PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering1Microstructure + Phase Transformations in Multicomponent SystemsChapter Outline: Phase DiagramsDefinitions and basic concepts Phases and microstructure Binary isomorphous systems (complete solid solubility) Binary eutectic systems (limited solid solubility)Binary systems with intermediate phases/compoundsThe iron-carbon system (steel and cast iron) Not tested: The Gibbs Phase RuleIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering2Component - chemical species (Fe + C in steel; H2O + NaCl in salt water). Binary alloy 2 two components, Ternary alloy – 3, etc.Phase – a portion that has uniform physical and chemical characteristics Two phases in a system have distinct physical or chemical characteristics (e.g. water and ice) Separated by a phase boundaryA phase may contain one or more components. A single-phase system = Homogeneous.System with two or more phases = mixture or heterogeneous system.Components and PhasesIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering3Solvent - host or major component Solute - minor component (Chapter 4).Solubility Limit = maximum amount that can be dissolved in a phase (e.g. alcohol has unlimited solubility in water, sugar has a limited solubility, oil is insoluble).Same concepts for solid phases: Cu and Ni are mutually soluble in any amount (unlimited solid solubility), while C has a limited solubility in Fe. Solubility LimitIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering4Microstructure Properties of alloy depend on proportions of the phases and on how they are arranged at the microscopic level. Microstructure given by number of phases, their proportions, and their arrangement in space.Microstructure of cast IronAlloy of Fe with 4 wt.% C. There are several phases. The long gray regions are flakes of graphite. The matrix is a fine mixture of BCC Fe and Fe3C compound.Phase diagrams help understand and predict microstructuresIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering5Equilibrium: at constant temperature, pressure and composition system is stable(Equilibrium is achieved given sufficient time, but that may be very long. )Metastable: System appears to be stableIn thermodynamics: equilibrium is state corresponds to the minimum of the free energy. Equilibrium and Metastable States•Under conditions of constant temperature, pressure and composition, change is toward lower free energy. •Stable equilibrium is state with minimum free energy. •Metastable state is a local minimum of free energy.metastableequilibriumFree EnergyIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering6Phase diagram - graphical representation of the combinations of temperature, pressure or composition for which specific phases exist at equilibrium. H2O: diagram shows temperature and pressure at which ice (solid),water (liquid) and steam (gas) exist. Phase diagramIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering7Phase diagrams show what phases exist at equilibrium and what transformations we can expect when we change one parameter (T, P, composition). Will discuss phase diagrams for binary alloys only Will assume pressure to be constant at one atmosphere. Phase diagramIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering8Complete solubility of the two components (both in the liquid and solid phases). Binary Isomorphous System (I)Three phase regions : Liquid (L) , solid + liquid (+L), solid ()Liquidus line separates liquid from liquid + solidSolidus line separates solid from liquid + solid + LLIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering9Binary Isomorphous Systems (II)Example: Cu-Ni Complete solubility occurs because Cu and Ni have the same crystal structure (FCC), similar radii, electronegativity and valenceIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering10Binary Isomorphous System (III)One-component: melting occurs at a well-defined temperature.Multi-component: melting occurs over range of temperatures between solidus and liquidus lines. Solid and liquid phases are in equilibrium in this temperature range. + LLLiquid solutionLiquid solution +Crystallites ofSolid solutionPolycrystalSolid solutionIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering11Interpretation of Phase DiagramsFor given temperature and composition can determine:1) Phases present2) Compositions of phases3) Relative fractions of phasesFinding the composition in a two phase region:1. Locate composition and temperature 2. In two phase region draw tie line or isotherm 3. Note intersection with phase boundaries. 4. Read compositions at the intersections. Liquid and solid phases have these compositions.Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering12The Lever RuleFind amounts of phases in two phase regionLocate composition and temperature Draw tie line or isotherm Fraction of a phase = length of tie line to other phase boundary divided by the length of tie lineThe lever rule is a mechanical analogy to the mass balance calculation. The tie line in the two-phase region is analogous to a lever balanced on a fulcrum.Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering13The Lever RuleMass