MSE2001H_8_2 - 1 Chapter 8 Kinetics d Mi Ki ti and...

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1 Chapter 8 Kinetics and Microstructure of Structural Transformations Structural transformations •D r i v ing force g • Homogeneous nucleation • Heterogeneous nucleation Matrix/Precipitate Interfaces • Coherent emi oherent • Semi-coherent • Incoherent Growth of a Phase Applications to Engineering Materials
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2 atrix/Precipitate Interfaces Matrix/Precipitate Interfaces Three general types of interfaces between two solids 1. Coherent interface 2. Semicoherent interface 3. Incoherent interference
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3 Coherent Interface rises when there is a one- - ne correspondence of atomic planes across arises when there is a one to one correspondence of atomic planes across the boundary separating two phases ame lattice lightly Same lattice parameter Slightly different lattice parameter TEM image Coherent interface between α -Al and Al 2 CuMg Coherent interface between α -Al and Al 3 Li Coherent interface between Al 3 Li and Al 2 CuMg
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4 Coherent Interface rises when there is a one- - ne correspondence of atomic planes across arises when there is a one to one correspondence of atomic planes across the boundary separating two phases ame lattice lightly Same lattice parameter Slightly different lattice parameter One or both phases must distort in order to maintain continuity of the plane across the interface. Æ coherency strain. TEM image Fully coherent Al 3 Li precipitates form in Al/Li alloys.
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5 Semicoherent Interface: Partially Coherent Interfaces As the lattice parameter difference increases, the coherency strains increase correspondingly. Thus, introducing the periodic dislocation arrays help the interface ccommodate the misfit accommodate the misfit. 0.1 μ m Dislocation interface Its interfacial energy has two components: Chemical mismatch 1. Chemical mismatch 2. Dislocations
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6 Incoherent Interfaces No atomic match across the interface I n t e r f a c Largest interfacial energy 200 mJ/m 2 for coherent interface 500 mJ/m 2 for semicoherent interface 1000 mJ/m 2 for incoherent interface
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7 Homogeneous/Heterogeneous Nucleation Type of Interface (relative) Interfacial energy Nucleation Process Coherent small Homogeneous arially Coherent termediate eterogeneous Parially Coherent intermediate Heterogeneous Noncoherent large Heterogeneous
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This note was uploaded on 06/29/2009 for the course MSE 2001 taught by Professor Tannebaum during the Spring '08 term at Georgia Institute of Technology.

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MSE2001H_8_2 - 1 Chapter 8 Kinetics d Mi Ki ti and...

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