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

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Unformatted text preview: 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. 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. 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 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 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 Homogeneous Nucleation At 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 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 lI s T m Driving Force for Solidification of a Pure Substance l s s l V V V l s l s l s V V V m l s V l s l s V V m G G G G H T S At T T G H S T =- = - = = = 1 l s p l s l s l s V V V m l s l s V V m l s l s m V V m l s l s V V m For any T when c H G H T T T G H T T T G H T H G T T = = - = - - = = Homogeneous Nucleation Solid Solid Liquid Liquid Liquid Liquid Solid/liquid Solid/liquid interface interface At some positive undercooling T 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 Homogeneous Nucleation...
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This note was uploaded on 09/12/2011 for the course MSE 2001 taught by Professor Tannebaum during the Fall '08 term at Georgia Institute of Technology.

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

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