Phase Diagrams
Phases of Matter Matter can exist in four phases (or states), solid, liquid, gas, and plasma plus a few other extreme phases, like critical fluids and degenerate gases. Generally, as a solid is heated (or as pressure decreases), it will change to a liquid form, and will eventually become a gas. For example, ice (frozen water) melts into liquid water when it is heated. As the water boils, the water evaporates and becomes water vapor. Sometimes, a solid will go directly from solid to gas - this is call subliming. An example of sublimation is dry ice, the solid (frozen) form of carbon dioxide, CO2, which turns into gaseous carbon dioxide at standard temperature and pressure - there is no liquid phase of CO2 at standard temperature and pressure.
Phases of Matter Solid: A solid is matter in which the molecules are very close together and cannot move around. Examples of solids include rocks, wood, and ice (frozen water). Liquid: A liquid is matter in which the molecules are close together and move around slowly. Examples of liquids include drinking water, mercury at room temperature, and lava (molten rock). Gas: A gas is matter in which the molecules are widely separated, move around freely, and move at high speeds. Examples of solids include the gases we breathe (nitrogen, oxygen, and others), the helium in balloons, and steam (water vapor).
Phases of Matter Plasma: A plasma is a gas that is composed of free-floating ions (atoms stripped of some electrons - positively charged) and free electrons (negatively charged). A plasma conducts electrical currents. There are many different types of plasmas. There is plasma in stars (including our Sun), and the solar wind in our Solar System is made of plasma. Supercritical Fluid: A supercritical (or critical) fluid is a liquid/gas under extreme pressure. These supercritical fluids have unique characteristics, the density of a liquid and the mobility of a gas. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth. Degenerate Gas: A degenerate gas is one that is super-compressed and very dense. The molecules of this degenerate gas are virtually touching one another and the gas acts much like a solid. Unlike gases under normal conditions, the temperature in a degenerate gas does not depend on the pressure. These gases follow quantum mechanical laws. http://www.enchantedlearning.com/physics/Phasesofmatter.shtml
Phase Diagrams Temperature vs. Pressure or Temperature vs. Composition Map of stable phases at equilibrium Information on phase transformations From a phase diagram you can identify Stable phases Composition of Phases Amounts of Phases
Pressure-temperature diagrams
Phase Diagram of H2O A critical point is where the properties of two phases become indistinguishable from each other
Phase Diagram of H2O
Phase Diagrams Lines of equilibrium or phase boundaries refer to the lines that identify where phase transitions occur. A triple point is, in a pressure-temperature phase diagram, the unique intersection of the lines of equilibrium between three states of matter, usually solid, liquid, and gas. For a phase diagram with temperature on the vertical axis, a solidus is a line below which the substance is stable in the solid state. A liquidus is a line above which the substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance is replaced by a mixture of solid crystals and liquid.
Iced Tea: Sugar-Water System Solubility limit- maximum concentration of solute (sugar) that can be dissolved into solvent (water) to form a solution
Binary Phase Diagram For a phase diagram with temperature on the vertical axis, a solidus is a line below which the substance is stable in the solid state. A liquidus is a line above which the substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance is replaced by a mixture of solid crystals and liquid.
Binary Phase Diagram L stands for liquid, and A and B are the two components and and are two solid phases rich in A and B respectively. The blue lines represent the liquidus and solidus lines, which are relatively simple to measure. The red lines, called the solvus boundaries, involve a solid-to-solid transition.
Two Component Eutectic Systems T > T1 -- all liquid T1 - TE -- liquid + A at TE -- liquid + A + B T < TE -- A + B all solid T2 % crystals of A = b/(a + b) x 100 % liquid = a/(a + b) x 100 T3 % crystals of A = d/(d + c) x 100 % liquid = c/(c + d) x 100
Definition of terms: Liquidus - The line separating the field of all liquid from that of liquid plus crystals. Solidus - The line separating the field of all solid from that of liquid plus crystals. Solvus - The line separating the field of a solid from another solid. Eutectic point - the point on a phase diagram where the maximum number of allowable phases are in equilibrium. When this point is reached, the temperature must remain constant until one of the phases disappears. A eutectic is an invariant point.
