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Phase. Any physically distinct, chemically homogeneous and mechanically separable portion of a substance Can be continuous or discontinuous Can be solid, liquid or gas Can be a pure substance or a solution. Multiple Solid Phases. Multiple solid phases are common in metals
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Phase • Any physically distinct, chemically homogeneous and mechanically separable portion of a substance • Can be continuous or discontinuous • Can be solid, liquid or gas • Can be a pure substance or a solution
Multiple Solid Phases • Multiple solid phases are common in metals • Phases defined by composition (not by state – solid, liquid, gas) • Example – two phases shown to the right, dark phase and then the lighter phase
Solubility • Unlimited Solubility • Hume Rothery’s Conditions • Similar Size • Same Crystal Structure • Same Valance • Similar Electronegativity • Implies single phase • Limited Solubility • Implies multiple phases • No Solubility (oil and water region)
Solid Solutions • Just like we have liquid solutions (ex mixing alcohol and water => liquid solution), we can have solid solutions. • Solid solution is mixed as a liquid and then allowed to solidify • May mix two metal elements together then allow to solidify => solid solution
Two Types of Solid Solutions Interstitial Solid Solution Substitutional Solid Solution
Equilibrium Phase DiagramCooling Curve DT/D t Superheat Thermal Arrest Temperature Local Solidification Time Total Solidification Time REMEMBER THIS? Time
Cooling Curves and the Phase Diagram In your groups, work on problem 10-57 in your book
Equilibrium Phase Diagram • Equilibrium: state of a system remains constant over an indefinite period of time • Binary phase diagram: diagram for alloy composed of two elements • Shows relationship in a metal among • temperature • pressure • composition • Gibb’s Phase Rule
Types of Phase diagrams Number of Constituents Number of phases Isomorphous – one solid phase Polymorphous – multiple solid phases • Unary (like the water one shown at the beginning) – one constituent • Binary - two constituents , like an alloy (we will focus on these) • Ternary - three constituents; requires 3-D diagram
Utilization of Phase Diagrams • For each point of temperature and composition, three pieces of information can be obtained • Phase present • The amount of each phase present ( how many ice cubes are in the glass) • lever law • Composition of each phase (chem make-up of phases) • tie line
Liquidus and Solidus Temperature • Liquidus – The temperature at which the first solid forms during solidification • Solidus – The temperature below which the liquid has completely solidified. liquidus solidus Wt % MgO
Tie Line –Composition of Phases • Draw a vertical line at the composition of the “alloy” of interest ex – 40 wt % MgO see purple line • For a given temperature draw an isotherm (horizontal line) ex at 2400 C red line • Where line crosses “phase boundary” (in this case the liquidus line or solidus line), draw a vertical line • Ex – composition of liquid phase is ~32% MgO; Composition of solid phase is ~ 60 % MgO liquidus solidus Wt % MgO
Lever Law – Amount of each phase Using the compositions of the phases determined from the tie line, calculate the amount of each phase as this: Opposite lever arm/total length of lever * 100% Therefore: Amt L = (60-40)/(60-32) * 100 = 71% Amt S = (40-32)/(60-32)*100 = 29% Solid C = 60 %MgO Liquid C = 32 %MgO **Notice the composition of the phases does not have to add up to 100% but the amount does!!***** Alloy C = 40 %MgO
What’s the difference between Amount and Composition???? • Composition is the chemical make-up of the phase (if we had a glass of coke with ice in it, our liquid phase would have one chemical make-up – won’t begin to think what it might be, the liquid phase would have a chemical make-up of ~ H2O. The composition of the “alloy” would be the total chemical make-up of the ice and coke combined (which would be watery coke?)
What’s the difference between Amount and Composition???? • Amount is the quantity of the phase (if we had a glass of coke with ice in it, we could take the ice cubes out and weigh them and then weigh the coke to figure out the amount of the solid and liquid phase or use the lever law) http://michellegeller.typepad.com/michelle_geller_weblog/images/2007/03/28/gluttony.jpg
Phase Diagram and Microstructure • Can you relate this to nucleation and growth? • Where does nucleation occur? • Where does growth occur?
Three-Phase Reactions • Involves 3 distinct phases • Occurs at a single “point” • Often times is associated with special properties or characteristics for that material • Eutectic L => S1 + S2 • Eutectoid S1 => S2+ S3 • Peritectic L + S1 => S2 • Peritectoid S1 + S2 => S3 • Monotectic L1 => S1 + L2 • Miscibility gap; Oil and water region, where two liquid phases coexist
Intermetallic • A compound formed of two or more metals that has its own unique composition, structure and properties. • Exists as a single phase region where you would expect a two phase region • Typically hard, brittle and strong • Stoichiometric – single composition • Non Stoichiometric – range of compositions
Solvus • Indicates solubility limit
Group Work ID all of the three phase reactions by name and “reaction”
Hyper-Eutectic vs. Hypo-Eutectic Microstructural Differences
Lead-Tin Eutectic Microstructure Temperature
Iron- Carbon Equilibrium Diagram Phases d - delta ferrite g - austenite a - alpha ferrite (ferrite) Fe3C - cementite Reactions Peritectic (L+ S1 > S2) Eutectic ( L > S1 + S2) Eutectoid (S1 > S2 + S3)
