Lecture07 - VK250 Lecture 07 EQUILIBRIUM PHASE DIAGRAMS •...

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VK250 Lecture 07 1 E QUILIBRIUM P HASE D IAGRAMS Cu / Ni completely soluble in both liquid and solid form Solubility limits in both phases during melting / solidification Solidus line - indicates solubility limit for freezing / melting solid Liquidus line - indicates solubility limit for freezing / melting liquid Cool a molten alloy of 60 wt% Ni & 40 wt% Cu: Above 1350 ° C - liquid is 60 wt%Ni & 40 wt% Cu At 1350 ° C - α phase (a solid solution) starts to precipitate with a composition of 74 wt% Ni & 26 wt% Cu Liquid phase is still 60 wt% Ni & 40 wt% Cu At 1320 ° C - Lever rule indicates that the alloy is 59 wt% α & 41 wt% liquid α is 67 wt% Ni % 33 wt% Cu Liquid phase is 50 wt% Ni & 50 wt% Cu At 1280 ° C - Last liquid to freeze is 42 wt % Ni & 58 wt % Cu α phase is 60 wt% Ni & 40 wt% Cu Below 1280 ° C - Solid is a single phase ( α phase) that is a solid solution with a composition of 60 wt% Ni & 40 wt% Cu
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VK250 Lecture 07 2 Note: Assume cooling rates are so slow that there is time for the equilibrium concentration to be formed throughout the solid. During cooling, Ni must diffuses from the Ni-rich regions first formed. Otherwise a non-equilibrium microstructure will form. Tin-lead phase diagram Eutectic composition Intermediate composition with a single melting / freezing point Eutectic point : Melting / freezing temperature of eutectic composition Eutectic reaction : L α + β (two solid phases) Cool a molten Sn-Pb alloy of eutectic composition (38.1wt% Pb, 61.9 wt% Sn) Above 183 ° C: A single phase (liquid at the eutectic composition) Below 183 ° C : The eutectic solid consists of very fine precipitates of two phases
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VK250 Lecture 07 3 (i) A solid solution of 2.5 wt% Pb in 97.5 wt% Sn (ii) A solid solution of 19 wt% Sn in 81 wt% Pb Proportions of two phases given by lever rule: Pb-rich phase (19 wt% Sn): 38.1 ! 2.5 81 ! 2.5 = 45.4 wt% Sn-rich phase (2.5 wt% Pb): 81 ! 38.1 81 ! 2.5 = 54.6 wt% Cool a molten alloy 50 wt% Pb and 50 wt% Sn - Above 225 ° C: Liquid is 50 wt% Pb & 50 wt% Sn At 225 ° C: A Pb-rich solid phase (86 wt% Pb & 14 wt% Sn) starts to form The liquid is still 50 wt% Pb & 50 wt% Sn Just > 183 ° C: Two phases The Pb-rich solid phase contains 81 wt% Pb & 19 wt% Sn The liquid phase is 38.1 wt% Pb (eutectic composition) Proportions of two phases given by lever rule: 50 ! 38.1 81 ! 38.1 = 27.7 wt%
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This note was uploaded on 09/15/2010 for the course MATSCIE 250 taught by Professor Yalisove during the Fall '08 term at University of Michigan.

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Lecture07 - VK250 Lecture 07 EQUILIBRIUM PHASE DIAGRAMS •...

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