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EGN3365-9 - Chapter 9 Phase Diagrams ISSUES TO ADDRESS When...

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Chapter 9 - 1 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 Phase B Phase A Nickel atom Copper atom
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Chapter 9 - 2 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 C o =Composition (wt% sugar) L (liquid solution i.e., syrup) Solubility Limit L (liquid) + S (solid sugar) 20 40 60 80 100 Pure Water Adapted from Fig. 9.1, Callister 7e.
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Chapter 9 - 3 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) Adapted from chapter-opening photograph, Chapter 9, Callister 3e. A phase maybe defined as a homogeneous portion of a system that has uniform physical and chemical characteristics.
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Chapter 9 - 4 Effect of T & Composition ( C o ) • Changing T can change # of phases: Adapted from Fig. 9.1, Callister 7e. D (100°C,90) 2 phases B (100°C,70) 1 phase path A to B . • Changing C o can change # of phases: path B to D . 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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Chapter 9 - 5 Phase Equilibrium Equilibrium: minimum energy state for a given T, P, and composition (i.e. equilibrium state will persist indefinitely for a fixed T, P and composition). Phase Equilibrium: If there is more than 1 phase present, phase characteristics will stay constant over time. Phase diagrams tell us about equilibrium phases as a function of T, P and composition (here, we’ll always keep P constant for simplicity).
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Chapter 9 - 6 Unary Systems Triple point
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Chapter 9 - 7 Phase Equilibria 0.1278 1.8 FCC Cu 0.1246 1.9 FCC Ni r (nm) electroneg Crystal Structure Both have the same crystal structure (FCC) and have similar electronegativities and atomic radii ( W. Hume – Rothery rules ) suggesting high mutual solubility. Simple solution system (e.g., Ni-Cu solution) Ni and Cu are totally miscible in all proportions.
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Chapter 9 - 8 Unary Systems Single component system Consider 2 metals: Cu has melting T = 1085 o C Ni has melting T = 1453 o C (at standard P = 1 atm) 1085 o C Cu solid liquid T 1453 o C Ni solid liquid T What happens when Cu and Ni are mixed?
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Chapter 9 - 9 Binary Isomorphous Systems 2 components Complete liquid and solid solubility 1085 o C Cu solid liquid T 1453 o C Ni solid liquid T wt% Ni 0 100 Expect T m of solution to lie in between T m of two pure components For a pure component, complete melting occurs before T
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