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09_-1 - E45 Lecture Chapter 6 Chapter 6 Diffusion ISSUES TO...

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Chapter 6 - E45 Lecture 9/30/09
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Chapter 6 - 2 ISSUES TO ADDRESS... How does diffusion occur? Why is it an important part of processing? How can the rate of diffusion be predicted for some simple cases? How does diffusion depend on structure and temperature? Chapter 6: Diffusion
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Chapter 6 - 3 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids – random (Brownian) motion Solids – vacancy diffusion or interstitial diffusion
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Chapter 6 - Gas A Gas B Diffusion of Gases partition
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Chapter 6 - Gas A Gas B Gas A+B Diffusion of Gases partition Remove partition and wait time t
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Chapter 6 - Gas A Gas B Gas A+B Diffusion of Gases partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 2 cm and D = 0.01 cm 2 /s, t = 100 s
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Chapter 6 - Liquid A Liquid B Diffusion of Liquids partition
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Chapter 6 - Liquid A Liquid B Liquid A+B Diffusion of Liquids partition Remove partition and wait time t
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Chapter 6 - Liquid A Liquid B Liquid A+B Diffusion of Liquids partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 2 cm and D = 10 -5 cm 2 /s, t = 10 5 s 28 h
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Chapter 6 - Solid A Solid B Diffusion of Solids partition
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Chapter 6 - Solid Cu Solid Ni Solid Cu+Ni Diffusion of Solids partition Remove partition and wait time t
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Chapter 6 - Solid Cu Solid Ni Solid Cu+Ni Diffusion of Solids partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 2 cm and D = 10 -23 cm 2 /s at room temperature t =3x10 19 s 5x10 15 y
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Chapter 6 - Solid Cu Solid Ni Solid Cu+Ni Diffusion of Solids partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 2 cm and D = 10 -12 cm 2 /s at T mp of Cu , t =10 12 s 5x10 4 y
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Chapter 6 - Solid Cu Solid Ni Solid Cu+Ni Diffusion of Solids partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 0.002 cm and D = 10 -12 cm 2 /s at T mp of Cu , t =10 6 s 15 d
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Chapter 6 - Solid Cu Solid Ni Solid Cu+Ni Diffusion of Solids partition Remove partition and wait time t Time for diffusion: x = diffusion distance = 2(Dt) 1/2 For x = 0.0002 cm and D = 10 -12 cm 2 /s at T mp of Cu , t =10 4 s 3 h
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Chapter 6 - Because diffusion is so slow in a solid, 1. It is usually impossible to homogenize a cast alloy by heat treatment alone. * 1. It is usually impossible to form a significant amount of a second phase by heat treatment alone.* In addition to heat treating a solid, other actions are often necessary in order to homogenize the material or to form a significant amount of a second phase.
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