Chapter Outline: Phase Diagrams Microstructure + Phase Transformations in Multicomponent Systems 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 Rule
Components and Phases Component - 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 boundary A phase may contain one or more components. A single-phase system = Homogeneous. System with two or more phases = mixture or heterogeneous system. Quaternary alloy…
Solubility Limit Solvent - 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. Ask what is one more difference between C-Fe and Cu-Ni solutions (interstitial vs substitutional)
Microstructure 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 Iron This is an alloy of iron (Fe) with 4% carbon (C) by weight. The microstructure has two main constituents. The long pale regions are flakes of graphite. They have a shape similar to the cornflake breakfast cereal. The background or matrix of the alloy is pearlite. This is a fine mixture of ferrite and iron carbide. A binary alloy may be a.a single solid solution b.two separated, essentially pure components. c.two separated solid solutions. d.a chemical compound, together with a solid solution. 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 help understand and predict microstructures
Equilibrium and Metastable States Equilibrium: 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 stable In thermodynamics: equilibrium is state corresponds to the minimum of the free energy. Under conditions of constant temperature, pressure and composition, change is toward lower free energy. equilibrium Stable equilibrium is state with minimum free energy. Metastable state is a local minimum of free energy. Free Energy metastable
Phase diagram Phase 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. Both the critical points are shown as red circles. The critical point and the orange line in the ice-one phase space refer to the low-density and high-density forms of amorphous water (ice). All the solid phases of ice involve the oxygen atoms being hydrogen bonded to four neighboring oxygen atoms. The high pressure phase lines of ice-ten (X) and ice-eleven (XI) are still subject to experimental verification. Two different forms of ice-eleven have been described by different research groups: the high-pressure form involves hydrogen atoms equally-spaced between the oxygen atoms (like ice-ten) whereas the lower pressure low temperature form utilizes the incorporation of hydroxide defect doping to order the hydrogen bonding of ice 1h. Another ice-ten has been described, being the proton ordered form of ice-six (VI) occurring below about 110 K. Only hexagonal ice-one (1h), ice-three (III), ice-five (V), ice-six (VI) and ice-seven (VII) can be in equilibrium with liquid water, whereas all the others ices, including ice-two (II), are not stable in its presence under any conditions of temperature and pressure. Ice-two, ice-eight (VIII), ice-nine (IX), ice-ten and ice-eleven (both) all possess (ice-nine mainly) ordered hydrogen-bonding whereas in the other ices the hydrogen-bonding is disordered even down to 0 K, where reachable. Ice-four (IV) and ice-twelve (XII) [81] are both metastable within the ice-five phase space. Cubic ice (1c) is metastable with respect to hexagonal ice (1h). It is worth noting that the water molecule is stable throughout the phase space above. Kurt Vonnegut's highly entertaining story concerning an (imaginary) ice-nine, which was capable of crystallizing all the water in the world [K. Vonnegut, Cat's Cradle, (Penguin, London, 1963) p. 34.], fortunately has no scientific basis (see also IE) as ice-nine, in reality, is a proton ordered form of ice-three, only exists at very low temperatures and high pressures and cannot exist alongside liquid water under any conditions. Ice 1c is a metastable form of ice that can be formed, by condensation of water vapor, at ambient pressure but low temperatures (less than -80°C, see Phase Diagram) or by reducing the pressure on high-pressure ices at 77 K. It converts, irreversibly but extremely slowly in the temperature range 170-220 K, to hexagonal ice with about 40 J mol-1 heat evolution. It consists of a face centered cubic lattice (Space group Fd3m) with half the tetrahedral holes filled.
Phase diagram Phase 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.
Binary Isomorphous System (I) Complete solubility of the two components (both in the liquid and solid phases). L + L Three phase regions : Liquid (L) , solid + liquid (+L), solid () Liquidus line separates liquid from liquid + solid Solidus line separates solid from liquid + solid
Binary Isomorphous Systems (II) Example: Cu-Ni Complete solubility occurs because Cu and Ni have the same crystal structure (FCC), similar radii, electronegativity and valence
Binary 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 Liquid solution + L Liquid solution + Crystallites of Solid solution Polycrystal Solid solution
Interpretation of Phase Diagrams For given temperature and composition can determine: 1) Phases present 2) Compositions of phases 3) Relative fractions of phases Finding the composition in a two phase region: 1. Locate composition and temperature 2. In two phase region draw tie line or isotherm Note intersection with phase boundaries. Read compositions at the intersections. Liquid and solid phases have these compositions.
The Lever Rule Find amounts of phases in two phase region Locate 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 line The 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.
The Lever Rule Mass fractions: WL = S / (R+S) = (C- Co) / (C - CL) W = R / (R+S) = (Co- CL) / (C - CL)
Derivation of the lever rule W and WL are fractions of and L phases 1) All material is in one phase or the other: W + WL = 1 2) Composition equal composition in one phase + composition second phase at given T: Co = WC + WLCL 3) Solution gives Lever rule. WL = (C- Co) / (C - CL) W = (Co- CL) / (C - CL)
Phase compositions and amounts. An example. Co = 35 wt. %, CL = 31.5 wt. %, C = 42.5 wt. % Mass fractions: WL = (C- Co) / (C - CL) = 0.68 W = (Co- CL) / (C - CL) = 0.32
Microstructure in isomorphous alloys Equilibrium (very slow) cooling Solidification in the solid + liquid phase occurs gradually upon cooling from the liquidus line. The composition of the solid and the liquid change gradually during cooling (as can be determined by the tie-line method.) Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line.