The structure of cast metals
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Metals Conservation Summer Institute The structure of cast metals Ralph E. Napolitano Department of Materials Science & Engineering Iowa State University Ames, Iowa Metals Conservation Summer Institute June 1, 2005 IOWA STATE UNIVERSITY Materials Science & Engineering 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) Metals Conservation Summer Institute Tm 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 Tm. Let’s do an experiment. Realistically, we do not observe an isothermal arrest. T (°C) Metals Conservation Summer Institute Tm ∆T t (sec) Even at the same temperature, the liquid phase “contains” more heat than the solid. This heat Is liberated upon freezing. Metals Conservation Summer Institute Driving force and the importance of rate Metals Conservation Summer Institute Driving force and the importance of rate In our freezing example, the heat may be liberated too quickly to be liberated efficiently. Mother Nature tries to optimize this efficiency using any and all means available. You are all very familiar with one consequence of such optimization… Still Metals Conservation Summer Institute How do metals freeze? Metals freeze in much the same way that water freezes into the familiar snowflakes. The Rasmussen & Libbrecht Collection Metals Conservation Summer Institute 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 Metals Conservation Summer Institute How do metals freeze? Here we compare the snowflake structures to a transparent organic metal-analog. M.E. Glicksman, NASA-IDGE, 1997. The Rasmussen & Libbrecht Collection Metals Conservation Summer Institute Perspective What is so special about the solid-liquid interface in metals? Metals Conservation Summer Institute Early observation of dendrites Metals Conservation Summer Institute Early observation of dendrites Metals Conservation Summer Institute Critical Issues The critical issues are essentially the same for all (most) phase transformations Thermodynamics Phase stability (phase diagrams) The energy of interfaces Quantification of driving forces Thermal and chemical partitioning Kinetics The diffusion of heat and solute The kinetics of atomistic processes Nucleation kinetics Interface kinetics Metals Conservation Summer Institute 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 Metals Conservation Summer Institute Natural selection If you want to study genetic – would you use antelope? Metals Conservation Summer Institute Natural selection Fruit flies Atoms vibrate at ~10000 GHz, - quite a prolific fruit fly! Metals Conservation Summer Institute Competition and “natural selection” In nature, everything is a competition, with many phenomena occurring simultaneously. Metals Conservation Summer Institute Dynamic Instability BUT – This is only a side view. Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Side view Front section view Metals Conservation Summer Institute Dynamic Instability Top view Side view Front section view Metals Conservation Summer Institute 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 Metals Conservation Summer Institute Dynamic Instability What happens in this case? The path might be straight. Side view Top view Front section view Metals Conservation Summer Institute Dynamic Instability Any small “perturbations” would be reinforced, and the path would diverge. Top view An unstable process Side view Front section view Metals Conservation Summer Institute Lesson learned During phase transformations (actually always) - The system relentlessly seeks the “best” path. - Perturbations are ubiquitous. Metals Conservation Summer Institute A simple (but useful) example The evolution of a grain structure illustrates instability, competition, and selection. Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute 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. Metals Conservation Summer Institute 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. Metals Conservation Summer Institute A simple (but useful) example The evolution of a grain structure illustrates instability, competition, and selection. Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute A simple example The evolution of a grain structure: Metals Conservation Summer Institute 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. Metals Conservation Summer Institute Solidification morphologies It is this competition within a growing grain that ultimately gives rise to most common solidification morphologies and casting microstructures. Metals Conservation Summer Institute Dendritic grains For dendritic solidification, the final branch spacing sets the scale of microsegregation and porosity. Metals Conservation Summer Institute 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. Metals Conservation Summer Institute 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) Metals Conservation Summer Institute Interface motion q S L q q q This heat must be conducted away from the interface. Metals Conservation Summer Institute Equiaxed vs directional growth q q Metals Conservation Summer Institute Equiaxed vs directional growth Metals Conservation Summer Institute Partitioning of solute In an alloy, suppose we extract some heat, reducing the temperature and moving the interface. S C0 L L L S CL C0 CS CS CL The excess solute is rejected into the liquid. Like the heat, this solute must be conducted away from the interface. Metals Conservation Summer Institute Partioning of solute Let’s now examine a full cooling path. L T S C C distance Metals Conservation Summer Institute Partioning of solute z C Distance (z) L T S z Region of constitutional supercooling. Metals Conservation Summer Institute 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? Metals Conservation Summer Institute 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. Metals Conservation Summer Institute Morphological instability Metals Conservation Summer Institute Morphological instability Metals Conservation Summer Institute 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? Metals Conservation Summer Institute Columnar to equiaxed transition Metals Conservation Summer Institute 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. Metals Conservation Summer Institute 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. Metals Conservation Summer Institute Examples of microstructure Metals Conservation Summer Institute Eutectic solidification β α Metals Conservation Summer Institute Eutectic solidification Arrows to illustrate solute diffusion… Metals Conservation Summer Institute Eutectic solidification Morphological selection Ω=Iv(Pe) ∆T = aλV + b/λ ∆Td α RV ∆Td α λV Observed behavior V Metals Conservation Summer Institute * Interfacial properties, γ and µ, play a critical role in this selection. λ Metals Conservation Summer Institute Summary of selection Partitioning of heat Extrinsic Contributors Partitioning of solute Diffusion of heat Diffusion of solute Fluid convection Nucleation of new phases (in Solid or Liquid) Interface response Liquid Solid Interface Stiffness & Interface Mobility Local interfacial Conditions (T,C,r,n) (G,Gc,V,K) Intrinsic Behavior Metals Conservation Summer Institute Examples of simulations Metals Conservation Summer Institute Dendritic grains 3-D alloy dendrite J. A. Warren and W. L. George Metals Conservation Summer Institute Prediction of grain structures Metals Conservation Summer Institute Casting simulations Metals Conservation Summer Institute Break time? Metals Conservation Summer Institute Cast microstructures Metals Conservation Summer Institute Dendrites in bronze Metals Conservation Summer Institute Dendrites in brass Metals Conservation Summer Institute Iron–carbon phase diagram Metals Conservation Summer Institute Gray cast iron Metals Conservation Summer Institute White cast iron This can be heat treated to yield malleable cast iron. Metals Conservation Summer Institute Nodular (ductile) cast iron Metals Conservation Summer Institute What can we measure in a cast microstructure? How can we measure it? Primary dendrite spacing Visual / Optical microscopy / SEM Secondary dendrite spacing Optical microscopy / SEM Dendritic chemical segregation profile EPMA / SEM-EDS-WDS Grain size Visual / Optical microscopy Shrinkage porosity Optical microscopy Percent of secondary phases Optical microscopy Composition of secondary phases SEM / TEM / EDS / WDS / EPMA Dendritic/Equiaxed transition Visual / Optical microscopy What can it tell us about the casting conditions? Chemical composition, Growth velocity, thermal gradient, Pouring temperature, mold materials, impurities, etc. Metals Conservation Summer Institute Diverse solidification morphologies All from the same composition of Al-Si.
