MSE2001H_8_1

MSE2001H_8_1 - 1 Chapter 8 Kinetics and Microstructure of...

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Unformatted text preview: 1 Chapter 8 Kinetics and Microstructure of Structural Transformations Structural transformations Driving force g Homogeneous nucleation Heterogeneous nucleation Matrix/Precipitate Interfaces Coherent emi oherent Semi-coherent Incoherent Growth of a Phase Applications to Engineering Materials 2 Kinetics Deals with the rate at which a process occurs. rimary variables: ime and (Temperature & Pressure) Primary variables: Time and (Temperature & Pressure) In this chapter, we will see the kinetics of phase transformations, especially, the liquid-solid phase transformation. F = C P + 2 C- Components P- Number of phases F- Degrees of freedom = 2 Q) What is the reason for the phase change? g F = 2 F = 1 F = 1 = 2 F = 2 What is the driving force causing the transformation? F = 0 F = 0 F = 2 Free Energy 3 Free Energy and Phase Transition Driving Force? G V G V A t T m , the liquid and the solid have the same free energy and coexist Energy, Energy, Above (to the right of) T m , the liquid exists because the energy f the liquid is lower s Free E G V S T s Free E G V S T of the liquid is lower Below (to the left of) T m , the olid exists because the energy Gibbs G V L T Gibbs G V L T solid exists because the energy of the solid is lower T m T m 4 Solidification of a Pure Substance- Driving Force for e Transformation the Transformation G V G V L S nergy, s Free En G V S T T T m = Gibbs G V L T Temperature, C T m T: extent of undercooling Driving Force for Solidification of a Pure Substance For any T l s s l V V V l s l s l s G G G H T S = = l s when c = V V V m s G H T S At T T = p l s l s l s V V V H G H T = l s V l s l s V G H = = 1 m l s l s V V T T G H = V m S T m l s l s m V T T T H = At T=T m V V m l s l s V G H T H T = ree Energy, G V G V S G V L S V m G T T = Gibbs F Temperature, C G V L T T m 6 Driving Force for Solidification of a Pure Substance l s l s V V H G T = nergy, G V G V L S m T T T T m = bs Free En G V S T If T is lowered ( T is increased), Gib emperature...
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MSE2001H_8_1 - 1 Chapter 8 Kinetics and Microstructure of...

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