Eutectic systems have a liquidus which contains a V to the eutectic point where it meets the eutectic invariant-reaction line. Here is an example of a eutectic phase diagram and are both solid phases. The two-phase solid region on the phase diagram will consist of a mixture of eutectic and either or phase depending on the whether the alloy composition is hypoeutectic (<eutectic) or hypereutectic (>eutectic). The constitution of an alloy under equilibrium conditions can be found from its phase diagram.
Cooling Curves Most systems consisting of two or more components exhibit a temperature range over which the solid and liquid phases are in equilibrium. Instead of a single melting temperature, the system now has two different temperatures, the liquidus temperature and the solidus temperature which are needed to describe the change from liquid to solid.
Cooling Curves When the liquidus temperature is reached, solidification begins and there is a reduction in cooling rate caused by latent heat evolution and a consequent reduction in the gradient of the cooling curve. Upon the completion of solidification the cooling rate alters again allowing the temperature of the solidus to be determined. As can be seen on the diagram below, these changes in gradient allow the liquidus temperature TL, and the solidus temperature TS to be identified.
Cooling Curves When cooling a material of eutectic composition, solidification of the whole sample takes place at a single temperature. This results in a cooling curve similar in shape to that of a single-component system with the system solidifying at its eutectic temperature.
Cooling Curves Formation of the eutectic causes the system to cease cooling until solidification is complete. The resulting cooling curve shows the two stages of solidification with a section of reduced gradient where a single phase is solidifying and a plateau where eutectic is solidifying.
Cooling Curves By taking a series of cooling curves for the same system over a range of compositions the liquidus and solidus temperatures for each composition can be determined allowing the solidus and liquidus to be mapped to determine the phase diagram.
Cooling Curves By removing the time axis from the curves and replacing it with composition, the cooling curves indicate the temperatures of the solidus and liquidus for a given composition. This allows the solidus and liquidus to be plotted to produce the phase diagram.
The lever rule If an alloy consists of more than one phase, the amount of each phase present can be found by applying the lever rule to the phase diagram. The lever rule can be explained by considering a simple balance. The composition of the alloy is represented by the fulcrum, and the compositions of the two phases by the ends of a bar. The proportions of the phases present are determined by the weights needed to balance the system. fraction of phase 1 = (C2 - C) / (C2 - C1) fraction of phase 2 = (C - C1) / (C2 - C1).
Lever rule applied to a binary system C = 65 weight% B A B Point 1 At point 1 the alloy is completely liquid, with a composition C. Let C = 65 weight% B.
Point 2 At point 2 the alloy has cooled as far as the liquidus, and solid phase b starts to form. Phase b first forms with a composition of 96 weight% B. The green dashed line below is an example of a tie-line. A tie-line is a horizontal (i.e., constant-temperature) line through the chosen point, which intersects the phase boundary lines on either side.
Point 3 A tie-line is drawn through the point, and the lever rule is applied to identify the proportions of phases present. Intersection of the lines gives compositions C1 and C2 as shown. Let C1 = 58 weight% B and C2 = 92 weight% B fraction of solid b = (65 - 58) / (92 - 58) = 20 weight% fraction of liquid = (92 - 65) / (92 - 58) = 80 weight%
Point 4 Let C3 = 48 weight% B and C4 = 87 weight% B fraction of solid b = (65 - 48) / (87 - 48) = 44 weight%. As the alloy is cooled, more solid phase forms. At point 4, the remainder of the liquid becomes a eutectic phase of + and fraction of eutectic = 56 weight%
Point 5 Let C5 = 9 weight% B and C6 = 91 weight% B fraction of solid = (65 - 9) / (91 - 9) = 68 weight% fraction of solid = (91 - 65) / (91 - 9) = 32 weight%.
