A Phase is a homogeneous por2on of a system that has uniform physical and chemical characteris2cs Mul2ple phase Single Phase pulp free 1 SINGLE PHASE Solid solu2on alloy TWO PHASE Pearlite a two phase mixture of ferrite BCC iron and Fe3C iron carbide 2 Phase Iden2 ca2on X ray Di rac2on di erent phases have di erent crystal structures and or quite di erent laZce parameters Each phase will have its own set of x ray di rac2on peaks Metallography polish and etch samples to examine microstructure using an op2cal or electron microscope Chemical Microanalysis using electron microprobes to determine the chemical composi2on of what you are viewing 3 Superalloys for Turbine Blades Airliner Skin 4 Phase Diagrams Pure Copper The furnace cools to below the mel2ng temperature of copper However if we have a thermocouple in the liquid copper it will con2nue to read 1085 C un2l the very last liquid has solidi ed even though the surroundings are at a temperature below 1085 C 5 Gibbs Phase Rule F C P 1 F Degrees of Freedom C number of components in the system P number of phases in equilibrium 1 represents the number of environmental variables in this case simply temperature For copper at 1085 C 2 phases are in equilibrium solid and liquid so F 1 2 1 0 This means that the only way the two phases can coexist is for the temperature to be xed at 1085 C Pure nickel does the same thing except at 1455 C 6 An isomorphous phase diagram Ni and Cu exhibit complete solid solubility across the range of composi2on from pure Cu to pure Ni Note that for mixtures there is no unique mel2ng temperature the mixtures melt over a range of temperature Instead there is a liquidus temperature above which everything is liquid and a solidus temperature below which everything is solid Pure Cu far leg and pure Ni far right s2ll behave as before e g the liquid and solid coexist at a single temperature but what about the solid solu2ons where Ni and Cu mix 7 In the 2 phase region F 1 2 2 1 This means that you can lower the temperature and s2ll have two phases in equilibrium unlike either pure Cu or pure Ni T liquid liquid 2me 8 A few things to no2ce about the 2 phase region A range of temperature over which a liquid and solid coexist As one moves lower in temperature through the 2 phase region the propor2on of solid increases phase frac2on The composi2on of the solid and liquid are di erent from the composi2on of the alloy phase composi2on 9 Reading Phase Diagrams Phase Composi2on To determine the composi2on of each phase in a two phase region 1 Draw a horizontal line at the desired temperature connec2ng the liquidus and solidus lines This is called a 2e line 2 Where the 2e line intersects the liquidus line drop a ver2cal line to the x axis This gives the composi2on of the liquid phase 3 Where the 2e line intersects the solidus line drop a ver2cal line to the x axis this gives the composi2on of the solid or phase 10 Determining how much of each phase you have is accomplished using the lever rule C Co Weight Frac2on of Liquid WL C C L Weight Frac2on of Solid Co C L W C C L Weight frac2on refers to the amount of each phase For example just below the liquidus temperature one expects a much larger weight frac2on of liquid than solid while the opposite is true just above the solidus temperature 11 Lever rule example for the weight frac2ons of liquid and solid at point c As one might expect given that we are closer to the liquidus line than the solidus line the weight frac2on of liquid is greater than the solid C Co 43 35 WL 0 73 C C L 43 32 Co C L 35 32 W 0 27 C C L 43 32 The numbers above are taken directly from the x axis we are using the x axis as a ruler CL C0 C 12 To Summarize Isomorphous Phase Diagrams In a single phase region the phase composi2on is equal to the alloy composi2on and the weight frac2on of the phase is 1 In a two phase region each phase has a composi2on di erent from the alloy composi2on The weight frac2on of each phase depends where we are temperature wise in the two phase region Friendly advice make sure you understand the di erence between the alloy composi on the phase composi on and the phase frac on or amount of each phase The alloy composi2on is determined by us when we decide on how much of each alloy cons2tuent to mix together The phase composi2on and frac2on are determined using the phase diagram 13
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