Chapter 7 - Deformation Mechanisms (Chapter 7) 1 What you...

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1 Deformation Mechanisms (Chapter 7)
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2 What you should understand by the end of this chapter: What are dislocations and why are they critical for plastic deformation? The basic mechanisms of dislocation slip . The role of dislocation and other major mechanisms responsible for strengthening metals. What happens to cold worked alloys during annealing .
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3 Why are Pure Metals So Soft? Very pure metals yield at low stress: σ y 20 – 40 Mpa Remember that for metals, E 100 – 200 GPa ⇒σ y 2 × 10 -4 E How much stress does it take to shear atomic planes? You have to break and reform many bonds simultaneously. This requires high levels of stress: ( σ y ) th 0.05 E Pure metals should be strong. .. … so why are they so soft? We need a mechanism to explain deformation.
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4 Dislocation Slip The concept of a line defect was introduced in chapter 4. There are edge and screw dislocations. Dislocation motion distorts the crystal. How does this work? What’s the mechanism? We call this slip or glide .
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5 Dislocations The concept of a dislocation was postulated in the 1930’s. Early evidence was indirect. Etch pits Slip trace on polished surface
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6 In the 1950s electron microscopy allowed us to “see” dislocations. Dislocation tangles in strained stainless steel Dislocation network in graphite Dislocations
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7 In Situ Electron Microscopy deformation during imaging Remember: You’re looking through a thin film approximately 100 nm thick.
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8 Modelling Dislocations in Nanocrystals Two-dimensional Crystal Dislocation starts here Dislocation glide path Downward Shearing force
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9 Dislocations & Yield Stress You’re breaking one bond (or one row of bounds) at a time. The applied force can be concentrated. Pure FCC crystals are very weak . The slip results are the same for edge or screw dislocations.
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10 Dislocation Concepts All crystals contain dislocations. They’re like growth defects. Let’s define dislocation density ( ρ ) , as the total length of a dislocation line / unit volume .
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Chapter 7 - Deformation Mechanisms (Chapter 7) 1 What you...

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