Chapter8 PRS

Chapter8 PRS - Introduction to Chapter 8 In Chapter 7 we...

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Introduction to Chapter 8 • In Chapter 7 we introduced the concepts associated with phase diagrams. • Phase diagrams are important, but because of the importance of – Time at temperature – Interfacial energies – Microstructure scale – Morphologies of the microstructural elements – Stress state phase diagrams are only guidelines when considering processing and stability of an engineering product.
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Introduction to Chapter 8 • Kinetics incorporates the influence of time and temperature on microstructure and thus properties. • Because the major emphasis of the work of a materials’ engineer is to improve properties by manipulating microstructure an understanding of the fundamentals of kinetics is necessary.
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Chapter 8 Kinetics of Microstructural Transformations • Structural transformations • Driving force • Homogeneous nucleation • Heterogeneous nucleation • Matrix/Precipitate Interfaces • Coherent • Semi-coherent • Incoherent • Growth of a Phase
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Chapter 8 • Applications to Engineering Materials • Phase transformations in steels – Decomposition of austenite to pearlite, bainite and/or martensite – Isothermal and athermal transformations – T-T-T curves- isothermal (IT) and continuous cooling (CT) – Hardenability and the the Jominy end- quench tests
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Chapter 8 • Applications to Engineering Materials • Precipitation Hardening – General Characteristics – Aluminum-Copper Alloys • Solidification- Coring • Recovery and Recrystallization • Crystallization (Devitrification) of an Oxide Glass • Crystallization of a Polymer
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Homogeneous Nucleation •A t T m , the liquid and the solid have the same energy and coexist • Above (to the right of) T m , the liquid exists because the energy of the liquid is lower • Below (to the left of) T m , the solid exists because the energy of the solid is lower Gibbs Free Energy, G V T G V s G V l T T m
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Solidification of a Pure Substance- Driving Force for the Transformation Gibbs Free Energy, G V Temperature, °C G V s G V l T G V l ² s T m
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Driving Force for Solidification of a Pure Substance 0 ls s l VV V V m V V V m GG G GH T S At T T G H S T →→ ∆= ∆− = 0 1 p V m m m m V V m For any T when c H T T T T TT T H GT T ⎛⎞ ⎜⎟ ⎝⎠ Driving “Force”
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Homogeneous Nucleation Solid Solid Liquid Liquid Liquid Liquid Solid/liquid Solid/liquid interface interface At some positive undercooling T
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• When a liquid is cooled below T m , there is a driving “force” for solidification • The volume free energy of the solid phase, G v S , is less than the volume free energy of the liquid phase, G v L • Conditions in the liquid are such that the solid forms directly from clusters • Homogeneous nucleation is relatively rare in practice but is well understood Homogeneous Nucleation
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Homogeneous Nucleation • Start off with a liquid that is below T m
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Chapter8 PRS - Introduction to Chapter 8 In Chapter 7 we...

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