ch8.-S10-1 - Chapter 8: Deformation and Strengthening...

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MY2100 – Ch 8, Part 1, Slide 1 Chapter 8: Deformation and Strengthening Mechanisms structure processing properties Performance ISSUES TO ADDRESS. .. What controls the strength of metals? How do we make a metal stronger? How can heating change strength and other properties?
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MY2100 – Ch 8, Part 1, Slide 2 Strengthening of metals - General Principle The yield strength is completely controlled by the onset of dislocation motion. Therefore, Anything that makes it harder for dislocations to move increases the yield strength of metals.
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MY2100 – Ch 8, Part 1, Slide 3 Strengthening mechanisms for metals There are 4 major ways to strengthen metals, and all work because they make dislocation motion more difficult. Unfortunately, they also reduce the toughness & ductility: 1. Reduce grain size 2. Add other elements in solid solution 3. Cold work (strain hardening) 4. Add second phase particles These mechanisms may be combined . For example, the world’s strongest structural material (with some ductility) is steel piano wire. It combines all four strengthening mechanisms , and can have a yield strength of 500,000 psi. One wire, 0.1” in diameter, can hold up a 4,000 lb SUV.
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MY2100 – Ch 8, Part 1, Slide 4 Grain size strengthening Dislocations cannot penetrate grain boundaries, because the crystal planes are discontinuous at the grain boundaries. Therefore, reducing the grain size increases strength because it creates more obstacles (in a given volume of material) resulting in shorter average slip distances for dislocations.
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MY2100 – Ch 8, Part 1, Slide 5 Quantification of grain size strengthening This effect of a reduction in grain size can be quantified using the so-called “Hall-Petch Equation: where σ yield is the yield strength, d is the average grain size, and σ o and k y are material constants. The increases in strength at very small grain sizes can be enormous. One area of current research is on so-called “nanostructured metals”, which have grain sizes from 20 to 200 nm. They can have very high strengths (increases by a factor of 15-20). Eq. 8.6
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MY2100 – Ch 8, Part 1, Slide 6 Strength of brass as a function of grain size [grain size (mm)] -0.5 50 10 0 150 20 0 0 4 8 12 16 10 -1 -2 5ξ10 -3 γραινσιζε,δ(μμ29 1 κ ψ 100 μ m 5 μ m Yield strength triples as the grain size is reduced from 100 μ m to 5 μ m.
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MY2100 – Ch 8, Part 1, Slide 7 Solid solution strengthening - Why it works!
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ch8.-S10-1 - Chapter 8: Deformation and Strengthening...

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