ch6-S10 - Chapter 6 Diffusion structure processing...

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MY2100 – Ch 6, Slide 1 Chapter 6: Diffusion structure processing properties Performance Diffusion involves atoms moving and changing places and is important in many types of material processes. You are probably more familiar with diffusion in gases and liquids but it also occurs in solids using the point defects we discussed in the last chapter.
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MY2100 – Ch 6, Slide 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?
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MY2100 – Ch 6, Slide 3 Why is diffusion important? One example Design application: transmission gear for a car or truck. Attributes needed at the surface of the gear teeth? Attributes needed at the base of the gear teeth?
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MY2100 – Ch 6, Slide 4 Here is the dilemma! Preview: In steel, strength goes up but toughness goes down with C.
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MY2100 – Ch 6, Slide 5 Processing solution using diffusion Case Hardening : -- Diffuse carbon atoms between the host iron atoms at the surface. -- Example of interstitial diffusion. • Result: The “case region” is -- hard to deform: C atoms "lock" planes from shearing . -- hard to crack: C atoms put the surface in compression. But, gear teeth interior is still -- ductile and tough
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MY2100 – Ch 6, Slide 6 increasing elapsed time First type of diffusion in solids: vacancy diffusion applies to host atoms or substitutional impurities atoms exchange positions with vacancies rate depends on: --number of vacancies --activation energy needed to cause the exchange (need to distort & break bonds)
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MY2100 – Ch 6, Slide 7 Second type of diffusion in solids: interstitial diffusion Interstitial Diffusion : Movement of smaller atoms into the empty spaces between host atoms (such voids are almost always “available”) - more rapid than vacancy diffusion.
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MY2100 – Ch 6, Slide 8 What is the driving force for diffusion in solids? Diffusion of an individual atom is random and probabilistic . Thus, the interstitial black atom has an equal probability of moving up, down, left or right.
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MY2100 – Ch 6, Slide 9 What if a concentration gradient exists?
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