1Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering1Phase DiagramsIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering2Microstructure and 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: 9.12 The Gibbs Phase Rule2Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering3Component - chemically recognizable species (Fe and C in carbon steel, H2O and NaCl in salted water). A binaryalloy contains two components, a ternary alloy – three, etc.Phase – a portion of a system that has uniform physical and chemical characteristics. Two distinct phases in a system have distinct physical or chemical characteristics (e.g. water and ice) and are separated from each other by definite phase boundaries. A phase may contain one or more components. A single-phase system is called homogeneous, systems with two or more phases are mixtures or heterogeneous systems.Definitions: Components and PhasesIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering4Solvent - host or major component in solution, solute -minor component (Chapter 4).Solubility Limit of a component in a phase is the maximum amount of the component that can be dissolved in it (e.g. alcohol has unlimited solubility in water, sugar has a limited solubility, oil is insoluble). The same concepts apply to solid phases: Cu and Ni are mutually soluble in any amount (unlimited solid solubility), while C has a limited solubility in Fe. Definitions: Solubility Limit3Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering5MicrostructureThe properties of an alloy depend not only on proportions of the phases but also on how they are arranged structurally at the microscopic level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.Microstructure of cast IronThis is an alloy 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 will help us to understand and predict the microstructures like the one shown in this pageIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering6A system is at equilibrium if at constant temperature, pressure and composition the system is stable, not changing with time. Equilibrium is the state that is achieved given sufficient time. But the time to achieve equilibrium may be very long (the kinetics can be slow) that a state along the path to the equilibrium may appear to be stable. This is called a metastable state. In thermodynamics, equilibrium is described as the state of system that corresponds to the minimum of the thermodynamic function called the free energy of the system. Thermodynamics tells us that:Equilibrium and Metastable States• Under conditions of a constant temperature and pressure and composition, the direction of any spontaneous change is toward a lower free energy. • The state of stable thermodynamic equilibrium is the one with minimum free energy. • A system at a metastable state is trapped in a local minimum of free energy that is not the global one.metastableequilibriumFree Energy4Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering7A phase diagram - graphical representation of the combinations of temperature, pressure, composition, or other variables for which specific phases exist at equilibrium. For H2O, a typical diagram shows the temperature and pressure at which ice (solid),water (liquid) and steam (gas) exist. Phase diagramIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering8A phase diagrams show what phases exist at equilibrium and what phase transformations we can expect when we change one of the parameters of the system (T, P, composition). We will discuss phase diagrams for binary alloys only and will assume pressure to be constant at one atmosphere. Phase diagrams for materials with more than two components are complex and difficult to represent. Phase diagram5Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering9Isomorphous system - complete solid solubility of the two components (both in the liquid and solid phases). Binary Isomorphous Systems (I)Three phase region can be identified on the phase diagram: Liquid (L) , solid + liquid (α +L), solid (α )Liquidus line separates liquid from liquid + solidSolidus line separates solid from liquid + solidα + LαLIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering10Binary Isomorphous Systems (II)Example of isomorphous system: Cu-Ni (the complete solubility occurs because both Cu and Ni have the same crystal structure, FCC, similar radii, electronegativity and valence).6Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering11Binary Isomorphous Systems (III)In one-component system melting occurs at a well-defined melting temperature.In multi-component systems melting occurs over the range of temperatures, between the solidus and liquidus lines. Solid and liquid phases are in equilibrium in this temperature range.α + LαLLiquid solutionLiquid solution +Crystallites ofSolid solutionPolycrystalSolid solutionIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Tennessee, Dept. of Materials Science and Engineering12Interpretation of Phase DiagramsFor a given temperature and composition we can use phase diagram to determine:1) The phases that are present2) Compositions of the phases3) The relative fractions of the phasesFinding the