Sample Problem
Lever Rule will give us the mass fractions or amounts of each phase W=(C-Co)/(C -C) W =(Co-C)/(C -C) check: (W x C)+(W x C)= Co
Microstructure Slow cooling, complete diffusion Uniform composition Fast cooling, incomplete diffusion segregation, non uniform
Eutectic Microstructure
The Iron-Iron Carbide Phase Diagram Steel: 0. 0008-2 The Iron-Iron Carbide Phase Diagram Steel: 0.0008-2.14 wt% C Cast Iron: 2.14-6.7 wt% C An allotropic transition is a transition of a pure element, at a defined temperature and pressure, from one crystal structure to another which contains the same atoms but which has different properties.
The Iron-Iron Carbide Phase Diagram
Eutectoid: 0.76 wt% C, 717C
Hypoeutectoid Steel: <0.8 wt% C
Hypereutectoid Steel: >0.8 wt% C
Sample Question
Sample Question Cementite is much harder but more brittle than ferrite. Thus increasing the amount of cementite in a steel alloy will result in a harder and stronger material
(Red) The specimen is cooled rapidly to 160C and left for 20 minutes (Red) The specimen is cooled rapidly to 160C and left for 20 minutes. The cooling rate is too rapid for pearlite to form at higher temperatures; therefore, the steel remains in the austenitic phase until the Ms temperature is passed, where martensite begins to form. Since 160C is the temperature at which half of the austenite transforms to martensite, the direct quench converts 50% of the structure to martensite. Holding at 160C forms only a small quantity of additional martensite, so the structure can be assumed to be half martensite and half retained austenite. (Green) The specimen is held at 250C for 100 seconds, which is not long enough to form bainite. Therefore, the second quench from 250C to room temperature develops a martensitic structure. 3. (Blue) An isothermal hold at 300C for 500 seconds produces a half-bainite and half-austenite structure. Cooling quickly would result in a final structure of martensite and bainite. 4. (Orange) Austenite converts completely to fine pearlite after eight seconds at 600C. This phase is stable and will not be changed on holding for 100,000 seconds at 600C The final structure, when cooled, is fine pearlite.
Bainite and Martensite Bainite forms when austenite is rapidly cooled past a critical temperature of 1333°F (about 723°C). A fine non-lamellar structure, bainite commonly consists of ferrite and cementite. It is similar in constitution to pearlite, but with the ferrite forming by a displacive mechanism similar to martensite formation, usually followed by precipitation of carbides from the supersaturated ferrite or austenite. When formed during continuously cooling, the cooling rate to form bainite is higher than that required to form pearlite, but lower than that to form martensite, in steel of the same composition. Bainite is generally stronger, harder, and more ductile than pearlite. Martensite is a metastable iron phase supersaturated in carbon that is the product of a diffusionless transformation from austenite. FCC austenite experiences a transformation to a body-centered tetragonal (BCT) martensite. All the carbon atoms remain as interstitial impurities in martensite. Martensite is the hardest and strongest, and in addition, the most brittle of all steel alloy microstructures.
Iron alloy phases Austenite (γ-iron; hard) Ferrite (α-iron; soft) Cementite (iron carbide; Fe3C) Bainite (ferrite and a fine dispersion of cementite) Martensite (diffusionless transformation from austenite) Pearlite (88% ferrite, 12% cementite) Types of Steel Plain-carbon steel (up to 2.1% carbon) Stainless steel (alloy with chromium) HSLA steel (high strength low alloy) Tool steel (very hard; heat-treated) Other Iron-based materials Cast iron (>2.1% carbon) Wrought iron (almost no carbon) Ductile iron (2% to 4% carbon)
TTT Diagrams Above 550C, austenite transforms completely to pearlite. Below 550C, both pearlite and bainite are formed and below 450C, only bainite is formed. The horizontal line C-D that runs between the two curves marks the beginning and end of isothermal transformations. The dashed line that runs parallel to the solid line curves represents the time to transform half the austenite to pearlite.