ch7 - Deformation Mechanisms Chapter 7 What you should...

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1 Deformation Mechanisms Chapter 7 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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2 Why are Pure Metals So Soft? ¾ 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 ¾ 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 ¾ Pure metals should be strong. .. … so why are they so soft? We need a mechanism to explain deformation.
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3 Dislocation Slip ¾ The concept of a line defect was introduced in chapter 5. ± There are edge and screw dislocations. ¾ Dislocation motion distorts the crystal. ± How does this work? What’s the mechanism? ¾ We call this slip or glide .
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4 Dislocations ¾ 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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5 Dislocations ¾ In the 1950s electron microscopy allowed us to “see” dislocations. Dislocation tangles in strained stainless steel Dislocation network in graphite
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6 In Situ Electron Microscopy ¾ deformation during imaging ± Remember: You’re looking through a thin film approximately 100 nm thick.
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7 Modelling Dislocations in Nanocrystals Two-dimensional Crystal Dislocation starts here Dislocation glide path Downward Shearing force
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8 Dislocations & Yield Stress ¾ You’re breaking one bond (or one row of bounds) at a time. ± The applied force can be concentrated. ¾ Pure FCC crystals are very weak . ¾ The slip results are the same for edge or screw dislocations.
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9 Dislocation Concepts ¾ All crystals contain dislocations. ± They’re like growth defects. ¾ Let’s define dislocation density ( ρ ) , as the total length of a dislocation line / unit volume .
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This note was uploaded on 09/28/2011 for the course 1M03 material 1 taught by Professor Llllll during the Spring '11 term at McMaster University.

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ch7 - Deformation Mechanisms Chapter 7 What you should...

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