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ch10 - ISSUES TO ADDRESS Chapter 9 Phase Diagrams for...

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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? Chapter 9: Phase Diagrams for Metallic Systems Phase B Phase A Nickel atom Copper atom
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Phase Equilibria: Solubility Limit Introduction Solutions – solid solutions, single phase Mixtures – more than one phase Solubility Limit : Max concentration for which only a single phase solution occurs. Question: What is the solubility limit at 20°C ? Answer: 65 wt% sugar . If C o < 65 wt% sugar: syrup If C o > 65 wt% sugar: syrup + sugar. 65 Sucrose/Water Phase Diagram Pure Sugar Temperature (°C) 0 20 40 60 80 100 =Composition (wt% sugar) L (liquid solution i.e., syrup) Solubility Limit L (liquid) + S (solid sugar) 20 40 60 80 100 Pure Water
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Components : The elements or compounds which are present in the mixture (e.g., Al and Cu) Phases : The physically and chemically distinct material regions that result (e.g., α and β ). Aluminum- Copper Alloy Components and Phases α (darker phase) β (lighter phase)
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Effect of T & Composition ( C o ) Changing T can change # of phases: path A to B . Changing C o can change # of phases: path B to D . D (100°C,90) 2 phases B (100°C,70) 1 phase A (20°C,70) 2 phases 70 80 100 60 40 20 0 Temperature (°C) C o =Composition (wt% sugar) L ( liquid solution i.e., syrup) 20 100 40 60 80 0 L (liquid) + S (solid sugar) water- sugar system
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Phase Equilibria Crystal Structure electro negativity r (nm) Ni FCC 1.9 0.1246 Cu FCC 1.8 0.1278 Both have the same crystal structure (FCC) and have similar electronegativities and atomic radii ( W. Hume – Rothery rules ) suggesting high mutual solubility. Ni and Cu are totally miscible in all proportions. Simple solution system (e.g., Ni-Cu solution)
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Phase Diagrams Indicate phases as function of T , C o , and P . For this course: -binary systems: just 2 components. -independent variables: T and C o ( P = 1 atm is almost always used). 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
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Phase Diagrams : # and types of phases Rule 1: If we know T and C o , then we know: --the # and types of phases present. Examples: A (1100°C, 60): 1 phase: α B (1250°C, 35): 2 phases: L + 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 Cu-Ni phase diagram B (1250°C,35) A (1100°C,60)
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Phase Diagrams : composition of phases Rule 2: If we know T and C o , then we know: --the composition of each phase.
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