ch7.3-S09 - Chapter 7: Mechanical Properties structure...

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MY2100 – Ch 7, Part 3, Slide 1 Chapter 7: Mechanical Properties structure processing properties Performance Mechanical behavior of ceramics and polymers ISSUES TO ADDRESS. .. Ceramics : How do their properties differ from metals? Polymers: How do their properties differ from metals?
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MY2100 – Ch 7, Part 3, Slide 2 Quick review of why metals deform plastically Remember that metals contain dislocations. A dislocation moving from left to right (because of a shear stress) can shear the crystal with less energy than shearing all the bonds across the slip plane; this causes a permanent shape change (plastic deformation).
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MY2100 – Ch 7, Part 3, Slide 3 Dislocations moving produce macroscopic strain Edge dislocation Doing this on large numbers of planes in different grains results in measurable shape changes in metals. This is why metals are ductile.
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MY2100 – Ch 7, Part 3, Slide 4 Can ceramics plastically deform? Ceramics are usually compounds of metals and non-metals. e.g. Al 2 O 3 , MgO, SiO 2 Si 3 N 4 , SiC Glasses Fired clays, porcelain In ceramics with ionic bonding, dislocations cannot move at low temperatures because the structure is made of cations & anions. Why? + - These materials have complicated crystal structures , as we found in Chapter 3. • (NaCl): Dislocation motion is hard because you -need to avoid ++ and -- neighbors.
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Ch 7, Part 3, Slide 5 Can ceramics plastically deform? When ceramics have strong covalent bonds, remember that these are strong , directional bonds and, as a result, dislocations cannot move at low temperatures.
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ch7.3-S09 - Chapter 7: Mechanical Properties structure...

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