gives rise to most common solidification morphologies and casting microstructures.

45 Dendritic grains For dendritic solidification, the final branch spacing sets the scale of microsegregation and porosity.

46 A closer look Let s look at such a location in more detail. S L Let s assume (momenarily) that the two phases are in equilibrium, so that the interface is not moving.

47 At equilibrium Typically, L-S interfaces in metals are atomistically rough. S L In addition, the interface continuously fluctuates with time. EAM for pure Al (J.R. Morris)

48 Interface motion q q S L q q This heat must be conducted away from the interface.

49 Equiaxed vs directional growth q q

50 Equiaxed vs directional growth

51 Partitioning of solute In an alloy, suppose we extract some heat, reducing the temperature and moving the interface. S L C 0 L L S C L C 0 C S C L C S The excess solute is rejected into the liquid. Like the heat, this solute must be conducted away from the interface.

52 Partioning of solute L Let s now examine a full cooling path. T S C C distance

53 Partioning of solute z C Distance (z) L T S z Region of constitutional supercooling.

54 Instability criterion T mgc > G z Region of constitutional supercooling. The driving force at the tips of the perturbations is greater than behind the tips. The interface is morphologically unstable. What is really happening here?

55 Common growth modes Constrained (directional) growth gives rise to certain typical solidification morphologies. liquidus G solidus Planar Cellular Dendritic Cooling rate is given by GV, and the local solidification time is T /GV. This is the time available for dendrite arm coarsening and therefore controls the final segregation length scale in dendritic growth.

56 Morphological instability

57 Morphological instability

58 Dendritic structure What is the length of the dendritic region (Mushy Zone)? How is this related to shrinkage porosity and hot tearing? When does branching stop? What is the final spacing? What solute distribution is observed in the casting?

59 Columnar to equiaxed transition

60 Branching limit When distance becomes on the order of D/V, there is no longer enough distance for the solute gradient to cause instability. We model such a small system by assuming perfect mixing in the liquid and no mixing in the solid phase.

61 The Gulliver-Scheil model L S S L This nonequilibrium solute distribution results in a higher amount of eutectic constituent than predicted by the phase diagram.

62 Examples of microstructure

63 Eutectic solidification β α

64 Eutectic solidification Arrows to illustrate solute diffusion

65 Eutectic solidification

66 Morphological selection Ω=Iv(Pe) T d α RV T = aλv + b/λ T d αλv Observed behavior V * Interfacial properties, γ and µ, play a critical role in this selection. λ

67 Summary of selection Partitioning of heat Partitioning of solute Diffusion of heat Diffusion of solute Fluid convection Nucleation of new phases (in Solid or Liquid) Extrinsic Contributors Local interfacial Conditions (T,C,r,n) (G,Gc,V,K) Liquid Solid Interface response Interface Stiffness & Interface Mobility Intrinsic Behavior

68 Examples of simulations

69 Dendritic grains 3-D alloy dendrite J. A. Warren and W. L. George

70 Prediction of grain structures

71 Casting simulations

72 Break time?

73 Cast microstructures

74 Dendrites in bronze

75 Dendrites in brass

76 Iron carbon phase diagram

77 Gray cast iron

78 White cast iron This can be heat treated to yield malleable cast iron.

79 Nodular (ductile) cast iron

80 What can we measure in a cast microstructure? How can we measure it? Primary dendrite spacing Secondary dendrite spacing Dendritic chemical segregation profile Grain size Shrinkage porosity Percent of secondary phases Composition of secondary phases Dendritic/Equiaxed transition Visual / Optical microscopy / SEM Optical microscopy / SEM EPMA / SEM-EDS-WDS Visual / Optical microscopy Optical microscopy Optical microscopy SEM / TEM / EDS / WDS / EPMA Visual / Optical microscopy What can it tell us about the casting conditions? Chemical composition, Growth velocity, thermal gradient, Pouring temperature, mold materials, impurities, etc.

81 Diverse solidification morphologies All from the same composition of Al-Si.