Lecture_09_Phase_Diagrams

# Lecture_09_Phase_Diagrams - MECH 340 Engineering Material...

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MECH 340 Engineering Material Phase Diagrams

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Issues to Address When we combine two elements. .. what equilibrium state do we get? In particular, if we specify. .. --a composition (e.g., wt%Cu - wt%Ni), and --a temperature (T) then. .. How many phases do we get? What is the composition of each phase? How much of each phase do we get? Phase B Phase A Nickel atom Copper atom
Solution and Solubility Example : Solubility of sugar in water There exists a maximum amount of sugar that can be completely dissolved in water; excess of sugar stays as solid. This maximum amount is the solubility of sugar in water. The solution containing the maximum concentration of sugar is a saturated solution . Cooling of saturated solution results in the formation of solid sugar from the solution, indicating that solubility decreases with decreasing T. This process is called precipitation and the solid formed is a precipitate . Heating the solution will lead to the dissolving of the precipitate back into solution. Sugary water – the solution Solid sugar – the precipitate The same concepts apply to solids: solid solution, saturation, solubility, precipitation In this example there exist two phases in the system and the two phases stay in equilibrium: Solution Solid dissolving precipitation

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Sugar Solubility Solubility Limit: Max concentration for which only a solution occurs. Ex: Phase Diagram: Water-Sugar System Question: What is the solubility limit at 20C? Solubility limit increases with T : e.g., if T = 100C, solubility limit = 80wt% sugar. Answer: 65wt% sugar. If C o < 65wt% sugar: sugar If C o > 65wt% sugar: syrup + sugar. Pure Sugar Temperature (°C) 0 20 40 60 80 100 C o =Composition (wt% sugar) L (liquid solution i.e., syrup) Solubility Limit L (liquid) + S (solid sugar) 65 20 40 60 80 100 Water
Components and Phases Components: The elements or compounds which are mixed initially (e.g., Al and Cu) Phases: The physically and chemically distinct material regions that result (e.g., α and β ). Aluminum- Copper Alloy α (darker phase) β (lighter phase)

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Effect of Temperature and Composition 70 80 100 60 40 20 0 Temperature (°C) C o =Composition (wt% sugar) L ( liquid solution i.e., syrup) A (70,20) 2 phases B (100,70) 1 phase 20 100 D (100,90) 2 phases 40 60 80 0 L (liquid) + S (solid sugar) Changing T can change # of phases: path A to B . Changing C o can change # of phases: path B to D . water-sugar system
Phase Diagrams Phase Diagram for Cu-Ni system • 2 phases: L (liquid) α (FCC solid solution) • 3 phase fields: L L + α α wt% Ni 20 40 60 80 100 0 1000 1100 1200 1300 1400 1500 1600 T(°C) L (liquid) α (FCC solid solution) L + α liquidus solidus Tell us about phases as function of T, C o , P. For this course: binary systems: just 2 components. independent variables: T and C o (P = 1atm is always used).

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Binary Phase Diagrams The simplest type of binary phase diagrams is the isomorphous system, in which the two constituents form a continuous solid solution over the entire composition range.
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## This note was uploaded on 02/07/2011 for the course MECH 340 taught by Professor Marwandarwiche during the Summer '07 term at American University of Beirut.

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Lecture_09_Phase_Diagrams - MECH 340 Engineering Material...

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